編輯:關於android開發
一、簡介
Android的消息機制主要是指Handler的運行機制,那麼什麼是Handler的運行機制那?通俗的來講就是,使用Handler將子線程的Message放入主線程的Messagequeue中,在主線程使用。
二、學習內容
學習Android的消息機制,我們需要先了解如下內容。
平常我們接觸的大多是Handler和Message,今天就讓我們來深入的了解一下他們。
三、代碼詳解
一般而言我們都是這樣使用Handler的
xxHandler.sendEmptyMessage(xxx);
當然還有其他表示方法,但我們深入到源代碼中,會發現,他們最終都調用了一個方法
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); }
sendMessageAtTime()方法,但這依然不是結束,我們可以看到最後一句enqueueMessage(queue, msg, uptimeMillis);按字面意思來說插入一條消息,那麼疑問來了,消息插入了哪裡。
1 boolean enqueueMessage(Message msg, long when) { 2 if (msg.target == null) { 3 throw new IllegalArgumentException("Message must have a target."); 4 } 5 if (msg.isInUse()) { 6 throw new IllegalStateException(msg + " This message is already in use."); 7 } 8 9 synchronized (this) { 10 if (mQuitting) { 11 IllegalStateException e = new IllegalStateException( 12 msg.target + " sending message to a Handler on a dead thread"); 13 Log.w(TAG, e.getMessage(), e); 14 msg.recycle(); 15 return false; 16 } 17 18 msg.markInUse(); 19 msg.when = when; 20 Message p = mMessages; 21 boolean needWake; 22 if (p == null || when == 0 || when < p.when) { 23 // New head, wake up the event queue if blocked. 24 msg.next = p; 25 mMessages = msg; 26 needWake = mBlocked; 27 } else { 28 // Inserted within the middle of the queue. Usually we don't have to wake 29 // up the event queue unless there is a barrier at the head of the queue 30 // and the message is the earliest asynchronous message in the queue. 31 needWake = mBlocked && p.target == null && msg.isAsynchronous(); 32 Message prev; 33 for (;;) { 34 prev = p; 35 p = p.next; 36 if (p == null || when < p.when) { 37 break; 38 } 39 if (needWake && p.isAsynchronous()) { 40 needWake = false; 41 } 42 } 43 msg.next = p; // invariant: p == prev.next 44 prev.next = msg; 45 } 46 47 // We can assume mPtr != 0 because mQuitting is false. 48 if (needWake) { 49 nativeWake(mPtr); 50 } 51 } 52 return true; 53 }
進入源代碼,我們發現,我們需要了解一個新類Messagequeue。
雖然我們一般把他叫做消息隊列,但是通過研究,我們發下,它實際上是一種單鏈表的數據結構,而我們對它的操作主要是插入和讀取。
看代碼33-44,學過數據結構,我們可以輕松的看出,這是一個單鏈表的插入末尾的操作。
這樣就明白了,我們send方法實質就是向Messagequeue中插入這麼一條消息,那麼另一個問題隨之而來,我們該如何處理這條消息。
處理消息我們離不開一個重要的,Looper。那麼它在消息機制中又有什麼樣的作用那?
Looper扮演著消息循環的角色,具體而言它會不停的從MessageQueue中查看是否有新消息如果有新消息就會立刻處理,否則就已知阻塞在那裡,現在讓我們來看一下他的代碼實現。
首先是構造方法
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
可以發現,它將當前線程對象保存了起來。我們繼續
Looper在新線程創建過程中有兩個重要的方法looper.prepare() looper.loop
new Thread(){ public void run(){ Looper.prepare(); Handler handler = new Handler(); Looper.loop(); } }.start();
我們先來看prepare()方法
1 private static void prepare(boolean quitAllowed) { 2 if (sThreadLocal.get() != null) { 3 throw new RuntimeException("Only one Looper may be created per thread"); 4 } 5 sThreadLocal.set(new Looper(quitAllowed)); 6 }
咦,我們可以看到這裡面又有一個ThreadLocal類,我們在這簡單了解一下,他的特性,set(),get()方法。
首先ThreadLocal是一個線程內部的數據存儲類,通過它可以在指定的線程中存儲數據,數據存儲後,只有在制定線程中可以獲取存儲的數據,對於其他線程而言則無法獲取到數據。簡單的來說。套用一個列子:
private ThreadLocal<Boolean> mBooleanThreadLocal = new ThreadLocal<Boolean>();// mBooleanThreadLocal.set(true); Log.d(TAH,"Threadmain"+mBooleanThreadLocal.get()); new Thread("Thread#1"){ public void run(){ mBooleanThreadLocal.set(false); Log.d(TAH,"Thread#1"+mBooleanThreadLocal.get()); }; }.start(); new Thread("Thread#2"){ public void run(){ Log.d(TAH,"Thread#2"+mBooleanThreadLocal.get()); }; }.start();
上面的代碼運行後,我們會發現,每一個線程的值都是不同的,即使他們訪問的是同意個ThreadLocal對象。
那麼我們接下來會在之後分析源碼,為什麼他會不一樣。現在我們跳回prepare()方法那一步,loop()方法源碼貼上
1 public static void loop() { 2 final Looper me = myLooper(); 3 if (me == null) { 4 throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); 5 } 6 final MessageQueue queue = me.mQueue; 7 8 // Make sure the identity of this thread is that of the local process, 9 // and keep track of what that identity token actually is. 10 Binder.clearCallingIdentity(); 11 final long ident = Binder.clearCallingIdentity(); 12 13 for (;;) { 14 Message msg = queue.next(); // might block 15 if (msg == null) { 16 // No message indicates that the message queue is quitting. 17 return; 18 } 19 20 // This must be in a local variable, in case a UI event sets the logger 21 Printer logging = me.mLogging; 22 if (logging != null) { 23 logging.println(">>>>> Dispatching to " + msg.target + " " + 24 msg.callback + ": " + msg.what); 25 } 26 27 msg.target.dispatchMessage(msg); 28 29 if (logging != null) { 30 logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); 31 } 32 33 // Make sure that during the course of dispatching the 34 // identity of the thread wasn't corrupted. 35 final long newIdent = Binder.clearCallingIdentity(); 36 if (ident != newIdent) { 37 Log.wtf(TAG, "Thread identity changed from 0x" 38 + Long.toHexString(ident) + " to 0x" 39 + Long.toHexString(newIdent) + " while dispatching to " 40 + msg.target.getClass().getName() + " " 41 + msg.callback + " what=" + msg.what); 42 } 43 44 msg.recycleUnchecked(); 45 } 46 }
首先loop()方法,獲得這個線程的Looper,若沒有拋出異常。再獲得新建的Messagequeue,在這裡我們有必要補充一下Messagequeue的next()方法。
1 Message next() { 2 // Return here if the message loop has already quit and been disposed. 3 // This can happen if the application tries to restart a looper after quit 4 // which is not supported. 5 final long ptr = mPtr; 6 if (ptr == 0) { 7 return null; 8 } 9 10 int pendingIdleHandlerCount = -1; // -1 only during first iteration 11 int nextPollTimeoutMillis = 0; 12 for (;;) { 13 if (nextPollTimeoutMillis != 0) { 14 Binder.flushPendingCommands(); 15 } 16 17 nativePollOnce(ptr, nextPollTimeoutMillis); 18 19 synchronized (this) { 20 // Try to retrieve the next message. Return if found. 21 final long now = SystemClock.uptimeMillis(); 22 Message prevMsg = null; 23 Message msg = mMessages; 24 if (msg != null && msg.target == null) { 25 // Stalled by a barrier. Find the next asynchronous message in the queue. 26 do { 27 prevMsg = msg; 28 msg = msg.next; 29 } while (msg != null && !msg.isAsynchronous()); 30 } 31 if (msg != null) { 32 if (now < msg.when) { 33 // Next message is not ready. Set a timeout to wake up when it is ready. 34 nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); 35 } else { 36 // Got a message. 37 mBlocked = false; 38 if (prevMsg != null) { 39 prevMsg.next = msg.next; 40 } else { 41 mMessages = msg.next; 42 } 43 msg.next = null; 44 if (DEBUG) Log.v(TAG, "Returning message: " + msg); 45 msg.markInUse(); 46 return msg; 47 } 48 } else { 49 // No more messages. 50 nextPollTimeoutMillis = -1; 51 } 52 53 // Process the quit message now that all pending messages have been handled. 54 if (mQuitting) { 55 dispose(); 56 return null; 57 } 58 59 // If first time idle, then get the number of idlers to run. 60 // Idle handles only run if the queue is empty or if the first message 61 // in the queue (possibly a barrier) is due to be handled in the future. 62 if (pendingIdleHandlerCount < 0 63 && (mMessages == null || now < mMessages.when)) { 64 pendingIdleHandlerCount = mIdleHandlers.size(); 65 } 66 if (pendingIdleHandlerCount <= 0) { 67 // No idle handlers to run. Loop and wait some more. 68 mBlocked = true; 69 continue; 70 } 71 72 if (mPendingIdleHandlers == null) { 73 mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)]; 74 } 75 mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers); 76 } 77 78 // Run the idle handlers. 79 // We only ever reach this code block during the first iteration. 80 for (int i = 0; i < pendingIdleHandlerCount; i++) { 81 final IdleHandler idler = mPendingIdleHandlers[i]; 82 mPendingIdleHandlers[i] = null; // release the reference to the handler 83 84 boolean keep = false; 85 try { 86 keep = idler.queueIdle(); 87 } catch (Throwable t) { 88 Log.wtf(TAG, "IdleHandler threw exception", t); 89 } 90 91 if (!keep) { 92 synchronized (this) { 93 mIdleHandlers.remove(idler); 94 } 95 } 96 } 97 98 // Reset the idle handler count to 0 so we do not run them again. 99 pendingIdleHandlerCount = 0; 100 101 // While calling an idle handler, a new message could have been delivered 102 // so go back and look again for a pending message without waiting. 103 nextPollTimeoutMillis = 0; 104 } 105 }
從24-30我們可以看到,他遍歷了整個queue找到msg,若是msg為null,我們可以看到50,他把nextPollTimeoutMillis = -1;實際上是等待enqueueMessage的nativeWake來喚醒。較深的源碼涉及了native層代碼,有興趣可以研究一下。簡單來說next()方法,在有消息是會返回這條消息,若沒有,則阻塞在這裡。
我們回到loop()方法27msg.target.dispatchMessage(msg);我們看代碼
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }
msg.target實際上就是發送這條消息的Handler,我們可以看到它將msg交給dispatchMessage(),最後調用了我們熟悉的方法handleMessage(msg);
三、總結
到目前為止,我們了解了android的消息機制流程,但它實際上還涉及了深層的native層方法,這裡有一篇博客專門講解這個轉載一下
http://www.cnblogs.com/angeldevil/p/3340644.html。
自定義控件之 圓形 / 圓角 ImageView,圓角imageview一、問題在哪裡? 問題來源於app開發中一個很常見的場景——用戶頭像要展示
《循序漸進Linux》第二版即將出版發行(附封面)從《循序漸進Linux》第一版發布,到現在已經近6年了,6年的時間,技術發生了很大的變化,Linux系統的內核版本從2.
計算機網絡和Internet之核心網絡,計算機網絡internet1.網狀的路由器 路由器的鏈接組成了網絡的核心。 任何一個路由器,或者某條線路掛掉,但是網絡還是通的,這
高靈活低耦合Adapter快速開發攻略,耦合adapter攻略Android開發中經常需要使用Adapter。 傳統方法是自定義一個Adapter並繼承AndroidSD