1. In the loading and drawing process of View, there is a choreographer class, mChoreographer.
mTraversalBarrier = mHandler.getLooper().postSyncBarrier();
Insert a synchronization barrier message into the MessageQueue, msg.target==null message, the return value mTraversalBarrier is an int token value.
void scheduleTraversals() { if (!mTraversalScheduled) { mTraversalScheduled = true; //向消息队列插入一个同步屏障的消息。msg.target==null的消息 mTraversalBarrier = mHandler.getLooper().postSyncBarrier(); mChoreographer.postCallback( Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null); } }
The mChoreographer.postCallback() method will execute the code in mTraversalRunnable.
mHandler.getLooper().removeSyncBarrier(mTraversalBarrier);
This will remove the synchronization barrier message in the MessageQueue based on the token value generated above.
final TraversalRunnable mTraversalRunnable = new TraversalRunnable(); final class TraversalRunnable implements Runnable { @Override public void run() { doTraversal(); } } void doTraversal() { if (mTraversalScheduled) { mTraversalScheduled = false; //移除同步屏障消息 mHandler.getLooper().removeSyncBarrier(mTraversalBarrier); //在这个方法中会调用 measure layout draw,view的绘制绘制流程的方法 performTraversals(); } }
Let’s look at this line of code mHandler.getLooper().postSyncBarrier() to see how the system handles it.
Obtained a Message without a handler set.
int enqueueSyncBarrier(long when) { // Enqueue a new sync barrier token. // We don't need to wake the queue because the purpose of a barrier is to stall it. synchronized (this) { final int token = mNextBarrierToken++; // 这个msg.target没有被赋值 final Message msg = Message.obtain(); msg.markInUse(); msg.when = when; msg.arg1 = token; Message prev = null; Message p = mMessages; if (when != 0) { while (p != null && p.when <= when) { prev = p; p = p.next; } } if (prev != null) { // invariant: p == prev.next msg.next = p; prev.next = msg; } else { msg.next = p; mMessages = msg; } return token; } }
Normally we send messages through the handler, and the handler is not allowed to be empty.
boolean enqueueMessage(Message msg, long when) { if (msg.target == null) { throw new IllegalArgumentException("Message must have a target."); } ........... }
Why does the system send a message with an empty handler?
Let’s first look at what mChoreographer did after sending the synchronization barrier message?
Another asynchronous message was sent: msg.setAsynchronous(true). The handler of this message is not null.
private void postCallbackDelayedInternal(int callbackType, Object action, Object token, long delayMillis) { synchronized (mLock) { final long now = SystemClock.uptimeMillis(); final long dueTime = now + delayMillis; mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token); if (dueTime <= now) { scheduleFrameLocked(now); } else { Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action); msg.arg1 = callbackType; //将消息设置为异步消息 msg.setAsynchronous(true); mHandler.sendMessageAtTime(msg, dueTime); } } }
Next, let’s take a look at how MessageQueue removes messages and how to process this synchronization barrier message.
Message next() { synchronized (this) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); Message prevMsg = null; Message msg = mMessages; //如果msg.target==null说明我们已经向消息队里中插入了一条屏障消息。 //此时会进入到这个循环中,找到msg.isAsynchronous==true的异步消息。 //通常我们发送的都是同步消息isAsynchronous = false的,并且msg.target不能为null的。 if (msg != null && msg.target == null) { // Stalled by a barrier. Find the next asynchronous message in the queue. do { prevMsg = msg; msg = msg.next; } while (msg != null && !msg.isAsynchronous());//msg.isAsynchronous==true时结束循环,说明找到了这个异步消息。 } if (msg != null) {//找到了同步屏障的异步消息后,直接返回 if (now < msg.when) { // Next message is not ready. Set a timeout to wake up when it is ready. nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); } else { // Got a message. mBlocked = false; if (prevMsg != null) { prevMsg.next = msg.next; } else { mMessages = msg.next; } msg.next = null; if (false) Log.v("MessageQueue", "Returning message: " + msg); return msg; } } else {//没有找到的话则进入休眠直到下一次被唤醒 // No more messages. nextPollTimeoutMillis = -1; } } }
When canceling, first determine whether msg.target is null, and then go through the while loop to find the message that msg.isAsynchronous() == true. That is the asynchronous message sent above. Usually the messages we send are synchronous messages, and msg.setAsynchronous(true); will not be set.
The purpose of the system is to process this asynchronous message first. All synchronization messages will be placed at the back, just like a barrier, so such an operation is called a synchronization barrier, and the processing of synchronization barrier messages has a higher priority.
Because the choreographer class mChoreographer is responsible for screen rendering, it needs to process signals from the bottom layer in a timely manner to ensure the frequency of interface refresh.
The above is the detailed content of Java Handler synchronization barrier example code analysis. For more information, please follow other related articles on the PHP Chinese website!

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于结构化数据处理开源库SPL的相关问题,下面就一起来看一下java下理想的结构化数据处理类库,希望对大家有帮助。

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于PriorityQueue优先级队列的相关知识,Java集合框架中提供了PriorityQueue和PriorityBlockingQueue两种类型的优先级队列,PriorityQueue是线程不安全的,PriorityBlockingQueue是线程安全的,下面一起来看一下,希望对大家有帮助。

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于java锁的相关问题,包括了独占锁、悲观锁、乐观锁、共享锁等等内容,下面一起来看一下,希望对大家有帮助。

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于多线程的相关问题,包括了线程安装、线程加锁与线程不安全的原因、线程安全的标准类等等内容,希望对大家有帮助。

本篇文章给大家带来了关于Java的相关知识,其中主要介绍了关于关键字中this和super的相关问题,以及他们的一些区别,下面一起来看一下,希望对大家有帮助。

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于枚举的相关问题,包括了枚举的基本操作、集合类对枚举的支持等等内容,下面一起来看一下,希望对大家有帮助。

封装是一种信息隐藏技术,是指一种将抽象性函式接口的实现细节部分包装、隐藏起来的方法;封装可以被认为是一个保护屏障,防止指定类的代码和数据被外部类定义的代码随机访问。封装可以通过关键字private,protected和public实现。

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于设计模式的相关问题,主要将装饰器模式的相关内容,指在不改变现有对象结构的情况下,动态地给该对象增加一些职责的模式,希望对大家有帮助。


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

AI Hentai Generator
Generate AI Hentai for free.

Hot Article

Hot Tools

DVWA
Damn Vulnerable Web App (DVWA) is a PHP/MySQL web application that is very vulnerable. Its main goals are to be an aid for security professionals to test their skills and tools in a legal environment, to help web developers better understand the process of securing web applications, and to help teachers/students teach/learn in a classroom environment Web application security. The goal of DVWA is to practice some of the most common web vulnerabilities through a simple and straightforward interface, with varying degrees of difficulty. Please note that this software

Atom editor mac version download
The most popular open source editor

Dreamweaver Mac version
Visual web development tools

PhpStorm Mac version
The latest (2018.2.1) professional PHP integrated development tool

SecLists
SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.
