最近正在找工作，正好也有時候整理整理知識點。今天整理一下Android的消息機制，這個也是面試當中必問的知識點了。
1.先看看我們是如何使用的
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mLooperThread = new LooperThread("回家");
mLooperThread.start();
}
/**
* 1.最常規用法，使用在主線程中直接new一個，然後發送消息
*/
private Handler mMainHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
Log.e("TAG", "handleMessage: " + Thread.currentThread().getName());
}
};
/**
* 2.在分線程中使用方法
*/
private LooperThread mLooperThread;
class LooperThread extends Thread {
public Handler mHandler;
public LooperThread(String name) {
super(name);
}
public void run() {
//第一步
Looper.prepare();
//第二步
mHandler = new Handler() {
public void handleMessage(Message msg) {
Log.e("TAG", "handleMessage: " + Thread.currentThread().getName());
}
};
//第三步
Looper.loop();
}
}
@Override
protected void onResume() {
super.onResume();
mLooperThread.mHandler.sendEmptyMessageDelayed(1, 1000);
mMainHandler.sendEmptyMessageDelayed(1, 1000);
}
}1）第一步Looper.prepare();方法，那麼他到底做了什麼呢？看源碼
final MessageQueue mQueue;
final Thread mThread;
private Printer mLogging;
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
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();
}2）第二步：創建handler，無參構造器最終會調用下面構造器
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;
} /**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}3）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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);
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();
}
}現在是消息隊列也有了，looper也開始工作不停的取消息了，handler也等待着處理消息了，那麼就差我們發消息了。
mMainHandler.sendEmptyMessageDelayed(1, 1000); /**
* Sends a Message containing only the what value, to be delivered
* after the specified amount of time elapses.
* @see #sendMessageDelayed(android.os.Message, long)
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
} public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
} 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);
}關鍵點來了，第一行代碼給mag設置了一個target，就是當前的handler，還記得剛纔取出來消息處理是不是調用的msg.target.dispatchMessage()方法，不就是又交給handler自己處理了嗎！這裏再說一下消息隊列裏的消息其實是根據延遲時間來進行排序，並不是真正的隊列先進後出原則。好了整個過程就分析完了，現在我們就可以在任意一個分線程中發送消息，然後會回到Looper所在的線程中處理結果，這樣就實現了跨線程的通信。
忘了把handler分發事件處理的代碼貼出來了：
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}4)爲什麼說Looper所在的線程纔是決定處理消息的線程呢，我麼可以驗證一下：
class LooperThread extends Thread {
public Handler mHandler;
public LooperThread(String name) {
super(name);
}
public void run() {
//第一步
Looper.prepare();
//第二步
mHandler = new Handler(getMainLooper()) {
public void handleMessage(Message msg) {
Log.e("TAG", "handleMessage: " + Thread.currentThread().getName());
}
};
//第三步
Looper.loop();
}
}2.使用中、面試中的問題：
1）既然looper裏面是個死循環，爲什麼不會阻塞主線程呢？這個問題上面已經說過了。
2）如果最開始上面的使用方法，AS會提示內存泄漏的，確實也會存在，解決方法有如下兩種：
第一種：
@Override
protected void onDestroy() {
mMainHandler.removeCallbacksAndMessages(null);
super.onDestroy();
}第二種方法：
private static class MyHandler extends Handler {
private final WeakReference<HandlerActivity2> mActivity;
public MyHandler(HandlerActivity2 activity) {
mActivity = new WeakReference<HandlerActivity2>(activity);
}
@Override
public void handleMessage(Message msg) {
if (mActivity.get() == null) {
return;
}
mActivity.get().todo();
}
}