Handler原始碼簡析

連結串列訊息系統安卓開發 · 發表 2018-09-15 22:30:50

摘要：訊息------Message 訊息的複用：Message.obtain() public static Message obtain() { synchronized (sPo...

訊息------Message

訊息的複用：Message.obtain()

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();
}

從Message 連結串列的頭節點-----sPool中獲取第一個訊息，然後更新頭節點
清空next資訊
重置in-use標誌位
更新訊息池size

訊息的回收：Message.recyclerUnchecked()

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

標識訊息used；
重置引數
加入到訊息池

訊息佇列-------MessageQueue

關鍵：使用連結串列儲存MessageQueue；使用一個阻塞標記標示是否列表是否為空；記錄next barrier的token

.....
Message mMessages;//連結串列頭

// Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout.
private boolean mBlocked; //阻塞標示

// The next barrier token.
// Barriers are indicated by messages with a null target whose arg1 field carries the token.
private int mNextBarrierToken; //分隔欄tekon
.....

>barrier是一個target為null的Message，使用postSyncBarrier(SystemClock.uptimeMillis())方法加入MessageQueue中，它的arg1引數儲存著token。
>
>當從MessageQueue中取出訊息時，遇到barrier會停止取出barrier後面的同步訊息，稱為“同步訊息分隔欄”。

訊息入隊：enqueueMessage()

boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {//訊息必須有一個處理它的目標Handler
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {//檢查"使用中"標記
throw new IllegalStateException(msg + " This message is already in use.");
}

synchronized (this) {
if (mQuitting) {//檢查訊息是否重複入隊，是則丟擲異常並回收該訊息
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();//訊息入隊後標記為--used
msg.when = when;
Message p = mMessages;
boolean needWake;
//新增訊息到連結串列中
if (p == null || when == 0 || when < p.when) {
//之前是空連結串列的時候讀取訊息會阻塞，新新增訊息後喚醒
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
//插入訊息到佇列時，若訊息佇列阻塞、佇列頭是barrier並且當前訊息是非同步的時，需要喚醒佇列
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;
}

訊息出隊：MessageQueue.next()

Message next() {
//如果訊息的 looper 退出，就退出這個方法
final long ptr = mPtr;
if (ptr == 0) {
return null;
}

int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
//迴圈取出訊息，沒有就阻塞
for (;;) {
if (nextPollTimeoutMillis != 0) { 
Binder.flushPendingCommands();
}

nativePollOnce(ptr, nextPollTimeoutMillis);

synchronized (this) {
//獲取下一個訊息
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;//當前的連結串列頭
if (msg != null && msg.target == null) {
//如果訊息沒有target---是barrier，向後遍歷找到第一個非同步的訊息
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {//如果這個訊息還沒到處理時間，就設定個時間過段時間再處理
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// 訊息可以立即處理
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();//標記這個訊息在被使用
return msg;
}
} else {
// 訊息連結串列裡沒有訊息了
nextPollTimeoutMillis = -1;
}

//如果收到退出的訊息，並且所有等待處理的訊息都處理完時，呼叫 Native 方法銷燬佇列
if (mQuitting) {
dispose();
return null;
}

if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
mBlocked = true;
continue;
}

if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}

for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler

boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}

if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}

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

mIdleHandlers：監聽MessageQueue阻塞的介面List，MessageQueue在阻塞時會遍歷並呼叫idler.queueIdle()方法，該方法返回false則會在一次呼叫後移除該idler。

訊息排程器----Looper

屬性：

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;//主執行緒的 Looepr

final MessageQueue mQueue;//與之管理的訊息佇列
final Thread mThread;//所在的執行緒

private Printer mLogging;
private long mTraceTag;

/* If set, the looper will show a warning log if a message dispatch takes longer than time. */
private long mSlowDispatchThresholdMs;

Looper初始化Looper.prepare() :

使用ThreadLocal保證每個執行緒各自擁有自己一個的Looper。

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));
}

迴圈處理訊息：Looper.loop()

public static void loop() {
final Looper me = myLooper();
if (me == null) {//當前執行緒必須建立 Looper 才可以執行
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;

Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();

for (;;) {//無限迴圈
Message msg = queue.next(); //從訊息佇列中讀取訊息，可能會阻塞
if (msg == null) {//當訊息佇列中沒有訊息時就會返回，不過這隻發生在 queue 退出的時候
return;
}

//...
try {
msg.target.dispatchMessage(msg);//呼叫訊息關聯的 Handler 處理訊息
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
//...
msg.recycleUnchecked();//標記這個訊息被回收
}
}

無限迴圈取出Message，呼叫對應Handler的dispatchMessage()方法處理訊息
標記訊息被回收

Looper.quit()方法退出迴圈，其呼叫的是MessageQueue.quit()方法。

void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}

synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;

if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}

// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}

判斷並更新標識位；
removeAllMessagesLocked();

立即把訊息連結串列中的所有訊息全部回收

- 在停止後如果 Handler 還發送訊息，會返回 false，表示入隊失敗
 - 這個方法是不安全的，一般建議使用下面那個

removeAllFutureMessagesLocked();

- 只會將還未執行的訊息回收掉
 - 在呼叫之後新增進入的訊息不會被處理，Handler.sendMessage 也會返回 false

Handler

主要屬性：

final Looper mLooper;
final MessageQueue mQueue;
final Callback mCallback;//構造Handler時可傳的自定義處理方法
final boolean mAsynchronous;
IMessenger mMessenger;`

訊息處理方法：dispatchMessage(Message msg)

public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

如果訊息定義了callback，則呼叫handleCallback(msg)，該方法是呼叫Message自身的callback方法；
```
private static void handleCallback(Message message) {
message.callback.run();
}
```
訊息的callback為null，呼叫mCallback的handleMessage(msg)方法，如果mCallback.handleMessage(msg)返回true，直接返回，不再呼叫handleMessage(msg)；

mCallback是構造Handler時可傳的自定義處理方法，其內部定義了一個handleMessage(Message msg)方法；
最後呼叫handleMessage(msg)方法。

傳送訊息

Message-----sendMessage(Message msg)
Runnable------post(Runnable r)

post方法會將Runnable包裝為Message，然後呼叫sendMessageDelayed()方法；

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;
}

sendMessage(Message msg)最後還是呼叫了訊息佇列的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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}

移除訊息，呼叫mQueue.removeMessages方法，從佇列中移除訊息。

public final void removeCallbacks(Runnable r){
mQueue.removeMessages(this, r, null);
}
public final void removeMessages(int what) {
mQueue.removeMessages(this, what, null);
}

引數r與Message.callback 比較

Handler原始碼簡析

訊息------Message

訊息佇列-------MessageQueue

訊息排程器----Looper

Handler

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