Handler类注释
/**
* A Handler allows you to send and process {@link Message} and Runnable
* objects associated with a thread's {@link MessageQueue}. Each Handler
* instance is associated with a single thread and that thread's message
* queue. When you create a new Handler, it is bound to the thread /
* message queue of the thread that is creating it -- from that point on,
* it will deliver messages and runnables to that message queue and execute
* them as they come out of the message queue.
*
* <p>There are two main uses for a Handler: (1) to schedule messages and
* runnables to be executed at some point in the future; and (2) to enqueue
* an action to be performed on a different thread than your own.
*
* <p>Scheduling messages is accomplished with the
* {@link #post}, {@link #postAtTime(Runnable, long)},
* {@link #postDelayed}, {@link #sendEmptyMessage},
* {@link #sendMessage}, {@link #sendMessageAtTime}, and
* {@link #sendMessageDelayed} methods. The <em>post</em> versions allow
* you to enqueue Runnable objects to be called by the message queue when
* they are received; the <em>sendMessage</em> versions allow you to enqueue
* a {@link Message} object containing a bundle of data that will be
* processed by the Handler's {@link #handleMessage} method (requiring that
* you implement a subclass of Handler).
*
* <p>When posting or sending to a Handler, you can either
* allow the item to be processed as soon as the message queue is ready
* to do so, or specify a delay before it gets processed or absolute time for
* it to be processed. The latter two allow you to implement timeouts,
* ticks, and other timing-based behavior.
*
* <p>When a
* process is created for your application, its main thread is dedicated to
* running a message queue that takes care of managing the top-level
* application objects (activities, broadcast receivers, etc) and any windows
* they create. You can create your own threads, and communicate back with
* the main application thread through a Handler. This is done by calling
* the same <em>post</em> or <em>sendMessage</em> methods as before, but from
* your new thread. The given Runnable or Message will then be scheduled
* in the Handler's message queue and processed when appropriate.
*/
翻译：
Handler是一个可以通过关联一个消息队列来发送和处理消息，发送或处理Runnable对象的一个处理程序，每个Handler都关联一个单个的线程和消息队列，当你创建一个新的Handler的时候它就将绑定到一个线程或线程上的消息队列，从那时起，这个Handler就将为这个消息队列提供消息或Runnable对象，处理消息队列释放出来的消息或Runnable对象。
处理程序有两种主要用途:
(1)调度消息和在将来的某个时候执行的runnables;
(2)排队在与您的线程不同的线程上执行的操作。
通过重写post(Runnable)、postAtTime(Runnable, long)、postDelayed(Runnable, long)、sendEmptyMessage(int)、sendMessage(Message)、sendMessageAtTime(Message, long)、sendMessageDelayed(Message, long)来完成发送消息。当前版本允许你通过消息队列接受一个Runnable对象，sendMessage方法当前版本允许你将一个包的数据通过消息队列的方式处理，但是你需要重写Handler的handleMessage方法
当你发送一个Handler时，你可以使消息队列(MessageQueue)尽快去处理已经准备好的条目，或者指定一个延迟处理的时间或者指定时间处理，后两者允许你实现超时，Ticks(系统相对的时间单位)和其他时间段为基础的行为。
即当一个进程被应用程序创建时，它的主线程会运行一个消息队列负责管理它创建的高层应用程序对象(如Activity、Broadcast Receiver等)和任何它的窗口创建的对象，你可以通过一个Handler，创建自己的线程来实现与主线程之间的交互，但前提是你得在你的线程重写sendMessage方法并写上一行行代码，这样你给定的Runnable或者Message将被MessageQueue(消息队列)预定，并在合适的时间处理
注意：有心的同学会注意，我们在进行Handler源码分析的时候，可以知道Handler中的Message是分2中类型的，一种是Message，就是Message对象，一种是CallMessage，就是Runnable对象，但是MessageQueue中只支持DataMessage，再插入到MessageQueue的时候，会把Runnable对象封装到Message对象中。
构造函数
public Handler() {
this(null, false);
}
public Handler(@Nullable Callback callback) {
this(callback, false);
}
@UnsupportedAppUsage
public Handler(boolean async) {
this(null, async);
}
public Handler(@Nullable 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());
}
}
//拿当前线程的Looper构建消息循环系统
//如果当前线程没有 初始化 looper 就会报错
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; // 这个就是消息队列
mCallback = callback; // 回调
mAsynchronous = async; // 异步
}
public Handler(@NonNull Looper looper) {
this(looper, null, false);
}
public Handler(@NonNull Looper looper, @Nullable Callback callback) {
this(looper, callback, false);
}
@UnsupportedAppUsage
public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
mLooper = looper; // 自定义looper
mQueue = looper.mQueue; // 消息队列
mCallback = callback; // 回调
mAsynchronous = async; // 异步
}
消息的处理
/**
* Handle system messages here.
* Looper.loop()里面调用了 msg.target.dispatchMessage(msg);
* 这一步 调用发送msg 的Handler 来处理msg (谁发送的谁处理)
*/
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
/**
* Subclasses must implement this to receive messages.
* 子类必须实现它来接收消息。
*/
public void handleMessage(@NonNull Message msg) {
}
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*/
public interface Callback {
/**
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
boolean handleMessage(@NonNull Message msg);
}
Handler提供的Message
/**
* Returns a new {@link android.os.Message Message} from the global message pool. More efficient than
* creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this).
* If you don't want that facility, just call Message.obtain() instead.
*/
@NonNull
public final Message obtainMessage() {
return Message.obtain(this);
}
/**
* Same as {@link #obtainMessage()}, except that it also sets the what member of the returned Message.
*
* @param what Value to assign to the returned Message.what field.
* @return A Message from the global message pool.
*/
@NonNull
public final Message obtainMessage(int what) {
return Message.obtain(this, what);
}
/**
* Same as {@link #obtainMessage()}, except that it also sets the what and obj members
* of the returned Message.
*
* @param what Value to assign to the returned Message.what field.
* @param obj Value to assign to the returned Message.obj field.
* @return A Message from the global message pool.
*/
@NonNull
public final Message obtainMessage(int what, @Nullable Object obj) {
return Message.obtain(this, what, obj);
}
/**
* Same as {@link #obtainMessage()}, except that it also sets the what, arg1 and arg2 members of the returned
* Message.
*
* @param what Value to assign to the returned Message.what field.
* @param arg1 Value to assign to the returned Message.arg1 field.
* @param arg2 Value to assign to the returned Message.arg2 field.
* @return A Message from the global message pool.
*/
@NonNull
public final Message obtainMessage(int what, int arg1, int arg2) {
return Message.obtain(this, what, arg1, arg2);
}
/**
* Same as {@link #obtainMessage()}, except that it also sets the what, obj, arg1,and arg2 values on the
* returned Message.
*
* @param what Value to assign to the returned Message.what field.
* @param arg1 Value to assign to the returned Message.arg1 field.
* @param arg2 Value to assign to the returned Message.arg2 field.
* @param obj Value to assign to the returned Message.obj field.
* @return A Message from the global message pool.
*/
@NonNull
public final Message obtainMessage(int what, int arg1, int arg2, @Nullable Object obj) {
return Message.obtain(this, what, arg1, arg2, obj);
}
通过源码知道了内部还是调用了Message的obtain函数。
post
public final boolean post(@NonNull Runnable r) {
return sendMessageDelayed(getPostMessage(r), 0);
}
public final boolean postAtTime(@NonNull Runnable r, long uptimeMillis) {
return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}
public final boolean postAtTime(@NonNull Runnable r, @Nullable Object token, long uptimeMillis) {
return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}
public final boolean postDelayed(@NonNull Runnable r, long delayMillis) {
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
public final boolean postDelayed(Runnable r, int what, long delayMillis) {
return sendMessageDelayed(getPostMessage(r).setWhat(what), delayMillis);
}
public final boolean postDelayed(@NonNull Runnable r, @Nullable Object token, long delayMillis) {
return sendMessageDelayed(getPostMessage(r, token), delayMillis);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r; // 将runable 封装到msg中了
return m;
}
@UnsupportedAppUsage
private static Message getPostMessage(Runnable r, Object token) {
Message m = Message.obtain();
m.obj = token;
m.callback = r; // 将runable 封装到msg中了
return m;
}
send
public final boolean sendMessage(@NonNull Message msg) {
return sendMessageDelayed(msg, 0);
}
public final boolean sendEmptyMessage(int what) {
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
// 表示系统开机到当前的时间总数，单位是毫秒，
// 但是，当系统进入深度睡眠（CPU休眠、屏幕休眠、设备等待外部输入）时间就会停止，
// 但是不会受到时钟缩放、空闲或者其他节能机制的影响。
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull 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);
}
public final boolean sendMessageAtFrontOfQueue(@NonNull Message msg) {
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, 0);
}
最终调用MessageQueue.enqueueMessage()
/**
* 将Msg添加到消息队列中 所有的发送消息都会走到这里
*
* @param queue
* @param msg
* @param uptimeMillis
* @return
*/
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg, long uptimeMillis) {
// Handler 保存进来了 looper的时候使用这个Handler 处理消息的
// 为什么使用匿名内部类来创建Handler的方法会有内存泄漏的风险？
// 内部类持有外部类的引用，如果有延时消息会导致外部类无法回收，导致内存泄漏
msg.target = this;
// 线程数据
msg.workSourceUid = ThreadLocalWorkSource.getUid();
// 异步
if (mAsynchronous) {
msg.setAsynchronous(true);
}
// 调用的是 Looper 的 MessageQueen 的函数
return queue.enqueueMessage(msg, uptimeMillis);
}
其他方法
移除消息 removeMessages()
public final void removeMessages(int what) {
mQueue.removeMessages(this, what, null);
}
public final void removeMessages(int what, @Nullable Object object) {
mQueue.removeMessages(this, what, object);
}
public final void removeCallbacksAndMessages(@Nullable Object token) {
mQueue.removeCallbacksAndMessages(this, token);
}
hasMessages()
public final boolean hasMessages(int what) {
return mQueue.hasMessages(this, what, null);
}
public final boolean hasMessagesOrCallbacks() {
return mQueue.hasMessages(this);
}
public final boolean hasMessages(int what, @Nullable Object object) {
return mQueue.hasMessages(this, what, object);
}
public final boolean hasCallbacks(@NonNull Runnable r) {
return mQueue.hasMessages(this, r, null);
}
通过上面的源码可知Handler操作Message最后调用的都是MessageQueue里面的方法对消息队列的操作。
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