Talking about kotlin process exception handling mechanism

1. Preface

If this is the first time you've heard of someone linking exception handling to event distribution , I believe you will be the same as when I first came into contact with the exception handling mechanism of a coroutine , A face of meng . Not to mention that many of you here Android old hand , Even if it is Android Adorable new , I should know , Exception handling is try catch Such a simple thing , How can it be related to the complex event distribution mechanism ？ Don't believe it's really related .

If you have been exposed to collaborative knowledge for some time , And we know that the exception handling mechanism of the collaboration process is different from the exception handling mechanism we have known in the past , But it doesn't quite work , I'll take you step by step to figure out what's going on .

If you have a good understanding of the process exception handling mechanism , So congratulations , I think at the current point in time , You have surpassed most people . I hope this article can be given to friends at different stages , Help in varying degrees .

Of course , This article does not cover event distribution in detail .Android Of View The structural concurrency of the system and the collaboration process is the same , Their corresponding data structures are tree.View Event distribution and coordination Cancel The communication mechanism is very similar , The depth traversal algorithm of the tree is used . For event distribution, please refer to Android Analysis of event distribution mechanism .

2. Exception trapping

1. Kotlin Exception capture and Java equally , use try catch You can catch exceptions . Let's take a simple example . adopt try catch Capture exception . Results the print "caught null pointer exception".
   

2. Start a process . Catch exceptions in the process body , The result is the same as expected , Print “caught null pointer exception”.
   

3. Suppose there is a method provided by a third party thirdApi.thirdApi Method starts a coroutine and does not catch an exception . As a result, the program crashes .
   

4. At first, I was confused To solve the above crash problem , Our first reaction is to call thirdApi Add try catch,so easy Well ！ Run code , Silly eyes , The program still crashes .try catch encase CoroutineScope.launch Unable to catch exception ？
   

5. Although I don't understand why the above code can't catch exceptions , But I know there are CoroutineExceptionHandler Can handle exceptions , Try to catch exceptions as follows ？ Results the print "caught null pointer exception".
   

6. Continue to look foolish If you put exceptionHandler Put it in the second launch In the method , I can't catch the exception , The program crashed again .
   

7. Continue to look foolish If you use coroutineScope Wrap the second launch Method , Things are different again , Results the print “caught exception in try catch exception null pointer exception”. The exception is try catch Capture , Rather than being exceptionHandler Capture .
   

8. Continue to look foolish If you use supervisorScope Instead of coroutineScope, Things are different again , Results the print “caught exception in handler null pointer exception”. By exceptionHandler Captured .
   

9. stay 7 Based on the code , If you put try catch Get rid of , In the outer layer launch Add outerExceptionHandler Parameters . The result is “caught exception in outer handler null pointer exception”. The exception is outerExceptionHandler Captured .
   

10. Continue to look foolish stay 9 Based on the code of coroutineScope Replace with supervisorScope. The result is “caught exception in inner handler null pointer exception”. The exception is innerExceptionHandler Captured .
    

above 10 A scenario , Whether you are a beginner or a veteran of Xiecheng , I believe you've lost your head . A simple exception handling , How come there are so many scenes in the process of cooperation , And the results are different . If you don't master the exception handling logic of the coordination process , I dare not cooperate in the project . Otherwise, I don't know how the project will collapse .

Don't worry about the above scene , That's just my list 10 A scenario , In theory , These scenarios can 72 change , There are countless scenes . But everything is the same , Only master the principle , To be able to cope freely .

3. Principle of exception handling

stay Chat Job and SupervisorJob The difference between In this article, I briefly introduced the principle of exception handling . To understand the principle of exception handling, you must also understand structured concurrency , You can refer to Kotlin How does a coroutine create structured concurrency ？

The exception handling logic can be explained by workplace examples . Suppose the unspoken rule of the workplace is , Any employee made a mistake , The first thing is to report to the superior , If your superiors are willing to deal with your mistakes , Then the employees don't have to care , If the supervisor calls back the problem to the employee , Then the mistakes have to be handled by the employees themselves .

The following figure illustrates the exception handling mechanism as an example ,Job5 An exception occurred at （ Here refers to non CancellationException）.

1. Job5 First, the exception will be passed to Job2 Handle .Job2 There are two options , Select No. 1 to not handle the exception , Call the exception back to Job5,Job5 call handleJobException Method . Option 2 yes Job2 Select to handle the exception , Then the exception is passed to Job2 了 ,Job5 No more chance to handle exceptions .
2. Job2 The logic for handling exceptions is the same as Job5, First, the exception will be passed to Job0.Job0 There are also two options , Return the call to Job2 Handle , call Job2 Of handleJobException Method or by Job0 Handle .
3. Middle root of coordination process Job Will not handle exceptions , It will return the exception to the cause Job Handle . In this case Job0 Will give the exception to Job2 Handle .
4. Job handleJobException The logic for handling exceptions is , First of all, judge the context Whether it has been set CoroutineExceptionHandler, If there is one, it should be handed over to CoroutineExceptionHandler Handling exceptions . If it is not set, it will be handed over to AndroidExceptionPreHandler Handle .

There are several points to pay attention to

1. CancellationException Spread to the father Job, It must be called back . In other words , It does not affect the parent Job.
2. SupervisorJob and supervisorScope Will also return the exception to the child Job Handle , Both of them can strangle the abnormality inside , Do not broadcast to the outside .
3. coroutineScope and withContext You can put the exception of the coordination process rethrow, That is, they can be coated with try catch Capture .

4. Last

Finally, click on the question . Event distribution Down Event is a tree traversal algorithm ( Root right left ). Instead of Down event ( Such as Move、Up Other events ) It's based on TouchTarget The generated event linked list traverses the linked list . The coordination process is similar ,cancel event , First, cancel yourself and the child cooperation process , Is traversal according to the preorder ( Root left and right ), Then cancel the parent association ( The parent coroutine is also a preorder traversal cancellation ). Exception handling is based on child nodes -> The linked list traversal request of the parent node is abnormal . It's not too much to say that you are a mother . If you don't understand , It shows that your knowledge of data structure and algorithm is still relatively deficient , It is suggested to make up lessons .

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