承接上一篇Handler系列一,上篇主要总结了Handler如何通信,这篇来介绍Handler怎么通信。
- Handler的通信机制
- Handler,Looper,MessageQueue如何关联
Handler 通信机制
- 创建Handler,并采用当前线程的Looper创建消息循环系统;
- Handler通过sendMessage(Message)或Post(Runnable)发送消息,调用enqueueMessage把消息插入到消息链表中;
- Looper循环检测消息队列中的消息,若有消息则取出该消息,并调用该消息持有的handler的dispatchMessage方法,回调到创建Handler线程中重写的handleMessage里执行。
Handler 如何关联 Looper、MessageQueue
Handler及其关联的类图
以上类图可以快速帮助我们理清Handler与Looper、MessageQueue的关系,以下从源码的角度慢慢分析:
1、Handler 发送消息
上一段很熟悉的代码:
Message msg =Message.obtain(); //从全局池中返回一个message实例,避免多次创建message(如new Message)
msg.obj = data;
msg.what=1; //标志消息的标志
handler.sendMessage(msg);
从sendMessageQueue开始追踪,函数调用关系:sendMessage -> sendMessageDelayed ->sendMessageAtTime,在sendMessageAtTime中,携带者传来的message与Handler的mQueue一起通过enqueueMessage进入队列了。
对于postRunnable而言,通过post投递该runnable,调用getPostMessage,通过该runnable构造一个message,再通过 sendMessageDelayed投递,接下来和sendMessage的流程一样了。
2、消息入队列
在enqueueMessage中,通过MessageQueue入队列,并为该message的target赋值为当前的handler对象,记住msg.target
很重要,之后Looper取出该消息时,还需要由msg.target.dispatchMessage
回调到该handler中处理消息。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
在MessageQueue中,由Message的消息链表进行入队列
boolean 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 (mQuitting) {
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();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
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.
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.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
3、Looper 处理消息
再说处理消息之前,先看Looper是如何构建与获取的:
-
构造Looper时,构建消息循环队列,并获取当前线程
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
-
但该函数是私有的,外界不能直接构造一个Looper,而是通过Looper.prepare来构造的:
public static void prepare() {
prepare(true);
}
private 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));
}
-
这里创建Looper,并把Looper对象保存在sThreadLocal中,那sThreadLocal是什么呢?
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
它是一个保存Looper的TheadLocal实例,而ThreadLocal是线程私有的数据存储类,可以来保存线程的Looper对象,这样Handler就可以通过ThreadLocal来保存于获取Looper对象了。
-
TheadLocal 如何保存与获取Looper?
public void set(T value) {
Thread currentThread = Thread.currentThread(); Values values = values(currentThread); if (values == null) { values = initializeValues(currentThread); } values.put(this, value);
}
public T get() {
// Optimized for the fast path. Thread currentThread = Thread.currentThread(); Values values = values(currentThread); if (values != null) { Object[] table = values.table; int index = hash & values.mask; if (this.reference == table[index]) { return (T) table[index + 1]; } } else { values = initializeValues(currentThread); } return (T) values.getAfterMiss(this);
}
在 set 中都是通过 values.put
保存当前线程的 Looper 实例,通过 values.getAfterMiss(this)
获取,其中put
和getAfterMiss
都有key
和value
,都是由Value对象的table数组保存的,那么在table数组里怎么存的呢?
table[index] = key.reference;
table[index + 1] = value;
很显然在数组中,前一个保存着ThreadLocal对象引用的索引,后一个存储传入的Looper实例。
接下来看Looper在loop中如何处理消息
在loop
中,一个循环,通过next
取出MessageQueue中的消息
-
若取出的消息为null,则结束循环,返回。
- 设置消息为空,可以通过MessageQueue的quit和quitSafely方法通知消息队列退出。
- 若取出的消息不为空,则通过msg.target.dispatchMessage回调到handler中去。
public 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.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// This must be in a local variable, in case a UI event sets the logger
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
msg.recycleUnchecked();
}
}
4、handler处理消息
Looper把消息回调到handler的dispatchMessage中进行消息处理:
- 若该消息有callback,即通过Post(Runnable)的方式投递消息,因为在投递
runnable
时,把runnable
对象赋值给了message的callback
。 - 若handler的mCallback不为空,则交由通过
callback
创建handler方式去处理。 - 否则,由最常见创建handler对象的方式,在重写handlerMessage中处理。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
总结
以一个时序图来总结handler的消息机制,包含上述如何关联Looper和MessageQueue的过程。
Handler-Looper-MessageQueue时序图
本文发表于个人博客:http://lavnfan.github.io/,欢迎指教。
**粗体** _斜体_ [链接](http://example.com) `代码` - 列表 > 引用
。你还可以使用@
来通知其他用户。