android開發經驗談：你到底有多瞭解handler
摘要：
記早年的一次面試：
面試官：說說你對Handler的瞭解？
我：new 一個handler，然後在某個執行緒裡傳送message，在handler的handleMessage裡接收message，message不要new 用Message.obtain()獲取池子裡的，來避免新建。
...
記早年的一次面試：
面試官：說說你對Handler的瞭解？
我：new 一個handler，然後在某個執行緒裡傳送message，在handler的handleMessage裡接收message，message不要new 用Message.obtain()獲取池子裡的，來避免新建。
面試官：就這麼多了麼？
我：。。。。。。。。。。。。。。。。。
以前我對handler的理解很薄，在開發多年，深入瞭解多年後，細細的瞭解了handler
一、Looper
looper是關聯message與handler的重要部分。
你在Android的子執行緒中new handler為啥報錯，和looper關係不淺，據如下個例子，在子執行緒中呼叫handler
new Thread(new Runnable() {
public void run() {
Looper.prepare();
Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
Toast.makeText(getApplicationContext(), "handler msg", Toast.LENGTH_LONG).show();
}
};
handler.sendEmptyMessage(1);
Looper.loop();
};
}).start();
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或者在子執行緒直接使用Looper.getMainLooper()；
new Thread(new Runnable() {
public void run() {
Handler handler = new Handler(Looper.getMainLooper()){
@Override
public void handleMessage(Message msg) {
Toast.makeText(getApplicationContext(), "handler msg", Toast.LENGTH_LONG).show();
}
};
handler.sendEmptyMessage(1);
};
}).start();
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Looper.getMainLooper()實際就是主執行緒的looper，主執行緒預設都會建立looper
建立looper時會建立messagequeue
注：每一個執行緒只能有一個looper，looper是執行緒持有
Looper實際上是給當前子執行緒建立的looper，為了能夠遍歷當前子執行緒的訊息佇列，
原始碼
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();
}
}
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二、MessageQueue
MessageQueue是一個訊息佇列，用於存放message，looper來把其中的message傳送到handlermessage
採用先進先出的方式來管理message，內部採用單鏈表來進行維護，插入刪除比較快
三、Message
就是咱最常用的訊息了，按照我上面的說法建議使用Message.obtain()來獲取Message例項，應為message有個pool
原始碼
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();
}
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