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android消息线程和消息队列
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基于消息队列的线程通信：
消息队列与线程循环
MessageQueue:
利用链表来管理消息。
Message：
通常由Handler的子类的handleMessage（）函数来处理该消息。
由android.os.Handler类来处理消息Message：
Handler：
由Handler来处理消息，由其函数dispatchMessage()完成。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg); //若消息没有指定处理消息的对象，则由Handler的子类来完成。其子类必须实现handleMessage()方法。
}
}
Looper：
Looper与线程关联，一个线程最多有一个Looper。Looper实例保存到宿主线程的线程局部存储中。
指定了Looper就意味着指定了在哪个线程中处理；为消息指定的Handler则决定如何处理消息。
Looper中拥有一个消息队列MessageQueue，该消息队列由Looper创建，属于它所有。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);//创建Looper要处理的消息队列，是否运行线程退出，由消息队列决定。
mRun = true;
mThread = Thread.currentThread();//得到当前线程
}
mQueue = new MessageQueue(quitAllowed);//创建Looper要处理的消息队列，是否运行线程退出，由消息队列决定。
mRun = true;
mThread = Thread.currentThread();//得到当前线程
}
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//取得一个消息
if (msg == null) {//消息体为空则表示退出
// 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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);//调用消息指定的接收者Handler来处理它
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
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.recycle();//丢弃消息,循环再利用
}
}
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//取得一个消息
if (msg == null) {//消息体为空则表示退出
// 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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);//调用消息指定的接收者Handler来处理它
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
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.recycle();//丢弃消息,循环再利用
}
}
Handler：消息的处理者。一旦为消息指定了某个特定的处理者Handler，也就确定了在哪个线程的上下文中去处理以及如何处理消息。
实际上是根据Handler的变量mLooper所代表的Looper所在的线程，来决定该Handler运行在哪个线程中。
在一个线程中创建的Handler并不一定运行在该线程中，需要判断该Handler的Looper变量在哪个线程中。
public Handler() {
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();//通过线程局部存储，获得本线程的Looper：当程序没有另启一个线程时，该Looper就是主线程的Looper。
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = null;
}
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);//消息所指定的CallBack，实际就是Runnable
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);//Handler的方法
}
}
