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This article is about what you need to pay attention to when using View.post (). The editor thinks it is very practical, so share it with you as a reference and follow the editor to have a look.

I. Preface

Sometimes we need to use the View.post () method to send a Runnable to the main thread for execution. All of this, which seems beautiful, will eventually be executed through a Handler.post () method that prevents us from redefining a Handler object.

However, starting with Android 7. 0 (Api level 24), View.post () will no longer be so reliable, and the Runnable that you post () out may never have a chance to implement.

2. The difference of post in 7. 0.

2.1 differences in post methods

As mentioned earlier, this problem occurs only on Android 7.0s. So let's first analyze what changes Android 7. 0 made to View.post () from the source code.

Take a look at their differences with Diff. On the left is the code for Api Level 24 + (hereinafter referred to as Api24), and on the right is the code for Api level 23-(hereinafter referred to as Api23).

It is obvious that the logic of execution is different only when the mAttachInfo is null.

In Api24, getRunQueue (). Post (action) is called, while Api23 calls the ViewRootImpl.getRunQueue (). Post (action) method, and that's the difference.

2.2 details of Api23 post

Let's have a brief understanding of what a ViewRootImpl is.

ViewRootImpl can be understood as an instance of the root node of Activity's ViewTree. Each ViewRootImpl is used to manage DecorView and ViewTree.

The queue used to host Runnable in ViewRootImpl is sRunQueues, which is a static variable, that is, the message queue in ViewRootImpl is the same throughout the life cycle of App.

Let's take a look at what ViewRootImpl.getRunQueue (). Post () did earlier.

The post () method simply wraps it as a HandlerAction object and puts it into the ArrayList of mActions. If you continue to follow up, you need to know when the HandlerAction added to mActions was consumed.

The place to consume HandlerAction is the executeActions () method.

In the end, it is the handler.postDelayed () that is called, which is nothing to say, but the key point is when the executeAction () method is called.

ExecuteAction () is called by TraversalRunnable doTraversa (), in the doTraversa () method. TraversalRunnable, on the other hand, is called iteratively through Choreographer.postCallBack (). This Choreographer sends a message loop call of type MSG_DO_SCHEDULE_CALLBACK through doScheduleCallback (), and the interval is the interval of a VSync.

About Choreographer, it is not the focus of this article, if you are interested, you can learn about it separately.

So, under the Api23, executeAction () is called loopingly, and basically the mActions in it is consumed as soon as there is an unexecuted Runnable.

So on devices below Api23, View.post () is basically reliable, and Runnable that goes out of post will have a chance to execute it.

2.3 details of Api24

Let's take a look at the implementation details in Api24, where Api24 calls the getRunQueue (). Post () method, which operates on a HandlerActionQueue object.

The internal structure is actually very similar to Api23, which also maintains an array mActions of HandlerAction.

The place where you end up consuming mActions is still an executeActions () method.

Going back to the fundamental question of when the executeActions () method will be called, follow up and see that it will be called in the View.dispatchAttachedToWindow () method.

Since the executeActions () method, in Api24 and above, will only have a chance to be called in the method of dispatchAttachedToWindow (), while the View.dispatchAttachedToWindow () method will only be called when the View is added to a ViewGroup through methods such as addView (). As a result, the control written in the Layout layout will never have a chance to call the addView () method again, so it will never be executed. This is also the reason that the phenomenon of View.post () is inconsistent under Api24.

Thank you for reading! This is the end of the article on "what you need to pay attention to when using View.post ()". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, you can share it for more people to see!

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