概述
Handler无疑是Android的异步消息处理机制的核心，这遍文章将通过分析Handler的源码来系统的解析Android异步消息机制。
Handler的使用
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";
private Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
switch (msg.arg1){
case 1:
if (BuildConfig.DEBUG) Log.d(TAG, msg.getData().getString("firstName"));
break;
case 2:
if (BuildConfig.DEBUG) Log.d(TAG, msg.getData().getString("lastName"));
break;
default:
break;
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new Thread(new Runnable() {
@Override
public void run() {
Message msg1 = new Message();
msg1.arg1 = 1;
Bundle bundle1 = new Bundle();
bundle1.putString("firstName", "xiao");
msg1.setData(bundle1);
mHandler.sendMessage(msg1);
Message msg2 = new Message();
msg2.arg1 = 2;
Bundle bundle2 = new Bundle();
bundle2.putString("lastName", "zhang");
msg2.setData(bundle2);
mHandler.sendMessage(msg2);
}
}).start();
}
}
打印结果：
image.png
问题来了：
1.消息是怎样传递到handleMessage()方法里面的。
2.多个消息是如何保证正确的顺序。
首先看一下Handler()
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
//省略部分代码
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
跟一下Looper.myLooper()
/**
* 返回当前线程绑定的Looper,如果没有绑定，则返回null
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
继续跟一下Looper.prepare()
/** 为当前线程初始化一个Looper
*在调用prepare()之后，可以创建一个绑定了该Looper的Handler
* 通过loop()方法，开启循环
* 通过quit()方法，结束循环
*/
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));
}
1."Can't create handler inside thread that has not called Looper.prepare()"，表示在初始化Handler之前必须先调用Looper .prepare()。但是通常在使用Handler的时候并没有去调用Looper.prepare()，这是因为在ActivityThread（app启动的时候会初始化）中已经调用了，如果想看一下，可以看ActivityThread类的main()方法，这里点到为止，只要记住一点：在非主线程中初始化Handler,必须先调用Looper.prepare()。
2."Only one Looper may be created per thread"，表示Looper和线程是一对一的。
接下来我们看一下Handler的sendMessageAtTime(Message msg, long uptimeMillis),sendMessage最终调用的都是该方法
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
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);
}
通过sendMessage将消息存入queue中，并且通过一个时间参数uptimeMillis来表示延时。
跟一下enqueueMessage(msg,uptimeMillis)
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();
/*************************重点关注*****************************/
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;
}
/*************************重点关注*****************************/
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
两个“重点关注”中间的代码表示通过时间参数when(也就是之前传入的uptimeMillis)，将msg按照时间顺序插入到queue中。
接下来，我们通过Looper.loop()看一下msg是怎么被取出的。
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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 (;;) {//--------------------------->(1)
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 {
msg.target.dispatchMessage(msg);//---------------------------------->(2)
} 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);
}
msg.recycleUnchecked();
}
}
(1) 表示死循环，queue.next()是一个阻塞的方法，一直等待消息。
(2) msg.target表示对应的Handler。
跟一下Handler.dispatchMessage(Message msg)
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {//msg.callback对应public static Message obtain(Handler h, Runnable callback){...}
handleCallback(msg);
} else {
if (mCallback != null) {//mCallback对应public Handler(Looper looper, Callback callback) {...}
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
如果Msg有初始化一个callback则执行Msg.callback.handleCallback(Message msg)，否则进行下面的判断
如果Handler有初始化一个callback则执行Handler.callback.handleCallback(Message msg)，否则执行Handler.handleMessage(Message msg)。
以上就是从Handler.sendMessage(Message msg)到Handlerd.handleMessage(Message msg)的全部过程。流程图如下
Handler (1).png
发送异步消息的其它方式：
1. Handler的post(Runnable r)方法
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
最终调用的MessageQueue.enqueueMessage(msg, uptimeMillis)。
2. View的post()方法
public boolean post(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
// Postpone the runnable until we know on which thread it needs to run.
// Assume that the runnable will be successfully placed after attach.
getRunQueue().post(action);
return true;
}
最终是调用上面的Handler.post(runnable r)。
3. Activity的runOnUiThread()方法
public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}
如果当前线程是UI线程，直接运行。如果当前线程不是UI线程，则通过Handler.post(Runnable r)发送到UI线程。
