写在前面
Handler消息传递机制.png
图片来源
源码分析
首先创建Handler
Handler handler = new Handler();
查看Handler.java$handler()构造方法的核心源码
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
...... //代码省略
// mLooper是一个Looper对象，获取Looper
mLooper = Looper.myLooper();
//在主线程中可以直接创建Handler，原因下面会分析，
//在子线程中需要先初始化Looper也就是调用Looper.prepare()方法,否则就会报下面异常
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//mQueue是一个MessageQueue对象
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
上面说到在主线程中可以直接创建Handler，其原因是因为应用程序的入口为ActivityThread类的main方法。
查看ActivityThread.java$main方法的核心源码
public static void main(String[] args) {
...... //代码省略
//创建主线程的Looper
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
//对MessageQueue消息进行循环将取出的Message交付给相应的Handler ，后面会对其源码进行分析
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
上面解释了在主线程创建Handler是因为在程序创建的时候已经创建了主线程的Looper 。
我们知道Handler要发送消息的话需要调用sendMessage方法
那我们接着查看Handler.java$sendMessage方法源码
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
//继续查看
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
//继续查看
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);
}
//继续查看
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
//msg.target是一个Handler对象，this指的就是Handler因为该方法在Handler类中
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//这里调用的是MessageQueue消息队列的enqueueMessage方法
return queue.enqueueMessage(msg, uptimeMillis);
}
上面说到msg.target是handler 我们查看Message.target源码
Handler target;
发现target确实是Handler对象，其作用是为了记录所发消息对应的handler,也是为了把消息分发到对应的Handler
继续分析MessageQueue消息队列enqueueMessage方法源码
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) {
//判断是否调用了quit()方法如果调用了则无法发送消息
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
// 把传进来的message按照延迟时间的先后添加到mMessage中
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;
}
//如果Looper.loop()是休眠状态则执行nativeWake方法唤醒Looper
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
在Looper.prepare()的同时，总会执行looper.loop()语句与之对应。
接着查看Looper.java$loop方法的源码
public static void loop() {
//获取looper对象确保Looper的唯一性
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();
//循环并分发message
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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
//分发msg给给对应的Handler msg.target为handler对象
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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);
}
//message数据回收
msg.recycleUnchecked();
}
}
Looper.loop()方法作用主要是 for循环不断从MessageQueue队列中获取Message，并分发给对应target的Handler。
先查看MessageQueue$next方法的核心代码
Message next() {
......//代码省略
//0为出队状态，-1为等待状态
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
//按时间顺序将message取出
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// 消息队列中没有信息,则将nextPollTimeoutMillis 设置为1，下次循环时消息队列则处于等待状态
nextPollTimeoutMillis = -1;
}
...... //代码省略
}
接着查看Handler.java$dispatchMessage方法源码
public void dispatchMessage(Message msg) {
//Message对象的callback不为空(runnable)，交给callback处理，
//这种大多使用post方法传入runnable对象时会调用
if (msg.callback != null) {
handleCallback(msg);
} else {
//handler的callback不为空，交给callback处理，callback
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//前两种都没有的情况下交给handlerMessage处理
//也就是我们在代码中重写的handlerMessage方法
handleMessage(msg);
}
}
以上就是我对handler消息传递机制的理解。
最后如果有理解错误之处欢迎指正。