Android消息机制学习小结
Android中的消息机制
在Android中,只能在UI线程对控件进行操作,只能在子线程进行耗时操作,例如IO和网络请求,所以需要一个消息通知方式,让耗时操作结束后,主线程能够及时响应.这个机制就是Android中的消息机制.
Android的消息机制主要是指Handler的运行机制以及Handler所附带的MessageQueue和Looper的工作过程.
示例图
图1-1 示例图
Handler基本使用,在子线程断续休眠2S,通过handler向主线程发送消息通知UI刷新,通过查看log和界面就能观察到变化
private static final String TAG = HandlerTestActivity.class.getSimpleName();
private Handler handler;
private ExecutorService executorService;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handler);
executorService = Executors.newCachedThreadPool();
final TextView tvMsg = findViewById(R.id.tv_msg);
handler = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
switch (msg.what) {
case 1:
String s = msg.getData().getString("msg");
tvMsg.setText("Content: " + s);
Log.d(TAG, "handleMessage: "+s+Thread.currentThread());
break;
default:
break;
}
return false;
}
});
}
public void sendMessage(View view) {
executorService.execute(new Runnable() {
@Override
public void run() {
//other thread
int i = 0;
try {
while (i < 5) {
TimeUnit.SECONDS.sleep(2);
Message message = new Message();
message.what = 1;
Bundle bundle = new Bundle();
bundle.putString("msg", "" + i);
message.setData(bundle);
handler.sendMessage(message);
i++;
Log.d(TAG, "run: "+Thread.currentThread());
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
}
1消息队列的工作原理
消息队列在Android中指的是MessageQueue,在MessageQueue中主要包含两个操作:插入和读取,在读取的过程中,会伴随着删除的操作,插入和读取对应的方法分别是enqueueMessage和next.尽管叫做消息队列,但是内部的数据结构是单链表,而不是队列,因为单链表在插入和删除的性能较好.
在MessageQueue方法中,enqueueMessage方法只是一个的链表插入操作.但是next方法是一个无限循环,如果队列中没有消息,就会一直阻塞,当有新的消息来时,next方法就会返回这条消息,并从链表中删除.
Message next() {
...
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;
}
}
....
}
2Looper的工作原理
Looper在消息机制中扮演着消息循环的角色,会不停地从MessageQueue中查看是否有新消息,如果有新消息,就会马上进行处理,否则也会一直阻塞.在Looper的构造方法中,会构造一个新的MessageQueue,通过ThrealLocal来保证每一个线程有且只有一个Looper
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));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Handler的工作需要Looper,在没有Looper的线程使用Handler发送消息,就会抛出一个运行时异常.
No Looper; Looper.prepare() wasn't called on this thread.
在主线程中我们没有主动调用Looper.prepare也能够正常使用Handler是因为在应用启动的时候,系统已经在ActivityThread的main方法中主动为我们调用了一个Looper.prepare(),因为四大组件的注册和使用都需要handler的支持.
public static void main(String[] args) {
...
Looper.prepareMainLooper();
...
}
在prepareMainLooper方法的注释上就为标明了这一点,
图2-1 prepareMainLooper.png
在子线程中创建了一个Looper,需要在使用结束的时候通过Looper的quit或者quitSafely方法终止消息循环,否则这个线程会一直处于等待状态,如果调用了quit方法,这个线程就会自动终止.
Looper还有一个很重要的loop方法,只有调用了loop之后,消息循环才会真正起作用,在loop中,当Message不为空,就会调用到msg持有的handler对象的dispatchMessage方法,Handler中的dispatchMessage方法是在创建Handler时所使用的Looper中执行,所以就可以将方法回调到主线程中.
所以在UI线程创建Handler,那么dispatchMessage就会在UI线程被调用,最终会调用的到handler的handleMessage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();
}
}
在loop方法中,也是一个死循环,不断从MessageQueue调用next方法获取Message,直到Message为空,才跳出循环.当Looper的quit方法被调用时,会调用MessageQueue的quit方法,将消息队列标记为退出状态,next方法就会返回null.由于MessageQueue的next方法是阻塞方法,当没有消息时会一直阻塞,也就导致了looper的loop方法阻塞,所以必须对用Looper的quit方法.
3. Handler工作原理
Handler的工作主要包含消息的发送和接收过程.消息的发送可以通过各种post方法,和各种send方法实现,post方法最终调用的也是send.所以主要看sendMessage方法就行.
public final boolean sendMessage(Message msg){
return sendMessageDelayed(msg, 0);
}
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);
}
通过源码可以发现,无论handler通过什么方式发送一条消息,最终都是将这条消息插入到MessageQueue的链表中,MessageQueue的next会返回这条消息给Looper,Looper收到消息之后进行处理,最终由Looper交给Hanlder的dispatchMessage进行消息接收处理,这样Handler就进入到消息处理的阶段.
4.消息处理
消息处理是在Handler的dispatchMessage中执行
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
在dispatchMessage方法中可以看到,会先判断Message的callback方法是否为空,Message的callback实际上是一个Runnable对象,就是通过handler.post(Runnable r)的Runnable对象.在第二个判断中mCallback对象是构造Handler时传入的Callback接口对象,通过这种方式创建的Handler就不需要继承Handler
Handler handler = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
return false;
}
});
dispatchMessage根据不同的handler创建方式最终回调到各个的handleMessage方法,
Handler发送和处理消息的过程基本就是这样.
