深入理解android Handler機制
通過原始碼理解android 中Handler,Message,Looper,MessageQueue之間的關係,之前說到handler機制,都是在網上看到得別人是怎麼講解的,自己就機械的記下來,回來忘了再到網上搜一堆資料看看,對自己本身能力沒有太大的提升。紙上得來終覺淺 絕知此事要躬行。開始試著自己閱讀原始碼,進行深入理解一下。
首先分別介紹一些handler,message,Looper,MessageQueue
handler 一般是我們進行非同步任務的完成時,通過handler通知主執行緒更新UI.
Message 是Handler傳送的訊息內容。
Looper 是一個輪詢器負責,負責輪詢MessageQueue裡面的訊息,然後將looper輪詢的訊息傳送到Handler裡面,一個Handler是對應一個Looper.
MessgeQueue 存放Message的訊息的連結串列,通過Handler.send。
下面通過原始碼介紹一下handler傳送和接收訊息的完整流程就明白他們之間的關係了。
1.建立一個Hanlder
Handler mHandler=new Handler()
我們呼叫的是一個無引數的構造方法,其實在原始碼裡無引數的構造方法呼叫了含有兩個引數的構造方法
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;
}
從這裡看出兩個引數的構造方法中通過 mLooper = Looper.myLooper()獲取了一個looper例項,又通過mQueue = mLooper.mQueue;獲得了一個MessageQueue例項。每個Handler都會有一個唯一對應的Looper和MessageQueue.
2.handler傳送訊息
一般我們會通過sendMessage(Message msg)傳送訊息,而sendMessage是直接呼叫的sendMessageDelayed
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
sendMessageAtTime方法又呼叫的sendMessageAtTime,在這裡就出現了我們建立handler時獲取的MessageQueue例項。
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);
}
下面就是把Message新增到MessageQueue裡面
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
msg.target = this;是將當前的Handler賦值給msg.targer,訊息傳送會用到。這就是一個完整的從訊息傳送到新增到MessageQueue過程。
有時候我們傳送的內容不是一個Message,例如使用的是sendEmptyMessage(int what),他會呼叫sendEmptyMessageDelayed 內部會幫我們建立一個Message物件。
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
- Looper是如何獲取MessageQueue裡面的訊息併發送給Handler的。
Looper在建立完成後就開始了一個無限for迴圈,不斷的從MessageQueue中取Message。
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.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();
}
}
如果獲取的Message為空就return 等待,如果不為空就msg.target.dispatchMessage(msg);將message分發出去,dispatchMessage()是handler中的方法,我們在傳送訊息的時候將handler賦值給了msg.target.
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
dispatchMessage方法會呼叫Callback中的方法handleMessage(msg),就是直接傳送給了我們建立的handler,也就是在handleMessage方法裡接收訊息
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
這個handler訊息傳送到接收的流程就完成了。
我們在理一下最後在理一下思路,
建立一個Handler會呼叫Handler兩個引數的構造方法,兩個引數的構造方法會通過mLooper = Looper.myLooper()獲取一個looper物件,然後通過mQueue = mLooper.mQueue;獲取一個MessageQueue物件,和一個介面回撥
mCallback = callback;這個介面就是定義的handlermessage方法。我們進行handler.sendMessage的時候會將Message新增到MessageQueue裡面,建立looper時,looper就開始了他的for迴圈,不斷的從MessageQueue中獲取方法,Message msg = queue.next()獲取訊息,如果獲取的訊息為null,就return,不為空,執行msg.target.dispatchMessage(msg);進行訊息的分發,也就是傳送到handleMessage方法裡面。Msg.target是一個Handler物件,因為我們在enqueueMessage裡msg.target = this;將當前的handler賦值給了msg.target.