相关词汇
a finite state:suspend 函数使用1-2-3 3个状态
Continuation：协程挂起函数之间相互通信的方式是Continuation接口对象。
recursively:递归调用
youtube视频链接
Kotlin中的协程简化了Android上的异步操作。在这个视频中，我们将更深入地了解为什么协程是重要的，它们在幕后是如何工作的，协程如何在不阻塞线程的情况下挂起，等等!
Suspend functions - Kotlin Vocabulary
协程Codelab
前言
Coroutine are a future of Kotlin that simplify asynchronous operations on Android.
A new suspend modifer was introduced in the language,and it is used for functions that need to be run inside the coroutine.
In this video ,I will tell you more about why coroutines are important and how they work under the hood.
This will help you better understand why a suspend function won't return until all the work that it has started has completed, and also how a coroutine can suspend without blocking threats.
If you learn something new,like the video and subscribe, but only if you think we're earned it.
Use coroutine to manage asynchronous tasks that might otherwise block the main thread and cause you app to freeeze.
Coroutines are also helpful to replace callback-based APIs with imperative looking code.
Let's see an example of asynchronous code that uses callbacks.
loginUser-with-callback
Here ,we have the function,loginUser that,after making an echo request to get user information from the internet,it saves the result to the local database and returns the result of that call--all of the using callbacks.
The result of the computation is returns using the userResult callback that is passed in as parameter of the function.
Code that heavily uses callbacks can become hard to read and understand.
Kotlin coroutines let you convert callback-based code to sequential code.
Code read in sequentianlly is typically easier to read and ,unlike callbacks,coroutines provide an easy way to swap between threads and handle exceptions.
See the same function return with coroutines.
2中实现方式比较
We added the suspend modifer to the function,and now it returns user instead of having that callback we used to pass in as a parameter.
As you can see from the wiggly icon in the code there,the other functions call from the suspend function body are also suspend functions.
That suspend modifer tells the compiler that this function needs to be executed inside a coroutine. As a developer,you can think of a suspend function as a regular function whose execution might be suspended and resumed at some point.
If you're new to coroutines in Android and want to learn more about them, I would recommend going throught the coroutine Codelabs first.
But what's the compiler actually doing under the hood when we mark the function as suspend?
Under the hood,the kotlin compiler takes suspended functions and coverts them to an optimized version of callbacks using a finite state machine.
So yes ,you are right,the kotlin compiler will write those callbacks for you.
Continuation接口
There way suspend functions communicate with each other is continueation objects.
A continuation is just a generic callback interface with some extra information.
- Context will be the CoroutineContext to be used in that continuation.
- resumeWith resumes execution of the coroutine with a result that can contian either a value,which is the result of the computation that caused the suspension, or an exception.
kotlin1.3 新增continuation方法
With Kotlin 1.3,you also have convenient extension functions called resume and resumeWithException that are specialized versions of the resumeWith function.
Back to our suspend function,how is compiler is going to modify it ?
kotlin-compiler-rewrite-suspend-function
It will replace the suspend modifier with an extra parmater called completion of type continuation in the function signature.
That will be used to communicate the result of the suspend function computation to the coroutine that called it ,as you can see in the code.
Also,the return type of the transformed function is unit instead of user. The user object will be returned in the added continuation parameter.
Time out.
As a disclaimer,the code we are showing will not fully match bytecode generated by the compiler.
It would be Kotlin code accurate enough to allow you to understand what's really happening internally. This representation is generated by coroutines version 1.3 and might change in future versions of the library.
suspend函数被编译成的代码
Back to the code again.
The kotlin compiler will identify when the function can suspend internally. Every suspension point will be represented as a state in the finite state machine. And these states are represented with lables by the compiler.
suspend函数被编译成的代码2
For a better representation of the state machine, the compiler will use a when statement to implement the different states.
- Notice that this code is incomplete, since the different states have no may to share information.
How is that problem solved?
The compiler will use the continuation parameter to do it. This is why the generic of the continuation is [inaudible] any instead of the return type of the original function that was user.
The compiler will create a private class that ,first,hold the required data ,and second,calls the loginUser function recursively to resume the execution of the function that was suspended.
Let's see what that class looks like.
image.png
Let's call that generated class,loginUserStateMachine. It is a private class that extends from CoroutineImpl,which is a subtype of continuation. In the constructor,it takes this continuation object,named completion ,that will be used to communicate back the result of this function to the function that called it .
image.png
This is the same continuation that we called before in the last state of the state machine.
But also,this class saves the variables that were declared in the original suspend function.
And there are other variable that are common for all CoroutineImples. The reuslt variable is the result from the previous continuation and label keeps the state of the state machine.
Also,it overrides the invoke suspend function that is used to resume the state machine. It will call the loginUser function to trigger the state machine again.
It calls it with just information of the continuation object.
The rest of parameters in the loginUser function signature become nullable.
At that point ,lable will be already in the next state to execute ,and the result of the previous state's continuation will be assigned.
image.png
An instance of this class is added to loginUser. The first thing it needs to do is knowing if it is the first time the function is called or if the function has resumed from a previious state.
It does it by checking if the continuation passed in is of type loginUserStateMachine or not.
If it's the first time ,it will create a new loginUserStateMachine instance and will store the completion instance received as a parameter.
If it's not ,it will just carry on executing the state machine.
image.png
For completion ,this is what the rest of the function looks like.
image.png
You can see how the rest of code uese the continuation variable to read the result of the last state of the state machine.
image.png
But also ,for every state,it checks if I never happened while this function was suspended.
image.png
In the last state, it calls resume on the continuation of the function that called this one.
And that's it. As you can see, the kotlin compiler does a lot under the hood.
Because of the implementation of the generated state machine,a suspend function won't return until all the work that it has started has completed.
Well,everything needed to resume
How can the code suspend without blocking the thread? the execution of a suspend function is in the continuation object that is passed around,so it can be resumed at any point .
That's all we have to say about suspend for now.
Thanks for watching ,and go write better Android apps with Kotlin.
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