ReactiveCocoa Ash Furrow 

— Shunryu Suzuki “In the beginner’s mind there are many possibilities, in the expert's mind there are few.” 

Agenda • Imperative programming is antiquated and dangerous • Functional reactive programming mitigates those dangers • ReactiveCocoa makes FRP easy 

BNE ADD1 BRA NONE1 ADD1 INY ;increment our counter of 1's NONE1 LDAA TEMP ;reload accumulator A LSL MASK ;Shift the mask's 1 bit left BNE LOOP1 ;If we haven't finished our loop, branch LDAA #$01 ;load new mask into A STAA MASK ;store the reset mask into MASK TSX ;pull of return address and store in X PULA ;pull off A STAA TEMP ;store the value into temp TXS ;push return address back onto the stack LOOP2 LDAA TEMP ;Load A into TEMP ANDA MASK ;logical AND MASK with A BNE ADD2 ;add one if we need to BRA NONE2 ADD2 INY ;increment our counter of 1's NONE2 LDAA TEMP LSL MASK ;shift our mask left by one BNE LOOP2 ;loop back until we've exhausted positions 

Functional Reactive Programming 

• Functional • Typically, functions have no side-effects • Programs are written as a series of data transformations 

Demo https://github.com/AshFurrow/FunctionalDemo 

• Reactive • When one thing changes, it effects changes in other things • Like a spreadsheet 

Functional programming has limits. ! Reactive programming is limited by itself. ! FRP is peanut butter and chocolate of programming paradigms. 

Functional Reactive Programming • In functional programming, things get hard because functions can’t have side-effects • (Every time a function is called, it must return the same result for the same parameters) • In functional reactive programming, time is an implicit parameter • This makes things easy to manage, without using monads 

Signals • At the core of FRP are signals • Signals send values over time, until they complete or error out • A signal never sends anything after completing or erring • A signal either completes, or errors out, but never both • A signal has no concept of a “current value” or “past values” • It’s like a pipe 

Signals • Signals can be chained to transform the values that the send • This is core to FRP: data transformation of values sent over time • In math terms, it’s f(g(y)) 

• ReactiveCocoa is a framework developed by GitHub • ReactiveCocoa is an implementation of FRP on iOS/OS X • It’s currently being revamped to version 3.0 

Signals • Signals are built on KVO • Creating a signal is easy • RACObserve macro • Wraps a @property • Sends a new value for updated property values 

RACSignal *signal = RACObserve(object, number); [signal subscribeNext:^id(NSNumber *value) { NSLog(@”%@”, value); }]; 

Signals • Signals always send id, never primitives • Will wrap primitives in NSNumber for you 

Map • Mapping performs a value transformation on the given signal • It “maps” each value sent along the signal to a new value • It then passes that new value along, creating a new signal • This is the canonical value transformation 

RACObserve map: subscribeNext: 

RACSignal *signal = RACObserve(object, number); [[signal map:^id(NSNumber *value) { return [value stringValue]; }] subscribeNext:^id(NSString *string) { NSLog(@”%@”, string); }]; 

Combine Latest • Combines two or more signals’ values into one • Useful for waiting for conditions to be met • Useful for deciding between values 

RACObserve combineLatest: reduce: RACObserve 

[RACSignal combineLatest:@[someNumberSignal, someStringSignal] reduce:^id(NSNumber *number, NSString *string){ return [NSString stringWithFormat:@"%@:%@", number.stringValue, string]; }]; 

Filter • Filtering allows values that pass some test to “pass through” 

RACObserve filter: subscribeNext: 

[[someNumberSignal filter:^BOOL(NSNumber *value){ return value.integerValue > 5; }] map:^id(NSNumber *value) { return [value stringValue]; }]; 

Bindings • Bindings can bind a property to the latest value sent on a signal • Bindings are typically created with the RAC macro • All bindings created with the RAC macro are one-way • Bindings are the secret sauce that hold apps together 

RAC(self.textField, text) = RACObserve(self, someString); 

NSArray *signalArray = @[self.firstNameField.rac_textSignal, self.lastNameField.rac_textSignal]; ! RAC(self.button, enabled) = [RACSignal combineLatest:signalArray reduce:^(NSString *firstName, NSString *lastName){ return @(firstName.length > 0 && lastName.length > 0); }]; 

Derived State • State is avoided in ReactiveCocoa, but is unavoidable in Cocoa • Instead of hacky workarounds, we derive the state from signals • Similar to the last example’s enabled state binding 

Other Operators • takeUntil: • scheduleOn: • throttle: • not, and, or 

Commands • A command represents a unit of work • Typically, work that’s done in response to user action • Commands are … confusing • They’ve been replaced in ReactiveCocoa 3.0 by Actions 

Two-Way Bindings • Two-way bindings exist, but their use is more rare under MVC • They’re more frequently used with MVVM • Created with RACChannelTo macro 

Demo https://github.com/AshFurrow/FunctionalReactiveDemo 

Demo • Notice subscribeNext: is used for side-effects • Cocoa Touch isn’t built on FRP, so we need side-effects • RACObserve captures self in its scope implicitly • Avoid reference cycles! • Network results are delivered on a background scheduler • Use deliverOn: to reschedule them 

Model-View-ViewModel 

View View Controller Model View Model owns owns Updates Updates 

Getting Started • Getting started with ReactiveCocoa is easy • Install ReactiveCocoa in your project (CocoaPods or submodule) • Start by replacing KVO methods with RACObserve • Use subscribeNext: for side-effects 

http://leanpub.com/iosfrp 

@ashfurrow http://ashfurrow.com