記得之前面試時，面試官就問了Handler的工作流程，當時腦子里有MessageQueue、Looper等類名但具體怎么執(zhí)行的卻說不明白。于是乎面試也就失敗了。想想還是挺菜雞的。。以一個簡單的例子通過源碼來走一遍Handler的執(zhí)行流程。
Handler handler=new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
return false;
}
});
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test);
Message message=Message.obtain();
message.arg1=1;
handler.sendMessage(message);
}
1.MessageQueue
就是這么一個最簡單的例子先進入sendMessage方法,然后又跳到了sendMessageDelayed方法，最后跳到了sendMessageAtTime方法，在sendMessageAtTime方法中看到了熟悉的面孔MessageQueue，初始化了一下MessageQueue，之后進入了enqueueMessage方法。
public final boolean sendMessage(Message msg) {
return sendMessageDelayed(msg, 0);
}
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);
}
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);
}
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
在enqueueMessage方法中將msg.target設置為當前的Handler，也就是綁定了Handler，然后進入enqueueMessage中，通過模擬代碼執(zhí)行，有了一個重要的結論，我們每發(fā)送一個消息都被保存到了 MessageQueue 消息隊列中，消息隊列中采用的是單鏈表的方式。
// 發(fā)送 Message1
Message message1 = new Message();
mHandler.sendMessageDelayed(message1, 500);
// 發(fā)送 Message2
Message message2 = new Message();
mHandler.sendMessage(message2);
// 發(fā)送 Message3
Message message3 = new Message();
mHandler.sendMessageDelayed(message3, 1000);
主要看這段來模擬：
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
模擬如下：
message1.when=500；
Message p = mMessages;
而mMessages=null； 所以Message p =null；
因為p =null，所以進入if
message1.next = null;
mMessages = message1;
發(fā)送完message1
然后發(fā)送message2：
message2.when=0；
Message p = mMessages; //在發(fā)送message1是已經(jīng)將mMessages賦值為 message1
所以
p=message1；
if (p == null || when == 0 || when < p.when)
因為when=0，所以進入if
message2.next=p；即
message2.next=message1
mMessages =message2;
發(fā)送完message2
然后發(fā)送message2：
message3.when=1000；
Message p =message2;
if (p == null || when == 0 || when < p.when) 不滿足if條件，進入else
Message prev;
進入for（;;）循環(huán)
prev = message2;
p = p.next;即
p=message1;
if (p == null || when < p.when) // 不滿足if繼續(xù)循環(huán)
prev = p;
即
prev = message1;
p = p.next;即
p=null;
if (p == null || when < p.when) // 滿足條件進入if 跳出
msg.next = p;即
message3.next=null；
prev.next = msg; 即
message1.next=message3
所以發(fā)送完message3之后應該是這樣的：
發(fā)送完message3
得出結論消息在隊列中是按when和先后順序排列的。
2. Loop 消息循環(huán)
我們始終沒有看到 Handler 調(diào)用 handleMessage() 方法，到底什么時候會執(zhí)行這個方法?
書上說在子線程中使用handler要先
Looper.prepare();
Handler handler = new Handler();
Looper.loop();
否則會報錯，而在MainActivity即主線程中直接使用不會報錯。原因就是在啟動Activity過程中在ActivityThread幫我們已經(jīng)調(diào)用過了代碼如下：
ActivityThread.java main()
public static void main(String[] args) {
....
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
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));
}
public void set(T value) {
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values == null) {
values = initializeValues(currentThread);
}
values.put(this, value);
}
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();
}
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
在prepare(false)方法中創(chuàng)建一個Looper對象并把它存儲到sThreadLocal中并且與當前線程綁定。
當調(diào)用 loop()方法再在從sThreadLocal中取出Looper對象。ThreadLocal用來保證一個線程只有一個 Looper 對象，這樣就保證了線程的安全。接下來是一個for(;;)死循環(huán)調(diào)用隊列中msg.target.dispatchMessage。這樣就找到了在handleMessage中可以收到信息的原因。message的target是在加入隊列時設置的。
/**
* 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);
}
}
總結：Looper.prepareMainLooper() 創(chuàng)建了一個 Looper 對象，而且保證一個線程只有一個 Looper；Looper.loop() 里面是一個死循環(huán)，不斷的從 消息隊列 MessageQueue 中取消息，然后通過 Handler 執(zhí)行。
