Concurrency in iOS: #iOScon 2016 - Speaker Deck

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Concurrency on iOS Sam Davies @iwantmyrealname _ github.com/sammyd/iOSConcurrency

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concurrency is hard

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what is concurrency?

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why use concurrency?

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common concurrency problems

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race condition

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Race Condition time Value 1 Thread 1 Thread 2 inc

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Race Condition time Value 1 1 Thread 1 Thread 2 inc r1

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Race Condition time Value 1 1 1 Thread 1 Thread 2 inc r1 +1

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Race Condition time Value 1 1 1 2 Thread 1 Thread 2 inc r1 w2 +1

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Race Condition time Value 1 1 1 2 2 Thread 1 Thread 2 inc r1 w2 +1 inc

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Race Condition time Value 1 1 1 2 2 2 Thread 1 Thread 2 inc r1 w2 +1 inc r2

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Race Condition time Value 1 1 1 2 2 2 2 Thread 1 Thread 2 inc r1 w2 +1 inc r2 +1

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Race Condition time Value 1 1 1 2 2 2 2 3 Thread 1 Thread 2 inc r1 w2 +1 inc r2 w3 +1

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Race Condition time Value 1 Thread 1 Thread 2 inc

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Race Condition time Value 1 1 Thread 1 Thread 2 inc r1 inc

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Race Condition time Value 1 1 1 Thread 1 Thread 2 inc r1 inc r1

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Race Condition time Value 1 1 1 1 Thread 1 Thread 2 inc r1 inc r1 +1

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Race Condition time Value 1 1 1 2 1 Thread 1 Thread 2 inc r1 inc r1 w2 +1

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Race Condition time Value 1 1 1 2 1 2 Thread 1 Thread 2 inc r1 +1 inc r1 w2 +1

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Race Condition time Value 1 1 1 2 1 2 2 Thread 1 Thread 2 inc r1 w2 +1 inc r1 w2 +1

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priority inversion

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Priority Inversion time low priority resource

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Priority Inversion time low priority resource

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Priority Inversion time low medium priority resource

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Priority Inversion time low medium high priority resource

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Priority Inversion time low medium high priority resource

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Priority Inversion time low medium high priority resource

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Priority Inversion time low medium high priority resource

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deadlock

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Deadlock time resource1 resource2 Thread 1 Thread 2

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Deadlock time resource1 resource2 Thread 1 Thread 2

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Deadlock time resource1 resource2 Thread 1 Thread 2

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Deadlock time resource1 resource2 Thread 1 Thread 2

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Deadlock time resource1 resource2 Thread 1 Thread 2

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Deadlock time resource1 resource2 Thread 1 Thread 2

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Deadlock time resource1 resource2 Thread 1 Thread 2

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Deadlock time resource1 resource2 Thread 1 Thread 2

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$&'( queueueueueue paradigm

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Tasks & Queues Thread 1 Thread 2 Queue

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Tasks & Queues Thread 1 Thread 2 Queue

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Tasks & Queues Thread 1 Thread 2 Queue

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Tasks & Queues Thread 1 Thread 2 Queue

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Tasks & Queues Thread 1 Thread 2 Queue

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Tasks & Queues Thread 1 Thread 2 Queue

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Tasks & Queues Thread 1 Thread 2 Queue

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feeling brave?

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a touch more theory

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Thread Safety time Value 1111 Thread 1 Thread 2 Thread 3

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Thread Safety time Value 1111 Thread 1 Thread 2 r1111 Thread 3

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Thread Safety time Value 1111 1112 Thread 1 Thread 2 r1111 w2222 Thread 3

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Thread Safety time Value 1111 1112 1122 Thread 1 Thread 2 r1111 w2222 r1122 Thread 3

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Thread Safety time Value 1111 1112 1122 1223 Thread 1 Thread 2 r1111 w2222 r1122 w3333 Thread 3 r1223

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Thread Safety time Value 1111 1112 1122 1223 2233 Thread 1 Thread 2 r1111 w2222 r1122 w3333 Thread 3 r1223

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Thread Safety time Value 1111 1112 1122 1223 2233 2333 Thread 1 Thread 2 r1111 w2222 r1122 r2333 w3333 Thread 3 r1223

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Thread Safety time Value 1111 1112 1122 1223 2233 2333 3333 Thread 1 Thread 2 r1111 w2222 r1122 r2333 w3333 Thread 3 r1223

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Thread Safety time Value 1111 1112 1122 1223 2233 2333 3333 3333 3333 Thread 1 Thread 2 r1111 w2222 r1122 r2333 w3333 Thread 3 r1223 r3333 r3333

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Dispatch Barrier time task task task task task barrier task task task task task task task task

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Dispatch Barrier time read read read read read read write read read read read read read

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alternatives to 

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promises networkRequest(url, callback: { data in decompressor(data, callback: { image in imageResizer(image, callback: { small in filterImage(small, callback: { filtered in displayImage(filtered) }) }) }) })

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promises promise int error

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promises str

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promises promise str f

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promises promise str error f img

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promises promise str error f img error handler

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promises promise str error f img g promise error handler

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promises promise str error f img g promise error error handler img

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promises promise str error f img g promise error error handler img

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promises networkRequest(url) .then(decompressor) .then(imageResizer) .then({ (image) in return filterImage(image) }).then(displayImage) .catch({ (error) in print(error) })

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imagine the “promise pipeline” accepted a sequence of values…

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that’s reactive programming

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testing

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don’t do it

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test core, synchronous logic

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let libraries handle concurrency

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conclusion

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concurrency is hard

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you will get it wrong

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• raywenderlich.com/videos • github.com/sammyd/iOSConcurrency • @iwantmyrealname