最近深感对基础知识不够扎实, 所以回头看一下Handler的源码. 首先, 我们先从他的使用作为入口来
Handler的使用
先看下, 几种通过Handler发送消息的用法, 一共有四种, 当然最终调用的还是Handler#sendMessageDelayed(Message msg, long delayMillis), 这个我们放在后面去解析.1
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18private fun sendHandler(){
// 1
MyHandler().obtainMessage(1).sendToTarget()
// 2
val msg = Message()
msg.what = 2
MyHandler().sendMessage(msg)
// 3
MyHandler().post { Log.e(MainActivity::class.java.canonicalName, "3") }
// 4
val callback = Handler.Callback {
Log.e(MainActivity::class.java.canonicalName, it.what.toString())
true
}
val msg2 = Message()
msg2.what=4
MyHandler(callback).sendMessage(msg2)
}
然后是他通用的消息接收处理1
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10// kotlin内部类默认为静态类
private class MyHandler: Handler {
constructor()
constructor(callback: Callback)
override fun handleMessage(msg: Message?) {
super.handleMessage(msg)
Log.e(MainActivity::class.java.canonicalName, msg?.what.toString())
}
}
Handler构造
要了解Handler, 我们首先看他的构造函数, Handler的构造, 实际分为两种, 一种是传入Looper, 一种是直接使用当前线程的Looper, 可以看到, 主要就是通过默认当前线程Looper或者是提供的Looper获取messageQueue对象.1
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public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
// 使用当前线程的Looper对象
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
// 获取looper持有的messageQueue
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
// 使用提供的Looper对象
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
当我们在子线程内直接发送消息, 会抛出异常, 具体我们可以从Looper#myLooper()开始看1
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7/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();
}
ThreadLocal这里不细说, 反正知道他是每个线程独有的存储空间就行了.可以看到, looper是每个线程不共享的内存对象, 它存储在ThreadLocal内, 而在Looper#prepare会进行Looper的实例化, 和存储在threadLocal内1
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10private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
发送消息
Handler发送消息最终都会走到Handler#sendMessageDelayed方法, 会发现他最终调用的是MessageQueue#enqueueMessage(Message msg, long when)1
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26public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
MessageQueue虽然说是消息队列, 但是实际实现队列机制是依靠的Message的数据结构,每一个Message都会通过它内部的next字段指向另外一个Message, 最终实现了单向列表的数据结构, 而MessageQueue内部的mMessages就是队列头的消息对象, 具体可以看下, 插入消息时候的方法体1
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60boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
if (mQuittying) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
// 延迟时间
msg.when = when;
// 队列中上一条消息
Message p = mMessages;
boolean needWake;
// 无延迟 或者 是第一条消息// 或者需要比上一条消息早发送
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
// 将当前消息置于列表顶
// 当前消息的下一条消息指向之前的mMessages
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
// 中间插入当前的消息对象
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
// 线程唤醒
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
消息处理
关于消息的处理, 我们又要回头去看Looper, 关于MessageQueue它主要做了两个事情, 从Handler那边接收Message做队列托管, 然后将符合条件的Message返回给Looper, 具体我们可以看下Looper#loop()1
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29public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
// 校验当前线程在当前进程内
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
for (;;) {
// 1. 获取message
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// 2. handler的分发处理
msg.target.dispatchMessage(msg);
...
// 3. 消息的回收
msg.recycleUnchecked();
}
}
删除了一些无关的代码, 我们看下核心流程, Looper#loop就一直不断循环的在做三件事:
- 拿取
MessageQueue中的Message - 调用
Message持有的handler的分发处理事件 - 回收已处理掉的
Message
我们来看下MessageQueue给到Looper怎么样的Message1
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122Message 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;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
// 获取当前启动时间
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
// 当头部消息的target(携带handler)为空的情况下
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
// 遍历队列, 直到获取到的消息不为空并且是同步信息
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
// 延迟消息
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.
// 指向下一条message
// 该条消息需要被执行的时候
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
// 返回当前的message
return msg;
}
} else {
// No more messages.
// 如果当前队列头部没有消息, 则说明队列为空
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// 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.
// 当队列空闲(头部消息为空或者头部消息还未到时候执行), 并且没有闲置handler的时候
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
// 当没有闲置handler需要执行的时候
mBlocked = true;
continue;
}
// 如果闲置handlers数组为空
if (mPendingIdleHandlers == null) {
// 初始化, 闲置的handlers最大数为4
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
// 引用释放, 则这个循环只会执行mPendingIdleHandlers集合的第一个handler元素
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
// 执行空闲回调处理
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
// 假设空闲回调处理不保留这个闲置处理, 则移除对应的idleHandler
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
// 重置
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
// 重置
nextPollTimeoutMillis = 0;
}
}
相关的代码注释差不多都在上面了, 我们主要整理下, 这里在做的事情
- 获取队列头部消息, 如果头部消息存在, 并且当前消息持有handler为空的情况下, 则去取下一个非空的同步消息
- 当该条消息是当前正需要执行的时候(具体看
Message#when的时间戳), 则返回这条消息, 否则继续查找下一条消息 - 如果当前没有消息, 或者当前时间没有需要执行的消息, 则触发idleHandler的处理.
- 然后在下个时间戳继续找消息
回头看Handler#dispatchMessage(Message msg), 可以了解到handler处理事件的优先级1
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15public void dispatchMessage(Message msg) {
// msg.callback通过handler.post传入的runnable
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
// mCallback是通过Handler的构造函数传入
if (mCallback.handleMessage(msg)) {
return;
}
}
// 最后才是Message传入的处理, 一般是继承Handler, 重写handleMessage方法
handleMessage(msg);
}
}
关于Message的重复利用
主流程基本分析完毕, 我们现在再回头看使用API, 一般的业务场景, 我们用到比较多的可能是自己新建包装一个Message, 通过Handler来发送. 那么Handler#obtainMessage()做了什么呢, 为什么通过这个自己可以不用再新建一个Message了. 我们看API调取, 会发现他内部调用到了Message#obtain(), 而最终message对象是从这里获取到.1
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13public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
可以看到, 当sPool对象不为空的时候, 是复用了这个对象的内存空间.那么sPool又是什么时候被赋值的呢1
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24void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
// MAX_POOL_SIZE == 50
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
这个方法是不是有点眼熟, 就是前面分析Looper#loop()时候, 当消息处理完以后, 消息回收调用到的, 可以看到sPool作为需要被回收的对象, 当消息处理完后, 当前消息进入当前回收队列中, 这样可以达到Message的复用作用