Android基礎-Handler原始碼

private fun sendHandler(){
// 1
MyHandler().obtainMessage(1).sendToTarget()
// 2
val msg = Message()
msg.what = 2
MyHandler().sendMessage(msg)
// 3
MyHandler().post { Log.e(MainActivity::class.java.canonicalName, "3") }
// 4
val callback = Handler.Callback {
Log.e(MainActivity::class.java.canonicalName, it.what.toString())
true
}
val msg2 = Message()
msg2.what=4
MyHandler(callback).sendMessage(msg2)
}

// kotlin內部類預設為靜態類
private class MyHandler: Handler {
constructor()
constructor(callback: Callback)

override fun handleMessage(msg: Message?) {
super.handleMessage(msg)
Log.e(MainActivity::class.java.canonicalName, msg?.what.toString())
}
}

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());
}
}

// 使用當前執行緒的Looper物件
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
// 獲取looper持有的messageQueue
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}

// 使用提供的Looper物件
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.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();
}

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();
}

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);
}

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 (mQuittying) {
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();
// 延遲時間
msg.when = when;
// 佇列中上一條訊息
Message p = mMessages;
boolean needWake;
// 無延遲 或者 是第一條訊息// 或者需要比上一條訊息早傳送
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
// 將當前訊息置於列表頂
// 當前訊息的下一條訊息指向之前的mMessages
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.
// 中間插入當前的訊息物件
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.
// 執行緒喚醒
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}

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.
// 校驗當前執行緒在當前程序內
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();

for (;;) {
// 1. 獲取message
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}

// 2. handler的分發處理
msg.target.dispatchMessage(msg);
...

// 3. 訊息的回收
msg.recycleUnchecked();
}
}

Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}

int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}

nativePollOnce(ptr, nextPollTimeoutMillis);

synchronized (this) {
// Try to retrieve the next message.Return if found.
// 獲取當前啟動時間
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
// 當頭部訊息的target(攜帶handler)為空的情況下
if (msg != null && msg.target == null) {
// Stalled by a barrier.Find the next asynchronous message in the queue.
// 遍歷佇列, 直到獲取到的訊息不為空並且是同步資訊
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
// 延遲訊息
if (now < msg.when) {
// Next message is not ready.Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
// 指向下一條message
// 該條訊息需要被執行的時候
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
// 返回當前的message
return msg;
}
} else {
// No more messages.
// 如果當前佇列頭部沒有訊息, 則說明佇列為空
nextPollTimeoutMillis = -1;
}

// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}

// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
// 當佇列空閒(頭部訊息為空或者頭部訊息還未到時候執行), 並且沒有閒置handler的時候
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run.Loop and wait some more.
// 當沒有閒置handler需要執行的時候
mBlocked = true;
continue;
}

// 如果閒置handlers陣列為空
if (mPendingIdleHandlers == null) {
// 初始化, 閒置的handlers最大數為4
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}

// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
// 引用釋放, 則這個迴圈只會執行mPendingIdleHandlers集合的第一個handler元素
mPendingIdleHandlers[i] = null; // release the reference to the handler

boolean keep = false;
try {
// 執行空閒回撥處理
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}

// 假設空閒回撥處理不保留這個閒置處理, 則移除對應的idleHandler
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}

// Reset the idle handler count to 0 so we do not run them again.
// 重置
pendingIdleHandlerCount = 0;

// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
// 重置
nextPollTimeoutMillis = 0;
}
}

public void dispatchMessage(Message msg) {
// msg.callback通過handler.post傳入的runnable
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
// mCallback是通過Handler的建構函式傳入
if (mCallback.handleMessage(msg)) {
return;
}
}
// 最後才是Message傳入的處理, 一般是繼承Handler, 重寫handleMessage方法
handleMessage(msg);
}
}

public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}

void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;

synchronized (sPoolSync) {
// MAX_POOL_SIZE == 50
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}