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@sandimetz Apr 2013
The Magic Tricks of
Testing
Sandi Metz
Friday, April 5, 13
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@sandimetz Apr 2013
Why do
I hate my tests?
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@sandimetz Apr 2013
Why do
I hate my tests?
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@sandimetz Apr 2013
They’re slow.
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@sandimetz Apr 2013
They break every time
I change something.
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@sandimetz Apr 2013
They cost more than
they’re worth.
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@sandimetz Apr 2013
They are misery
incarnate.
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@sandimetz Apr 2013
It doesn’t have to be
this way.
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@sandimetz Apr 2013
You just need to
delete some tests.
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@sandimetz Apr 2013
Unit Tests: Goals
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@sandimetz Apr 2013
Thorough
Unit Tests: Goals
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@sandimetz Apr 2013
Thorough
Stable
Unit Tests: Goals
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@sandimetz Apr 2013
Thorough
Stable
Fast
Unit Tests: Goals
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@sandimetz Apr 2013
Thorough
Stable
Fast
Few
Unit Tests: Goals
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@sandimetz Apr 2013
What does your app
feel like?
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@sandimetz Apr 2013
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@sandimetz Apr 2013
Focus on messages.
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@sandimetz Apr 2013
Objects have a simple
understanding of
messages.
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@sandimetz Apr 2013
Object
Under
Test
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@sandimetz Apr 2013
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@sandimetz Apr 2013
Object
Under
Test
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@sandimetz Apr 2013
Object
Under
Test
Received from
others
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@sandimetz Apr 2013
Object
Under
Test
Received from
others
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@sandimetz Apr 2013
Object
Under
Test
Incoming
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@sandimetz Apr 2013
Sent to
others
Incoming
Object
Under
Test
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@sandimetz Apr 2013
Sent to
others
Incoming
Object
Under
Test
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@sandimetz Apr 2013
Incoming Outgoing
Object
Under
Test
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@sandimetz Apr 2013
Sent to self
Incoming Outgoing
Object
Under
Test
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@sandimetz Apr 2013
Sent to self
Incoming Outgoing
Object
Under
Test
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@sandimetz Apr 2013
Incoming
Sent to Self
Outgoing
Message Origin
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@sandimetz Apr 2013
Sent to self
Incoming Outgoing
Object
Under
Test
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@sandimetz Apr 2013
Sent to self
Incoming Outgoing
Object
Under
Test
Friday, April 5, 13
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@sandimetz Apr 2013
Sent to self
Incoming Outgoing
Object
Under
Test
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@sandimetz Apr 2013
Sent to self
Incoming Outgoing
Query Command
Object
Under
Test
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@sandimetz Apr 2013
Query
Command
Message Type
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@sandimetz Apr 2013
Query: Returns something
but changes nothing.
Command: Returns nothing
but changes something.
Type
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@sandimetz Apr 2013
Query: Returns something
but changes nothing.
Command: Returns nothing
but changes something.
Type
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@sandimetz Apr 2013
Query: Returns something
but changes nothing.
Command: Returns nothing
but changes something.
Type
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@sandimetz Apr 2013
We conflate
commands and queries
at our peril.
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@sandimetz Apr 2013
But we do it all the time.
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@sandimetz Apr 2013
3 message Origins
X
2 message Types
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Query Command
Message Origin x Type
Outgoing
Incoming
Type
POV
@sandimetz Apr 2013
Message
Sent to Self
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Part 1
Incoming
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Incoming
Query Messages
Incoming
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Incoming
Wheel
diameter
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Wheel
attr_reader
:rim,
:tire
def
initialize(rim,
tire)
#
...
end
def
diameter
rim
+
(tire
*
2)
end
#
...
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Wheel
attr_reader
:rim,
:tire
def
initialize(rim,
tire)
#
...
end
def
diameter
rim
+
(tire
*
2)
end
#
...
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Wheel
attr_reader
:rim,
:tire
def
initialize(rim,
tire)
#
...
end
def
diameter
rim
+
(tire
*
2)
end
#
...
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
WheelTest
<
MiniTest::Unit::TestCase
def
test_calculates_diameter
wheel
=
Wheel.new(26,
1.5)
assert_in_delta(29,
wheel.diameter,
0.01)
end
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
WheelTest
<
MiniTest::Unit::TestCase
def
test_calculates_diameter
wheel
=
Wheel.new(26,
1.5)
assert_in_delta(29,
wheel.diameter,
0.01)
end
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
WheelTest
<
MiniTest::Unit::TestCase
def
test_calculates_diameter
wheel
=
Wheel.new(26,
1.5)
assert_in_delta(29,
wheel.diameter,
0.01)
end
end
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@sandimetz Apr 2013
Rule
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@sandimetz Apr 2013
Test incoming query messages
Rule
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@sandimetz Apr 2013
Test incoming query messages
by making assertions
about what they send back.
Rule
Friday, April 5, 13
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Query Command
The Unit Testing Minimalist
Incoming
Type
POV
@sandimetz Apr 2013
Message
Sent to Self
Outgoing
Friday, April 5, 13
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Query Command
Assert
result
The Unit Testing Minimalist
Incoming
Type
POV
@sandimetz Apr 2013
Message
Sent to Self
Outgoing
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Incoming
Another
Incoming
Query Message
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Gear
gear_inches
Incoming
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
#
...
def
gear_inches
ratio
*
wheel.diameter
end
private
def
ratio
chainring
/
cog.to_f
end
#
...
end
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
#
...
def
gear_inches
ratio
*
wheel.diameter
end
private
def
ratio
chainring
/
cog.to_f
end
#
...
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
#
...
def
gear_inches
ratio
*
wheel.diameter
end
private
def
ratio
chainring
/
cog.to_f
end
#
...
end
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
#
...
def
gear_inches
ratio
*
wheel.diameter
end
private
def
ratio
chainring
/
cog.to_f
end
#
...
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
#
...
def
gear_inches
ratio
*
wheel.diameter
end
private
def
ratio
chainring
/
cog.to_f
end
#
...
end
Wheel
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@sandimetz Apr 2013
Sight along the edges of
the space capsule.
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@sandimetz Apr 2013
Sight along the edges of
the space capsule.
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
end
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
end
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
end
end
Friday, April 5, 13
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@sandimetz Apr 2013
Test the interface,
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@sandimetz Apr 2013
Test the interface,
not the implementation.
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Incoming
Command Messages
Incoming
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Gear
set_cog
Incoming
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
def
set_cog(new_cog)
@cog
=
new_cog
end
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
def
set_cog(new_cog)
@cog
=
new_cog
end
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
def
set_cog(new_cog)
@cog
=
new_cog
end
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_set_cog
gear
=
Gear.new
gear.set_cog(27)
assert(27,
gear.cog)
end
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_set_cog
gear
=
Gear.new
gear.set_cog(27)
assert(27,
gear.cog)
end
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_set_cog
gear
=
Gear.new
gear.set_cog(27)
assert(27,
gear.cog)
end
end
Send the message
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_set_cog
gear
=
Gear.new
gear.set_cog(27)
assert(27,
gear.cog)
end
end
Assert the side effect
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@sandimetz Apr 2013
Rule
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@sandimetz Apr 2013
Test incoming command messages
Rule
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@sandimetz Apr 2013
Test incoming command messages
by making assertions
about direct public side effects.
Rule
Friday, April 5, 13
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Query Command
Assert
result
The Unit Testing Minimalist
Incoming
Type
POV
@sandimetz Apr 2013
Message
Sent to Self
Outgoing
Friday, April 5, 13
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Query Command
Assert
result
The Unit Testing Minimalist
Incoming
Type
POV
@sandimetz Apr 2013
Message
Assert
direct public
side effects
Sent to Self
Outgoing
Friday, April 5, 13
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@sandimetz Apr 2013
DRY It Out
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@sandimetz Apr 2013
Receiver of incoming message
has sole responsibility
DRY It Out
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@sandimetz Apr 2013
Receiver of incoming message
has sole responsibility
for asserting
the result
direct public side effects.
DRY It Out
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Part 2
to Self
Incoming
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming to Self
Incoming
Messages Sent to Self
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
#
...
def
gear_inches
ratio
*
wheel.diameter
end
private
def
ratio
chainring
/
cog.to_f
end
#
...
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Anti-Pattern
to Self
Incoming
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_ratio
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(4.7,
gear.ratio,
0.01)
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_ratio
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(4.7,
gear.ratio,
0.01)
end
Test the private method?
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_ratio
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(4.7,
gear.ratio,
0.01)
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_ratio
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(4.7,
gear.ratio,
0.01)
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_ratio
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(4.7,
gear.ratio,
0.01)
end
Redundant:
Other tests already prove Gear works.
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Anti-Pattern
to Self
Incoming
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
#
...
def
gear_inches
ratio
*
wheel.diameter
end
private
def
ratio
chainring
/
cog.to_f
end
#
...
end
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
#
...
def
gear_inches
ratio
*
wheel.diameter
end
private
def
ratio
chainring
/
cog.to_f
end
#
...
end
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
#
...
def
gear_inches
ratio
*
wheel.diameter
end
private
def
ratio
chainring
/
cog.to_f
end
#
...
end
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
end
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
gear.expect(:ratio)
gear.verify
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
gear.expect(:ratio)
gear.verify
end
Expect to send
the private method?
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
gear.expect(:ratio)
gear.verify
end
Over Speci ed:
Adds no safety yet breaks every
time the implementation changes.
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@sandimetz Apr 2013
Rule
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@sandimetz Apr 2013
Do not test private methods.
Rule
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@sandimetz Apr 2013
Do not test private methods.
Do not make assertions about their result.
Rule
Friday, April 5, 13
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@sandimetz Apr 2013
Do not test private methods.
Do not make assertions about their result.
Do not expect to send them.
Rule
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@sandimetz Apr 2013
Temporarily breaking this rule
can save $$$
during development.
Caveat
Friday, April 5, 13
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Query Command
Assert
result
Assert
direct public
side-effects
The Unit Testing Minimalist
Incoming
Type
POV
@sandimetz Apr 2013
Message
Sent to Self
Outgoing
Friday, April 5, 13
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Query Command
Assert
result
Assert
direct public
side-effects
The Unit Testing Minimalist
Incoming
Type
POV
@sandimetz Apr 2013
Ignore
Message
Sent to Self
Outgoing
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Part 3
Outgoing
to Self
Incoming
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming Outgoing
to Self
Incoming
Outgoing
Query Messages
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming Outgoing
to Self
Incoming
Spoiler Alert
Same rules as ‘Sent to Self’
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@sandimetz Apr 2013
to Self Outgoing
Incoming
diameter
Gear
gear_inches
Wheel
Outgoing
to Self
Incoming
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@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
def
gear_inches
ratio
*
wheel.diameter
end
private
def
ratio
chainring
/
cog.to_f
end
#
...
end
Outgoing
to Self
Incoming
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@sandimetz Apr 2013
to Self Outgoing
Incoming
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
def
gear_inches
ratio
*
wheel.diameter
end
private
def
ratio
chainring
/
cog.to_f
end
#
...
end
Outgoing
to Self
Incoming
Wheel
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@sandimetz Apr 2013
to Self Outgoing
Incoming
Gear
Gear
Outgoing
Wheel
diameter
Incoming
Query
Outgoing
to Self
Incoming
gear_inches
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@sandimetz Apr 2013
to Self Outgoing
Incoming
Anti-Pattern
to Self
Incoming Outgoing
to Self
Incoming
Friday, April 5, 13
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
end
end
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
assert_in_delta(29,
gear.wheel.diameter,
0.01)
end
end
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
assert_in_delta(29,
gear.wheel.diameter,
0.01)
end
end
Assert result of
outgoing query?
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
assert_in_delta(29,
gear.wheel.diameter,
0.01)
end
end
Redundant:
This duplicates Wheel’s tests.
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Anti-Pattern
to Self
Incoming Outgoing
to Self
Incoming
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
gear.wheel.expect(:diameter)
gear.verify
end
end
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
gear.wheel.expect(:diameter)
gear.verify
end
end
Expect to send
outgoing query?
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_calculates_gear_inches
gear
=
Gear.new(
chainring:
52,
cog:
11,
wheel:
Wheel.new(26,
1.5))
assert_in_delta(137.1,
gear.gear_inches,
0.01)
gear.wheel.expect(:diameter)
gear.verify
end
end
Over Speci ed:
Adds no safety yet breaks every
time the implementation changes.
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@sandimetz Apr 2013
Rule
Friday, April 5, 13
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@sandimetz Apr 2013
Rule
Do not test
outgoing query messages.
Friday, April 5, 13
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@sandimetz Apr 2013
Rule
Do not test
outgoing query messages.
Do not make assertions about their result.
Friday, April 5, 13
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Slide 135 text
@sandimetz Apr 2013
Rule
Do not test
outgoing query messages.
Do not make assertions about their result.
Do not expect to send them.
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@sandimetz Apr 2013
If the message has
no visible side-effects
Friday, April 5, 13
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@sandimetz Apr 2013
If the message has
no visible side-effects
the sender
should not test it.
Friday, April 5, 13
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Query Command
Assert
result
Assert
direct public
side effects
The Unit Testing Minimalist
Incoming
Type
POV
@sandimetz Apr 2013
Message
Ignore
Sent to Self
Outgoing
Friday, April 5, 13
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Slide 139 text
Query Command
Assert
result
Assert
direct public
side effects
The Unit Testing Minimalist
Incoming
Type
POV
@sandimetz Apr 2013
Message
Ignore
Sent to Self
Outgoing
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Outgoing
Command Messages
to Self
Incoming Outgoing
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
diameter
gear_inches
set_cog
changed
to Self
Incoming Outgoing
Wheel
Gear
Obs
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
def
set_cog(new_cog)
@cog
=
new_cog
end
end
Change #set_cog to add observer.
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
Gear
attr_reader
:chainring,
:cog,
:wheel
def
initialize(args)
#
...
end
def
set_cog(new_cog)
@cog
=
new_cog
end
end
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
Gear
attr_reader
:chainring,
:cog,
:wheel,
:observer
def
initialize(args)
#
...
@observer
=
args[:observer]
end
def
set_cog(new_cog)
@cog
=
new_cog
end
end
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
Gear
#
...
def
set_cog(new_cog)
@cog
=
new_cog
end
end
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
Gear
#
...
def
set_cog(new_cog)
@cog
=
new_cog
end
end
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
Gear
#
...
def
set_cog(new_cog)
@cog
=
new_cog
changed
@cog
end
def
changed
observer.changed(chainring,
cog)
end
end
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
Gear
#
...
def
set_cog(new_cog)
@cog
=
new_cog
changed
@cog
end
def
changed
observer.changed(chainring,
cog)
end
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Gear
set_cog
changed
Obs
to Self
Incoming Outgoing
Friday, April 5, 13
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@sandimetz Apr 2013
to Self Outgoing
Incoming
Gear
set_cog
changed
Obs
to Self
Incoming Outgoing
Side
Effects
Friday, April 5, 13
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@sandimetz Apr 2013
to Self Outgoing
Incoming
changed
Gear
set_cog
Obs
Incoming
Command
to Self
Incoming Outgoing
Side
Effects
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@sandimetz Apr 2013
to Self Outgoing
Incoming
Gear
Outgoing
set_cog
Obs
changed
Incoming
Command
to Self
Incoming Outgoing
Side
Effects
Friday, April 5, 13
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_set_cog
gear
=
Gear.new
gear.set_cog(27)
assert(27,
gear.cog)
end
end
Make existing #set_cog test run.
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_set_cog
gear
=
Gear.new
@gear.set_cog(27)
assert(27,
@gear.cog)
end
end
Friday, April 5, 13
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_set_cog
@observer
=
Obs.new
gear
=
Gear.new
@gear.set_cog(27)
assert(27,
@gear.cog)
end
end
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_set_cog
@observer
=
Obs.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@gear.set_cog(27)
assert(27,
@gear.cog)
end
end
Friday, April 5, 13
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_set_cog
@observer
=
Obs.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@gear.set_cog(27)
assert(27,
@gear.cog)
end
end
Runs real @observer#changed
1st version
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_set_cog
@observer
=
Obs.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@observer.stub(:changed,
@gear.set_cog(27))
do
assert(27,
@gear.cog)
end
end
end
Variant
Runs fake @observer#changed
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
Gear
#
...
def
set_cog(new_cog)
@cog
=
new_cog
changed
@cog
end
def
changed
observer.changed(chainring,
cog)
end
end
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
Gear
#
...
def
set_cog(new_cog)
@cog
=
new_cog
changed
@cog
end
def
changed
observer.changed(chainring,
cog)
end
end
The app will be correct only if
this message gets sent.
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
Wheel
Incoming
Query
changed
Side
Effects
Gear
Incoming
Command
Outgoing
Obs
Outgoing
diameter
gear_inches
set_cog
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
Side
Effects
Gear Wheel
Obs
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
Side
Effects
Gear Wheel
Obs
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
Side
Effects
Gear Wheel
Obs
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self Outgoing
Incoming
Anti-Pattern
to Self
Incoming Outgoing
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_saves_changed_cog_in_db
@observer
=
Obs.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@gear.set_cog(27)
#
assert
something
about
the
state
of
the
db
end
end
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_saves_changed_cog_in_db
@observer
=
Obs.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@gear.set_cog(27)
#
assert
something
about
the
state
of
the
db
end
end
Friday, April 5, 13
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_saves_changed_cog_in_db
@observer
=
Obs.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@gear.set_cog(27)
#
assert
something
about
the
state
of
the
db
end
end
Friday, April 5, 13
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_saves_changed_cog_in_db
@observer
=
Obs.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@gear.set_cog(27)
#
assert
something
about
the
state
of
the
db
end
end
Depends on the distant side effect.
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_saves_changed_cog_in_db
@observer
=
Obs.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@gear.set_cog(27)
#
assert
something
about
the
state
of
the
db
end
end
Is this Gear’s responsibility?
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_saves_changed_cog_in_db
@observer
=
Obs.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@gear.set_cog(27)
#
assert
something
about
the
state
of
the
db
end
end
Is this Gear’s responsibility?
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_saves_changed_cog_in_db
@observer
=
Obs.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@gear.set_cog(27)
#
assert
something
about
the
state
of
the
db
end
end
This is an integration test.
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_notifies_observers_when_cogs_change
@observer
=
MiniTest::Mock.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@observer.expect(:changed,
true,
[52,
27])
@gear.set_cog(27)
@observer.verify
end
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_notifies_observers_when_cogs_change
@observer
=
MiniTest::Mock.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@observer.expect(:changed,
true,
[52,
27])
@gear.set_cog(27)
@observer.verify
end
Friday, April 5, 13
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_notifies_observers_when_cogs_change
@observer
=
MiniTest::Mock.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@observer.expect(:changed,
true,
[52,
27])
@gear.set_cog(27)
@observer.verify
end
Friday, April 5, 13
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@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_notifies_observers_when_cogs_change
@observer
=
MiniTest::Mock.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@observer.expect(:changed,
true,
[52,
27])
@gear.set_cog(27)
@observer.verify
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_notifies_observers_when_cogs_change
@observer
=
MiniTest::Mock.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@observer.expect(:changed,
true,
[52,
27])
@gear.set_cog(27)
@observer.verify
end
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_notifies_observers_when_cogs_change
@observer
=
MiniTest::Mock.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@observer.expect(:changed,
true,
[52,
27])
@gear.set_cog(27)
@observer.verify
end
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_notifies_observers_when_cogs_change
@observer
=
MiniTest::Mock.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@observer.expect(:changed,
true,
[52,
27])
@gear.set_cog(27)
@observer.verify
end
Depends on the interface.
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_notifies_observers_when_cogs_change
@observer
=
MiniTest::Mock.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@observer.expect(:changed,
true,
[52,
27])
@gear.set_cog(27)
@observer.verify
end
Gear is responsible for sending
#changed to @observer.
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@sandimetz Apr 2013
Rule
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@sandimetz Apr 2013
Expect to send
outgoing command messages.
Rule
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@sandimetz Apr 2013
Breaking this rule
can save $$$ if side effects
are both stable and cheap.
Caveat
Friday, April 5, 13
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Query Command
Assert
result
Assert
direct public
side effects
Ignore
The Unit Testing Minimalist
Incoming
Type
POV
@sandimetz Apr 2013
Message
Ignore
Sent to Self
Outgoing
Friday, April 5, 13
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Query Command
Assert
result
Assert
direct public
side effects
Ignore Expect
to send
The Unit Testing Minimalist
Incoming
Type
POV
@sandimetz Apr 2013
Message
Ignore
Sent to Self
Outgoing
Friday, April 5, 13
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@sandimetz Apr 2013
But
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_notifies_observers_when_cogs_change
@observer
=
MiniTest::Mock.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@observer.expect(:changed,
true,
[52,
27])
@gear.set_cog(27)
@observer.verify
end
What happens if
the Observer class
stops implementing #changed?
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@sandimetz Apr 2013
Stubs
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@sandimetz Apr 2013
Stubs
Stubs de ne context.
Friday, April 5, 13
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@sandimetz Apr 2013
Stubs
Stubs de ne context.
Stub as needed (but zillions is a smell).
Friday, April 5, 13
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@sandimetz Apr 2013
Stubs
Stubs de ne context.
Stub as needed (but zillions is a smell).
Avoid stubbing in the object under test.
Friday, April 5, 13
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@sandimetz Apr 2013
Stubs
Stubs de ne context.
Stub as needed (but zillions is a smell).
Avoid stubbing in the object under test.
Do not make assertions about stubs!
Friday, April 5, 13
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@sandimetz Apr 2013
Mocks
Friday, April 5, 13
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@sandimetz Apr 2013
Mocks
Mocks test behavior.
Friday, April 5, 13
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@sandimetz Apr 2013
Mocks
Mocks test behavior.
Set expectations on mocks to test
outgoing command messages.
Friday, April 5, 13
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@sandimetz Apr 2013
Mocks
Mocks test behavior.
Set expectations on mocks to test
outgoing command messages.
One expectation per test (do your best).
Friday, April 5, 13
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@sandimetz Apr 2013
Mocks
Mocks test behavior.
Set expectations on mocks to test
outgoing command messages.
One expectation per test (do your best).
Do not stub using a mock!
Friday, April 5, 13
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@sandimetz GoGaRuCo 2012
@sandimetz Apr 2013
to Self
Incoming Outgoing
class
GearTest
<
MiniTest::Unit::TestCase
def
test_notifies_observers_when_cogs_change
@observer
=
MiniTest::Mock.new
@gear
=
Gear.new(
chainring:
52,
cog:
11,
observer:
@observer)
@observer.expect(:changed,
true,
[52,
27])
@gear.set_cog(27)
@observer.verify
end
So really, what happens if
Observer
stops implementing #changed?
Friday, April 5, 13
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@sandimetz Apr 2013
Pain
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@sandimetz Apr 2013
API Drift
Pain
Friday, April 5, 13
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@sandimetz Apr 2013
Rule
Friday, April 5, 13
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@sandimetz Apr 2013
Honor the contract.
Rule
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@sandimetz Apr 2013
Honor the contract.
Ensure test doubles stay in sync with the API.
Rule
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@sandimetz Apr 2013
Automagically
minitest
requires
that
stubbed
methods
exist
https://github.com/benmoss/quacky
https://github.com/xaviershay/rspec-‐fire
https://github.com/cfcosta/minitest-‐firemock
Friday, April 5, 13
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@sandimetz Apr 2013
Summary
Friday, April 5, 13
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@sandimetz Apr 2013
Friday, April 5, 13
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@sandimetz Apr 2013
Friday, April 5, 13
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@sandimetz Apr 2013
Strive for
Thorough, Stable
Fast, Few
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@sandimetz Apr 2013
Test.
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@sandimetz Apr 2013
Test.
Everything.
Friday, April 5, 13
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@sandimetz Apr 2013
Test
.
Everything.
Once.
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@sandimetz Apr 2013
Test the
interface,
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@sandimetz Apr 2013
Test the
interface,
not the
implementation.
Friday, April 5, 13
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@sandimetz Apr 2013
Guard against
API drift.
Friday, April 5, 13
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Query Command
Assert
result
Assert
direct public
side effects
Ignore Expect
to send
Be a Minimalist
Incoming
Type
POV
@sandimetz Apr 2013
Message
Ignore
Sent to Self
Outgoing
Friday, April 5, 13
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@sandimetz Apr 2013
Use good judgement.
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@sandimetz Apr 2013
Insist on simplicity.
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@sandimetz Apr 2013
Practice the Tricks.
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@sandimetz Apr 2013
And go home
every day saying...
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@sandimetz Apr 2013
I love my tests.
Friday, April 5, 13
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Thanks
Sandi Metz
@sandimetz
Friday, April 5, 13
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http://www. ickr.com/photos/phil_shirley/5452562957/ thicket
http://www. ickr.com/photos/zebble/6080622/ one padlock
http://www. ickr.com/photos/chrisinplymouth/5543281168/ two padlocks
http://www. ickr.com/photos/donjohnson395/2915304926/ twins at beach
http://www.med.navy.mil/sites/nmcp/Patients/pediatrics/PublishingImages/binocular_boy.gif
http://upload.wikimedia.org/wikipedia/commons/e/e1/Ordinary_bicycle02.jpg ordinary bike
Magic Hat (82805980) Copyright Mmaxer - Shutterstock.com
Dogs in Hats (39263399) Copyright MLoiselle - Fotolia.com
Garden (35521385) Copyright Beboy - Fotolia.com
Space Walk - Nasa http://grin.hq.nasa.gov/copyright.html
Photo Credits
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@sandimetz Apr 2013
And
one
last
time...
http://poodr.info
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@sandimetz Apr 2013
http://poodr.info
Friday, April 5, 13
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@sandimetz Apr 2013
Hot
off
the
press
http://poodr.info
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Questions?
Friday, April 5, 13
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@sandimetz Apr 2013
http://poodr.info
Friday, April 5, 13