关于MessageQuene, 关于IdealHandler
最近在看android message处理机制。 以前看到有
Looper.myQueue().addIdleHandler(new IdleHandler() {
@Override
public boolean queueIdle() {}
}
这样的代码一直不懂，其原理是什么，只是从ADK中了解到其作用：
向消息队列中添加一个新的MessageQueue.IdleHandler。当调用IdleHandler.queueIdle()返回false时，此MessageQueue.IdleHandler会自动的从消息队列中移除。或者调用removeIdleHandler(MessageQueue.IdleHandler)也可以从消息队列中移除MessageQueue.IdleHandler。
此方法是线程安全的。
看到其他网友的使用心得： 这个一般用在activty中用，若想在activty界面显示后做一些事情,就在这个idlehandler中做。
比较常见的情况是，在有控制条的界面，在queueIdle中添加popupwindow,弹出控制条。
最近看到messageQuene的源码，才恍然大悟。贴出部分有用的源码（位于Android.Os.MessageQuene）：
这个next()函数的调用者是Looper.loop() -> Message.target.dispatchMessage() ; 具体课参考google源码
/*
* Copyright (C) 2006 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.os;
import android.util.AndroidRuntimeException;
import android.util.Log;
import java.util.ArrayList;
/**
* Low-level class holding the list of messages to be dispatched by a
* {@link Looper}. Messages are not added directly to a MessageQueue,
* but rather through {@link Handler} objects associated with the Looper.
*
* You can retrieve the MessageQueue for the current thread with
* {@link Looper#myQueue() Looper.myQueue()}.
*/
public class MessageQueue {
Message mMessages;
private final ArrayList mIdleHandlers = new ArrayList();
private IdleHandler[] mPendingIdleHandlers;
private boolean mQuiting;
boolean mQuitAllowed = true;
// Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout.
private boolean mBlocked;
@SuppressWarnings("unused")
private int mPtr; // used by native code
private native void nativeInit();
private native void nativeDestroy();
private native void nativePollOnce(int ptr, int timeoutMillis);
private native void nativeWake(int ptr);
/**
* Callback interface for discovering when a thread is going to block
* waiting for more messages.
*/
public static interface IdleHandler {
/**
* Called when the message queue has run out of messages and will now
* wait for more. Return true to keep your idle handler active, false
* to have it removed. This may be called if there are still messages
* pending in the queue, but they are all scheduled to be dispatched
* after the current time.
*/
boolean queueIdle();
}
/**
* Add a new {@link IdleHandler} to this message queue. This may be
* removed automatically for you by returning false from
* {@link IdleHandler#queueIdle IdleHandler.queueIdle()} when it is
* invoked, or explicitly removing it with {@link #removeIdleHandler}.
*
* This method is safe to call from any thread.
*
* @param handler The IdleHandler to be added.
*/
public final void addIdleHandler(IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}
/**
* Remove an {@link IdleHandler} from the queue that was previously added
* with {@link #addIdleHandler}. If the given object is not currently
* in the idle list, nothing is done.
*
* @param handler The IdleHandler to be removed.
*/
public final void removeIdleHandler(IdleHandler handler) {
synchronized (this) {
mIdleHandlers.remove(handler);
}
}
MessageQueue() {
nativeInit();
}
@Override
protected void finalize() throws Throwable {
try {
nativeDestroy();
} finally {
super.finalize();
}
}
//关键函数 next();
final Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(mPtr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
final Message msg = mMessages;
if (msg != null) {
final long when = msg.when;
if (now >= when) {
mBlocked = false;
mMessages = msg.next;
msg.next = null;
if (false) Log.v("MessageQueue", "Returning message: " + msg);
msg.markInUse();
return msg;
} else {
nextPollTimeoutMillis = (int) Math.min(when - now, Integer.MAX_VALUE);
}
} else {
nextPollTimeoutMillis = -1;
}
// If first time, then get the number of idlers to run.
//pendingIdleHandlerCount 的初始值是 -1
// 关键部分，当MessageQuene为空时 或者 MessageQuene中的 msg 没到执行时间，会走到这一步，并执行mIdleHandlers中的内容。这也正是其名字的由来，空闲时间执行的handler.
if (pendingIdleHandlerCount < 0) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount == 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
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];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf("MessageQueue", "IdleHandler threw exception", t);
}
if (!keep) {//当queueIdle()的返回值是false的时候，会将这个queueIdle从queue中删除
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.//只需在程序第一次运行的时候调用，调用一次后永不再调用
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;
}
}
final boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg
+ " This message is already in use.");
}
if (msg.target == null && !mQuitAllowed) {
throw new RuntimeException("Main thread not allowed to quit");
}
final boolean needWake;
synchronized (this) {
if (mQuiting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
} else if (msg.target == null) {
mQuiting = true;
}
msg.when = when;
//Log.d("MessageQueue", "Enqueing: " + msg);
Message p = mMessages;
if (p == null || when == 0 || when < p.when) {
msg.next = p;
mMessages = msg;
needWake = mBlocked; // new head, might need to wake up
} else {
Message prev = null;
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
msg.next = prev.next;
prev.next = msg;
needWake = false; // still waiting on head, no need to wake up
}
}
if (needWake) {
nativeWake(mPtr);
}
return true;
}
final boolean removeMessages(Handler h, int what, Object object,
boolean doRemove) {
synchronized (this) {
Message p = mMessages;
boolean found = false;
// Remove all messages at front.
while (p != null && p.target == h && p.what == what
&& (object == null || p.obj == object)) {
if (!doRemove) return true;
found = true;
Message n = p.next;
mMessages = n;
p.recycle();
p = n;
}
// Remove all messages after front.
while (p != null) {
Message n = p.next;
if (n != null) {
if (n.target == h && n.what == what
&& (object == null || n.obj == object)) {
if (!doRemove) return true;
found = true;
Message nn = n.next;
n.recycle();
p.next = nn;
continue;
}
}
p = n;
}
return found;
}
}
final void removeMessages(Handler h, Runnable r, Object object) {
if (r == null) {
return;
}
synchronized (this) {
Message p = mMessages;
// Remove all messages at front.
while (p != null && p.target == h && p.callback == r
&& (object == null || p.obj == object)) {
Message n = p.next;
mMessages = n;
p.recycle();
p = n;
}
// Remove all messages after front.
while (p != null) {
Message n = p.next;
if (n != null) {
if (n.target == h && n.callback == r
&& (object == null || n.obj == object)) {
Message nn = n.next;
n.recycle();
p.next = nn;
continue;
}
}
p = n;
}
}
}
final void removeCallbacksAndMessages(Handler h, Object object) {
synchronized (this) {
Message p = mMessages;
// Remove all messages at front.
while (p != null && p.target == h
&& (object == null || p.obj == object)) {
Message n = p.next;
mMessages = n;
p.recycle();
p = n;
}
// Remove all messages after front.
while (p != null) {
Message n = p.next;
if (n != null) {
if (n.target == h && (object == null || n.obj == object)) {
Message nn = n.next;
n.recycle();
p.next = nn;
continue;
}
}
p = n;
}
}
}
/*
private void dumpQueue_l()
{
Message p = mMessages;
System.out.println(this + " queue is:");
while (p != null) {
System.out.println(" " + p);
p = p.next;
}
}
*/
}