Android非同步訊息處理機制:Looper、Handler、Message
1 簡介
Handler,Looper,Message這三者都與Android非同步訊息處理執行緒相關,
Looper:負責建立一個MessageQueue,然後進入一個無限迴圈體不斷從該MessageQueue中讀取訊息;
Handler:訊息建立者,一個或者多個
2 原始碼解析
2.1 Looper
Looper主要方法就是:prepare()和loop()兩個方法:
- prepare()方法
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)); }
sThreadLocal是一個ThreadLocal物件,可以在一個執行緒中儲存變數,通過上述程式碼知道,一個Looper例項被放入了ThreadLocal,並且判斷sThreadLocal是否為null,否則丟擲異常,說明Looper.prepare()方法不能被呼叫兩次,同時也保證了一個執行緒中只有一個Looper例項。
下面看Looper的構造方法
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
在訊息佇列中,建立MessageQueue(訊息佇列)
- loop()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 traceTag = me.mTraceTag; if (traceTag != 0 && Trace.isTagEnabled(traceTag)) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } try { msg.target.dispatchMessage(msg); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } 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(); } }
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}如何me == null,則直接丟擲異常,也就是說looper方法必須在prepare方法之後執行
第14行,取出一條訊息,如果沒有訊息則阻塞
第32行,msg.target.dispatchMessage(msg); 把訊息交給msg的target的dispatchMessage方法處理,Msg的target是什麼?其實就是Handler物件
第54行,釋放訊息佔用的資源
Looper主要作用
1.與當前執行緒繫結,保證一個執行緒只有一個Looper例項,同時一個Looper例項只有一個MessageQueue;
2.loop()方法,不斷從MessageQueue中去取訊息,交給訊息target屬性的dispatchMessage去處理;
2.2 Handler
首先看Handler構造方法,看其如何與MessageQueue聯絡的,他在子執行緒中傳送的訊息(一般傳送訊息都在非UI執行緒),如何傳送到MessageQueue中去的。
public Handler(boolean async) {
this(null, async);
}
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;
}第15行:通過Looper.myLooper()獲取當前執行緒儲存的Looper例項;
第20行:通過Looper例項中儲存的MessageQueue(訊息佇列),這樣就保證了handler的例項與我們Looper例項中MessageQueue關聯上。
sendMessage方法
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}sendEmptyMessageDelayed方法
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);
}多次跳轉之後到sendMessageAtTime方法,此方法中呼叫了enqueueMessage方法
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}enqueueMessage中首先為msg.target賦值this,[Looper的loop方法取出每個msg,然後交給msg.target.dispatchMessage(msg)去處理訊息],也就是爸當前的handler作為msg的target屬性。最終呼叫queue.enqueueMessage的方法,說明handler發出的訊息,最終儲存到訊息佇列中去了。
過程:Looper呼叫prepare()和loop()方法,在當前執行的執行緒中儲存一個Looper例項,這個例項儲存一個MessageQueue物件,然後當前執行緒進入一個無限迴圈中去,不斷從MessageQueue中讀取Handler發來的訊息,然後在回撥建立這個訊息的handler中的dispatchMessage方法
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}第10行:呼叫handleMessage(msg),這個方法空實現:
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}可以看到這是一個空方法，因為訊息的最終回撥是由我們控制的，我們在建立handler的時候都是複寫handleMessage方法，然後根據msg.what進行訊息處理。
總結:
1.首先Looper.prepare()在本執行緒中儲存一個Looper例項,然後該例項中儲存一個MessageQueue物件;因為Looper.prepare(0在一個執行緒中只能呼叫一次,所以MessageQueue在一個執行緒中只會存在一個;
2.Looper.loop()會讓當前執行緒進入一個無限迴圈,不斷從MessageQueue的例項中讀取訊息,然後回撥msg.target.dispatchMessage(msg)方法;
3.Handler的構造方法,會首先得到當前執行緒中儲存的Looper例項,進而與Looper例項中的MessageQueue相關聯;
4.Handler的sendMessage方法,會給msg的target賦值為handler自身,然後加入MessageQueue;
5.在構造Handler例項時,我們會重寫handleMessage方法,也就是msg.target.dispatchMessage(msg)最終呼叫的方法.
2.3 Handler post
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}可以看到在getPostMessage中,得到了一個Message物件,然後將我們建立的Runnable物件作為callback屬性,賦值給message
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);
}最終和handler.sendMessage一樣,呼叫了sendMessageAtTime,然後呼叫了enqueueMessage方法,給msg.target賦值為handler,最終加入MessageQueue.