异步通信，消息传递public class HandlerActivity extends AppCompatActivity {
Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
Toast.makeText(HandlerActivity.this, "接收到了消息", Toast.LENGTH_SHORT).show();
}
};
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handler);
}
public void onClickView(View v) {
switch (v.getId()) {
case R.id.button: {
Thread thread = new Thread() {
@Override
public void run() {
Message msg = Message.obtain();
handler.sendMessage(msg);
}
};
thread.start();
break;
}
}
}
}这个界面只有一个Button, 点击事件是：启动一个线程，在这个线程里使用handler，发送一个消息；这个消息不带任何数据。
Message: 这个可以使用new来生成，但是最好还是使用Message.obtain()来获取一个实例。这样便于消息池的管理。
handler初始化的时候，我们复写了handleMessage方法，这个方法用来接收，子线程中发过来的消息。
public class HandlerActivity extends AppCompatActivity {
Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
Log.d("child", "----" + Thread.currentThread().getName());
}
};
Handler childHandler = null;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handler);
Thread child = new Thread("child thread") {
@Override
public void run() {
Looper.prepare();
childHandler = new Handler(Looper.myLooper()) {
@Override
public void handleMessage(Message msg) {
Log.d("child", "----" + Thread.currentThread().getName() + ", " + msg.obj);
}
};
Looper.loop();
}
};
child.start();
}
public void onClickView(View v) {
switch (v.getId()) {
case R.id.button: {
Thread thread = new Thread() {
@Override
public void run() {
Message msg = Message.obtain();
handler.sendMessage(msg);
}
};
thread.start();
break;
}
case R.id.button1: {
if (childHandler != null) {
Message msg = Message.obtain(childHandler);
msg.obj = "123---" + SystemClock.uptimeMillis();
childHandler.sendMessage(msg);
} else {
Log.d("-----", "onClickView: child Handler is null");
}
break;
}
}
}
}主线程向子线程发送消息时，我们需要使用的是handler,但是这个handler是需要在子线程中实例化，否则子线程无法接收到消息。
在child thread中准备的工作：
Looper.loop()调用
假如我们直接在子线程中直接实例化一个handler，而不传入Looper实例。程序会直接抛出异常：java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
这个异常是在hanlder源码：
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
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;
}
出现的，从这里可知，在线程中实例化一个handler需要一个Looper实例。
首先看Looper源码中是怎么写的。Looper的构造方法是私有的，这样我们只能通过Looper的静态方法来实例化一个Looper.在Looper类中还有一个ThreadLocal<Looper> sThreadLocal 变量，而在上面的代码中使用了Looper.myLooper()方法来给handler一个Looper实例。
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}这个方法返回了一个存放在ThreadLocal<Looper>中的Looper实例。
Looper.prepare源码：
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));
}
方法很简单，就是检查sThreadLocal里面是否有有值，然后将Looper的实例存放到ThreadLocal，如果不为空，直接就是 RuntimeException 异常。这也保证了一个线程中只能有一个Looper实例。因此，Looper.prepare方法只能调用一次。
在这个程序中，可以大概理解成线程间数据的隔离。意思就是我存放在ThreadLocal中的数据，只能在我本线程中可以获得到值，在其他线程中获取不到（其他线程中获取的是null）。
这样就能得出，其他线程中的Looper，在本线程中通过Looper.myLooper()获取不到数据。
通过查找Android源码，可以知道在ActivityThread中main方法里面，UI线程已经初始化了Looper.prepareMainLooper();这样就在UI线程中有Looper实例了。当然在main方法的下面，也调用了Looper.loop()方法。
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();
**}**}
这个方法里面，我们需要注意到：
msg.target.dispatchMessage(msg);
for循环是一个死循环，这个需要一直取出MessageQueue队列中的数据，也就是刚才所列的第三个 queue.next方法。这个方法会阻塞，直到有消息从消息队列中取出来。
next方法源码：
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;
}}
public void dispatchMessage(Message msg) {<br/>if (msg.callback != null) {<br/>handleCallback(msg);<br/>} else {<br/>if (mCallback != null) {<br/>if (mCallback.handleMessage(msg)) {<br/>return;<br/>}<br/>}<br/>handleMessage(msg);<br/>}<br/>}<br/>Handler 通过sendMessage方法发送消息。这个方法最终调用的是
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {<br/>msg.target = this;<br/>if (mAsynchronous) {<br/>msg.setAsynchronous(true);<br/>}<br/>return queue.enqueueMessage(msg, uptimeMillis);<br/>}<br/>
参数：MessageQueue queue则是Handler中mQueue变量，这个变量在Handler(Callback callback, boolean async)初始化完成。它是Looper中一个final MessageQueue的变量，在初始化Looper的时候，就开始初始化MessageQueue。这也是一个线程中只有一个MessageQueue的原因.
Message msg 是封装的消息。
long uptimeMillis 延迟发送时间。
之前分析looper.loop方法的时候，说了msg.target.dispatchMessage, 这个target就是在这个方法里面赋值的。
从这个方法里面，可以知道handler的sendMessage，只是把消息(Message实例)添加到了queue队列中。
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 {
// 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;
}通过handler发送Message，将Message压入MessageQueue队列中；而Looper.loop方法又在不停的循环这个消息队列，取出压入MessageQueue的Message， 然后dispatchMessage分发，最终会调用handler.handleMessage方法来处理发送过来的Message.
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