之前介绍过Android的Handler机制
https://www.jianshu.com/p/92b1f22dadeb
2020-02-12-Android消息机制Handler
今天简单看一下藏在MessageQueue里的另外一种消息处理机制IdleHandler。它的作用是在消息队列空闲的时候,执行一些不需要保证执行时机的任务。
MessageQueue
我们看到MessageQueue内部是定义了一个IdleHandler接口,并且维持了一个Array List数组,用来添加和管理Idle任务。
private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();
private IdleHandler[] mPendingIdleHandlers;
/**
* Callback interface for discovering when a thread is going to block
* waiting for more messages.
*/
public static interface IdleHandler {
/**
* Called when the message queue has run out of messages and will now
* wait for more. Return true to keep your idle handler active, false
* to have it removed. This may be called if there are still messages
* pending in the queue, but they are all scheduled to be dispatched
* after the current time.
*/
boolean queueIdle();
}
同时MessageQueue提供了两个方法来进行Idle任务的添加删除:
public void addIdleHandler(@NonNull IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}
public void removeIdleHandler(@NonNull IdleHandler handler) {
synchronized (this) {
mIdleHandlers.remove(handler);
}
}
根据前面的内容,我们知道Handler线程通过Looper.loop方法,不断从消息队列中取出消息执行。实际上,在Message.next方法中,也会进行IdleHandler任务的插入执行。
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
//1
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
//……
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {//2
pendingIdleHandlerCount = mIdleHandlers.size();//3
}
if (pendingIdleHandlerCount <= 0) {//4
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);//5
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {//6
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {//7
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;//8
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
1.注释1处定义了一个pendingIdleHandlerCount变量用来记录idleHandler的数量,并且初始化时赋值-1;
2.接着进入for循环,省略正常message的执行过程,如果当前消息等于null或者当前是一个延时消息,需要等待一段时间,就会进入idle任务的执行;
3.首先获取mIdleHandlers的数量;
4.如果当前没有idleHandler,进入BLOCKED状态;
5.将mIdleHandlers中的内容拷贝到mPendingIdleHandlers中,并且维持mPendingIdleHandlers的最小容量是4;
6.进入for循环,从mPendingIdleHandlers中取出任务并且释放该位置的对象,随后调用该idle任务的queueIdle方法;
7.如果queueIdle方法返回false,则在mIdleHandlers中保留该任务,否则删除;
8.所有idle任务执行完毕后将pendingIdleHandlerCount置为0,下一个循环就不会再次检查idleHandler,直到下一次next调用才会执行,保证了在idle的逻辑不会陷入死循环。
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