用過 okhttp 網絡請求架構的人都知道,okhttp 的 callback 接口不在主線程。是以要想在 callback 中進行 UI上更新需要切換到主線程,可以handler.post(runable),那麼,handler 是怎麼直接就切換到主線程的呢?handler 是怎麼處理線程之間的消息傳遞呢?
本文主要解決的幾個問題
- handler 是怎樣傳遞消息的?
- handler.post(runable) 方法是怎麼直接切換到主線程的?
- Handler,Looper,MessageQueue,Message 有什麼關系?
相關類介紹
Message 消息對象
public int what;
public int arg1;
public int arg2;
public Object obj;
public Messenger replyTo;
private static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
private static boolean gCheckRecycle = true;
這是Message的一些屬性,建立一個消息執行個體一般有兩種:
// 第一種,直接建立一個對象
Message msg = new Message();
// 第二種,通過Message 的靜态方法擷取一個對象
Message msg = Message.obtain();
Message msg = Message.obtain((Message orig);
Message msg = Message.obtain(Handler h);
Message msg = Message.obtain(Handler h, Runnable callback);
Message msg = Message.obtain(Handler h, int what);
Message msg = Message.obtain(Handler h, int what, Object obj);
Message msg = Message.obtain(Handler h, int what, int arg1, int arg2);
Message msg = Message.obtain(Handler h, int what, int arg1, int arg2, Object obj);
這裡推薦用第二種。那麼兩種方式有什麼不一樣嗎?Message.obtain(…)方法最終都是調用的是Message.obtain(),是以我們看一下 obtain() 的源碼。
/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
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();
}
先看注釋:從全局池中傳回一個新的消息對象,允許我們在很多情況下避免配置設定新的對象。其實就是複用以前用過的對象,亦可以看出Message是一個連結清單結構,如果 sPool 為空就建立一個,反之就從連結清單中取。注意到 sPoolSize 減一,那麼最多可以擷取多少個Message執行個體呢?Message 有一個屬性 MAX_POOL_SIZE ,值為50,這個屬性中在回收Message時被用到。
/**
* Recycles a Message that may be in-use.
* Used internally by the MessageQueue and Looper when disposing of queued Messages.
*/
void recycleUnchecked() {
...
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
如果此時消息池中的可用message數量小于MAX_POOL_SIZE,則該message加傳入連結表尾部,同時可用數量加一,否則舍棄。是以最多可擷取50個Message執行個體。
Handler 消息處理類
消息實體類有了,發送消息就簡單了,大概分為兩種,一種handler.postXXX(Runable, xxx),一種handler.sendMessageXXX(message);
// 一種 post
public final boolean post(Runnable r) {
return sendMessageDelayed(getPostMessage(r), 0);
}
public final boolean postAtTime(Runnable r, long uptimeMillis){
return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}
public final boolean postAtTime(Runnable r, Object token, long uptimeMillis) {
return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}
public final boolean postDelayed(Runnable r, long delayMillis){
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
public final boolean postAtFrontOfQueue(Runnable r) {
return sendMessageAtFrontOfQueue(getPostMessage(r));
}
// 一種sendMessage
public final boolean sendMessage(Message msg) {
return sendMessageDelayed(msg, 0);
}
public final boolean sendEmptyMessage(int what){
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
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);
}
public final boolean sendMessageAtFrontOfQueue(Message msg) {
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, 0);
}
其實調到最後也是一種而已,因為 post 類最後也是把參數轉為Message,然後調用sendMessage()方法的,看源碼由 getPostMessage(Runable)這個方法。
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
而 sendMessageXXX()方法最後會調用 enqueueMessage 方法,是以我們重點看一下這個方法。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
可以看出 msg.target 是個 handler 類型的對象,把目前 handler 指派給它。最後調用 queue.enqueueMessage 方法。quene 是一個MessageQueue 類型的對象。MessageQueue,顧名思義就是消息隊列,我們通過 handler 發送的消息都會被放到這樣的一個隊列裡面。我們繼續看 enqueueMessage 的源碼。
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;
// 如果這是第一個消息,或者這個消息需要馬上執行,或者這個消息要比之前的消息先執行
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
// 把這個消息放在最前面
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;
}
前面做一些狀态判斷,不符合就抛出異常。後面把 mMessages 指派給 p。前面我們說過,Message 其實是一個連結清單結構。這裡後面的兩個if - else 裡面就是把發來的消息按延遲的時間排序。 if 裡面的條件 (p == null || when == 0 || when < p.when),如果這是第一個消息,或者這個消息需要馬上執行,或者發來的消息要比隊列裡第一個消息先執行,那麼把這個消息放在連結清單的第一個位置(原來是msg ,p,變成 msg —> p)。else 裡面有一個循環,先把 p 的頭部指派給 prev ,然後 p.next 指派給 p ,如果 p 為空或者目前消息比 p 先執行,那麼就跳出循環。此時的 p 是執行時間比 msg 要晚,而prev 又比目前 msg 執行時間早。然後做一個連結清單的插入操作。插入前順序為prev---->p,插入後順序為 prev ----> msg ----> p。
發來的消息順序也排好了,那麼什麼時候去處理消息呢?我們使用 handler 的時候一般會這樣使用:
private Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
if (msg.what == 1) {
Toast.makeText(MainActivity.this, "重新整理UI", Toast.LENGTH_SHORT).show();
}
}
};
重寫 handlerMessage 方法,去實作自己的操作。那麼什麼時候調用的這個方法呢?從 handler 類中有一個dispatchMessage的方法。
/**
* 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);
}
}
這個方法裡面最後調用了 handlerMessage 方法。又是誰調用這個方法呢?handler 類中并沒有什麼地方調用,一頓搜尋之後,發現是 Looper 裡面的 loop() 方法調用了該方法。但是我們平時使用handler 的時候并沒有接觸到這個類,是什麼地方使用的呢?這裡就要看 handler 的構造函數了。
public Handler() {
this(null, false);
}
public Handler(Callback callback) {
this(callback, false);
}
public Handler(Looper looper) {
this(looper, null, false);
}
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
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;
}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
大概分為兩類,參數傳 looper 和不傳 looper,最後都是得到一個 Looper 執行個體。不傳 looper 的最後會調用 Looper.myLooper() 去得到一個 looper,同時也得到一個 MessageQueue 。如果 looper 為空就會抛出異常,看這個異常描述是不是很熟悉?如果你在子線程中直接建立一個 handler 就會抛這個異常。為什麼在主線程中不會報異常呢?因為主線程啟動的時候,系統已經幫我們調用了這個方法,是以就不會報錯了。接下來我們看一下 Looper 的 loop() 方法。
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
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();
}
}
先看注釋:在目前線程運作消息隊列,確定使用{@link #quit()} 結束循環。是以如果我們是在主線程中執行個體化 handler,那麼這個消息隊列就運作在主線程中了,那麼用 handler.post() 就可以直接操作 UI了。
總結
總結一下:
- 在建立 handler 的時候,會在 handler 所在的線程中建立一個Looper 對象,同時Looper 也會建立一個 MessageQueue 對象。通過 Looper.loop() 開啟一個死循環,從MessageQueue 中不斷取出 Message,然後通過 handler 将消息分發傳回handler所在的線程。
- MessageQueue 名字叫做消息隊列,實質上是一個連結清單結構,傳來的消息都按照執行時間進行了排序。
另外,關于 handler 還有很多需要我們注意的地方,具體内容參考下一篇,handler 消息機制需要注意的地方有哪些?