編輯:關於android開發
Handler的主要用途有兩個:(1)、在將來的某個時刻執行消息或一個runnable,(2)把消息發送到消息隊列。
主要依靠post(Runnable)、postAtTime(Runnable, long)、postDelayed(Runnable, long)、sendEmptyMessage(int)、sendMessage(Message)、sendMessageAtTime(Message)、sendMessageDelayed(Message, long)這些方法來來完成消息調度。post方法是當到Runable對象到達就被插入到消息隊列;sendMessage方法允許你把一個包含有信息的Message插入隊列,而且它會Handler的handlerMessage(Message)方法中執行(該方法要求在Handler的子類中實現)。
/** 默認的構造方法,handler是和當前線程的隊列關聯在一起,如果隊列不存在,那麼handler就不能接受消息。 * Default constructor associates this handler with the {@link Looper} for the * current thread. * 如果線程沒有looper,就會拋出異常 * If this thread does not have a looper, this handler won't be able to receive messages * so an exception is thrown. */ public Handler() { this(null, false); } //傳入一個callback接口用於處理handler傳遞的Message。 public Handler(Callback callback) { this(callback, false); } //給變量賦值 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; } public Handler(Looper looper, Callback callback, boolean async) { mLooper = looper; mQueue = looper.mQueue; mCallback = callback; mAsynchronous = async; }
方法都差不多,主要是完成了賦值的過程,還有幾個沒粘貼,但是都差不多。。。
/* 設置這個標記為true來檢測不是靜態的匿名,本地或成員類繼承Handler類。這些類型的類可以帶來潛在的洩漏。在Handler的構造方法裡面使用到了這個參數,目的就如上所述。 */ private static final boolean FIND_POTENTIAL_LEAKS = false; final MessageQueue mQueue; final Looper mLooper; final Callback mCallback; //回調接口 final boolean mAsynchronous; IMessenger mMessenger;
接著下面就是Callback接口:
/** * Callback interface you can use when instantiating(實例化) a Handler to avoid * having to implement(實現) your own subclass of Handler. * @param msg A {@link android.os.Message Message} object * @return True if no further handling is desired */ public interface Callback { public boolean handleMessage(Message msg); }
callback接口你可以在實例化的時候用,避免去實現你自己的handler子類
這個Callback接口裡只有一個handleMessage方法返回boolean值,在後面Handler的ctor會用到,一般情況下都是null。這個接口的存在
沒什麼特殊的含義,只是為了讓你不extends Handler就能處理消息而已(正如此方法的doc所說),類似Thread和Runnable接口的關系。
/** * Subclasses must implement this to receive messages. */ public void handleMessage(Message msg) { }
子類需要實現這個方法,因為這是個空方法。
/** * 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); } } private static void handleCallback(Message message) { message.callback.run(); }
它的處理是如果message自身設置了callback,則
直接調用callback.run()方法,否則Callback接口的作用就顯現了;如果我們傳遞了Callback接口的實現,即mCallback非空,則調用它處理
message,如果處理了,返回true,則直接返回,否則接著調用Handler自己的handleMessage方法,其默認實現是do nothing,如果你
是extends Handler,那麼你應該在你的子類中為handleMessage提供自己的實現。
public final Message obtainMessage() { return Message.obtain(this); } public final Message obtainMessage(int what) { return Message.obtain(this, what); } public final Message obtainMessage(int what, int arg1, int arg2, Object obj) { return Message.obtain(this, what, arg1, arg2, obj); }
從上面可以看出來還是調用的Message的obtain方法,來構造message。
/** * Causes the Runnable r to be added to the message queue. * The runnable will be run on the thread to which this handler is * attached. * * @param r The Runnable that will be executed. * * @return Returns true if the Runnable 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 post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); } public final boolean postAtTime(Runnable r, long uptimeMillis) { return sendMessageAtTime(getPostMessage(r), uptimeMillis); } public final boolean postAtTime(Runnable r, Object token, long uptimeMillis) { return sendMessageAtTime(getPostMessage(r, token), uptimeMillis); } public final boolean postDelayed(Runnable r, long delayMillis) { return sendMessageDelayed(getPostMessage(r), delayMillis); } public final boolean postAtFrontOfQueue(Runnable r) { return sendMessageAtFrontOfQueue(getPostMessage(r)); }
上面方法的作用就是:把Runnable發送到消息隊列,執行的時候實行Runnable的run方法。。
下面看一系列sendXXX方法,和上邊對應的。。。
//把消息入隊 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); } public final boolean sendEmptyMessage(int what) { return sendEmptyMessageDelayed(what, 0);
上面所有的postXXX,sendXXX方法最後都會調用這個方法:enqueueMessage
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
最後調用queue的enqueueMessage方法把msg入隊,對這個方法不了解的,可以看前邊的MessageQueue源碼解析。
SystemClock.uptimeMillis(),就是表示系統開機到當前的時間總數,如果有延遲,就加上延遲時間,分析到現在,我們也能發現,postDelayed不是延遲多少秒發送消息,這個消息是直接發送給隊列的,不過在MessaegQueue中,消息按時間排放的,不到時間不會把它取出來,所以應該說延遲多少秒取出消息更合適。。
至於上邊的getPostMessage(r)就是把r設置給callback。
private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m; }
調用MQ的removeMessages方法。就是移出messageQueue中所有滿足條件的message,當然前提是消息還沒取走。
/** * Remove any pending posts of Runnable r that are in the message queue. */ public final void removeCallbacks(Runnable r) { mQueue.removeMessages(this, r, null); } /** * Remove any pending posts of Runnable <var>r</var> with Object * <var>token</var> that are in the message queue. If <var>token</var> is null, * all callbacks will be removed. */ public final void removeCallbacks(Runnable r, Object token) { mQueue.removeMessages(this, r, token); }
分析結束,鼓勵自己下。。哈哈哈。。。。
前三篇傳送門: Looper源碼解析: http://www.cnblogs.com/jycboy/p/5787443.html
MessageQueue源碼解析: http://www.cnblogs.com/jycboy/p/5786682.html
Message源碼解析:http://www.cnblogs.com/jycboy/p/5786551.html
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