Handler简介
Handler是Android消息机制的上层接口，多数开发者会使用Handler去更新UI或做延时处理。如下代码所示：
new Thread(new Runnable() {
@Override
public void run() {
// do something
handler.sendMessage(message)
}
}).start();new Handler().postDelayed(new Runnable() {
@Override
public void run() {
//do something
}
}, 2000);Handler分析
Handler类位于android.os包下，继承Object类
其中可见的构造方法有三个
/**
* Constructor associates this handler with the {@link Looper} for the
* current thread and takes a callback interface in which you can handle
* messages.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Callback callback) {
this(callback, false);
}
/**
* Use the provided {@link Looper} instead of the default one.
*
* @param looper The looper, must not be null.
*/
public Handler(Looper looper) {
this(looper, null, false);
}
/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}其中Callback是Handler内部定义的接口，只有一个回调方法handleMessage
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public interface Callback {
public boolean handleMessage(Message msg);
}这里先来分析无参构造方法
public Handler() {
this(null, false);
}在无参构造方法中调用了重载方法,如下，省略部分代码：
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(Callback callback, boolean async) {
....//省略
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;
}这里需要注意下@hide修饰为隐藏方法，不能直接调用，可以通过反射调用。
这里需要一个Looper对象，如果mLooper属性为空，会抛出异常。如果在主线程(UI线程)中创建Handler对象，默认在主线程中已经调用了Looper.prepareMainLooper()方法，详细代码在ActivityThread类中main方法中。Looper.myLooper()方法会获取当前线程的Looper对象，当当前线程中没有Looper对象时，创建Handler会抛出异常，这就是常见于在子线程中创建Handler之前没有调用Looper.prepare()方法出现的崩溃，有问题代码如下：
new Thread(new Runnable() {
@Override
public void run() {
Handler mHandler = new Handler();
}
}).start();正确在子线程中创建Handler如下：
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
Handler mHandler = new Handler();
Looper.loop();
}
}).start();这里需要注意的是在创建完Handler后，这里又调用了Looper.loop()方法，这个方法是为了在这个线程中运行消息队列。
Looper分析
我们可以查看Looper源码中prepare方法可知，如下：
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));
}
当我们调用Looper.prepare方法时，调用重载方法，为当前线程关联一个新的Looper对象，这里使用的是ThreadLocal，有兴趣的可以查看下源码。我们接着看Looper.loop方法：
/**
* 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 (;;) {
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 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);
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);
}
}
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();
}
}这里主要逻辑是获取当前Looper对象的MessageQueue队列，for死循环不断的从队列中取出消息，调用msg.target.dispatchMessage(msg);这个的target就是Handler对象，这个target是在Handler中sendMessage系列方法中设置引用的，代码如下：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}msg.target = this，到这里对Handler的调用关系就分析完毕了，有兴趣的可以查看源码，谢谢。