Android 消息机制Handler

Android的消息机制主要是指Handler的运行机制以及Handler所附带的MessageQueue和Looper的工作过程。Handler的主要作用是将一个任务切换到某个指定的线程中去执行，常用来更新UI。

Handler的简单使用

public class MainActivity extends Activity {

private Handler handler;
private int i = 1;
private TextView textView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
textView = (TextView) findViewById(R.id.tv_process_id);
handler = new Handler() {

@Override
public void handleMessage(Message message) {
Log.e("id -->", message.arg1+"");
textView.setText(message.arg1+"");
}

};

findViewById(R.id.bt_add_process).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
send(i);
i++;
}
});
}

//在新的线程中可以执行如下载之类的耗时任务，然后把结果发送给Handler，在Handler中更新UI
private void send(final int id) {
new Thread(new Runnable() {
@Override
public void run() {
try {
Log.e("thread id is:" ,Thread.currentThread().getId()+"");
Message message = new Message();
message.arg1 = id;
handler.sendMessage(message);
} catch (Exception e) {
e.printStackTrace();
}
}
}).start();
}
}

原理分析

Handler的发送消息

消息发送通过post的一系列方法以及send的一些列方法实现，post的一系列方法最终是通过send的一系列方法实现。

public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}

public 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);
}

由以上的源码可以发现，Handler发送消息的过程仅仅只是向消息队列（也就是MessageQueue)插入一条消息

MessageQueue插入消息

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 me
4000
ssage 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;
}

MessageQueue的enqueueMessage主要操作是单链表的插入操作

MessageQueue的next()方法从消息队列中取出一条消息并从消息列表中删除

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);

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());
}
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 {
// 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) {
// 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;
}
}

从上面next()的方法中可以看出，next()是一个无限循环的方法，如果消息队列中没有消息，next()方法就会一直阻塞在这里

MessageQueue的quit()方法退出消息队列

void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}

synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;

if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}

// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}

Looper的工作过程

Looper会不停地从MessageQueue中查看是否有新消息，如果有新消息就立即处理，否则一直阻塞在那里

Looper在构造方法中会创建一个MessageQueue

private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}

Looper主要有2个方法，Looper.prepare()方法主要是为当前线程创建Looper，Looper.loop()开启消息循环，只有调用了loop()方法后，消息循环系统才开始起作用

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
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.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}

if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);

bc94
}

// 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();
}
}

从上面的源码可以看出，loop方法是一个死循环，一直不停查看MessageQueue中是否有消息，唯一跳出循环的条件是MessageQueue的next()方法返回null。

从MessageQueue的next()获得消息后，调用msg.target.dispatchMessage(msg)处理消息，msg.target就是发送消息的Handler对象，可以看出Handler的dispatchMessage()方法是在Looper线程中执行。

Looper的停止

public void quit() {
mQueue.quit(false);
}

public void quitSafely() {
mQueue.quit(true);
}

Looper的quit和quitSafely调用的是MessageQueue的quit方法，不同之处在于，quit()方法直接退出Looper，quitSafely()只是设定 一个退出标记，把消息队列的已有消息处理完毕后才安全退出。MessageQueue的quit方法被调用后，next()方法就会返回null，loop方法就退出阻塞状态，所以Looper必须退出。

消息的处理

public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

处理的优先级是Message的callback,然后是mCallback ，最后再是Handler的handleMessage，也就是我们需要重写的处理消息的逻辑。

简要分析一下开篇例子中的处理过程

首先当主线程中需要执行耗时任务时，开启新的子线程执行任务

子线程执行完任务后通过handler将处理的结果通过消息发送给主线程

主线程的MessageQueue收到消息，Looper（主线程中自带Looper)获取消息并通过msg.target.dispatchMessage方法调用handler的handleMessage方法执行我们的逻辑，比如更新UI