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用于同进程的不同线程进行通讯，不能用于进程间通讯，Handler机制中管道作用就是当一个线程A准备好Message， 并放入消息池，这时需要通知另一个线程B去处理这个消息。线程A向管道的写端写入数据1（对于老的Android版本是写入字符W）， 管道有数据便会唤醒线程B去处理消息。管道主要工作是用于通知另一个线程的，这便是最核心的作用。
主线程中的MessageQueue是唯一的，所有的消息都往里面插入，但在子线程中使用需要重新创建
在主线程中的prepare()和loop()有调用，所以我们不需要额外的调用
//ActivityThread.java的 main()方法中
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
...
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
}
//创建Looper[线程和Queue]
Looper.prepare();//调用
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));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
一、消息的发送
//消息发送
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);
}
//向队列里插入数据
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 {
//插入队列内部，通常我们不必唤醒队列，除非队列头部阻塞并且之前有异步消息在队列中
needWake = mBlocked && p.target == null && msg.isAsynchronous();之前消息阻塞，当前是异步消息
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) {//调用Native去唤醒队列处理消息
nativeWake(mPtr);
}
}
return true;
}
二、消息轮训
//消息进行轮训
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(); //从消息队列中提取消息进行处理 下一条Message
if (msg == null) {//没有消息停止循环
// No message indicates that the message queue is quitting.
return;
}
final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);／／调用handler的dispatchMessage(msg)方法
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
...
msg.recycleUnchecked();
}
}
//下一条Message
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;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);//提取消息队列中的消息,调用Native层代码
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;//mMessages为我们插入的消息
if (msg != null && msg.target == null) {//消息不为空，handler为空[msg.target 是handler对象]
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());//msg非空，且不是异步消息停止
}
if (msg != null) {
if (now < msg.when) {//when为 SystemClock.uptimeMillis() + delayMillis，如果小于给定的延时时长，则先不处理消息
// 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);//记录计算的消息处理延时
} else {
// Got a message.
mBlocked = false;//不阻塞，即唤醒
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 {
// No more messages.
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.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {//消息队列为空或者第一条消息在未来处理的时候，将阻塞loop
// 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);
}
// 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];
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) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
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.
nextPollTimeoutMillis = 0;
}
}
三、消息的处理
//handler进行处理
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);//调用Message的内部回调
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {//借口回调处理消息
return;
}
}
handleMessage(msg);//内部类重写handleMessage(Message msg)方法处理消息，可能造成内存泄漏
}
}
//三种处理方式
//1、通过Message 回调发送处理问题
private Handler handler = new Handler();
Message message = Message.obtain(handler, new Runnable() {
@Override
public void run() {
Log.i("cxd", "run: message callback");
}
});
message.sendToTarget();
//2、通过回调
private Handler handler = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
return false;
}
});
//3、内部类实现
private Handler handler = new Handler(){
@Override
public void dispatchMessage(Message msg) {
super.dispatchMessage(msg);
}
};
四、在子线程中调用
//在子线程中使用 需要调用prepare()和loop()
Looper.prepare();
looper = Looper.myLooper();
mHandler = new Handler(looper, new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
Log.i("cxd", "handleMessage: "+msg.what);
return false;
}
});
mHandler.sendEmptyMessage(999999);
Looper.loop();