[源码分析]Android消息机制之MessageQueue类

package android.os;
import java.util.ArrayList;
import android.util.AndroidRuntimeException;
import android.util.Config;
import android.util.Log;
import com.android.internal.os.RuntimeInit;
public class MessageQueue {
Message mMessages;//消息对象;
private final ArrayList mIdleHandlers = new ArrayList();//空闲队列;
private boolean mQuiting = false;//是否正在退出;
boolean mQuitAllowed = true;//是否允许退出;
public static interface IdleHandler {//接口;
boolean queueIdle();
}

//增加一个IdleHandler对象;
public final void addIdleHandler(IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}

//清楚一个IdleHandler对象;
public final void removeIdleHandler(IdleHandler handler) {
synchronized (this) {
mIdleHandlers.remove(handler);
}
}

MessageQueue() {
}

final Message next() {
boolean tryIdle = true;

while (true) {
long now;
Object[] idlers = null;
synchronized (this) {
now = SystemClock.uptimeMillis();
Message msg = pullNextLocked(now);
if (msg != null) return msg;
if (tryIdle && mIdleHandlers.size() > 0) {
idlers = mIdleHandlers.toArray();
}
}
//如果现在消息队列中没有消息,而且mIdleHandlers队列中有数据,通知mIdleHandlers里面变量的回调;
// There was no message so we are going to wait...  but first,
// if there are any idle handlers let them know.
boolean didIdle = false;
if (idlers != null) {
for (Object idler : idlers)
{
boolean keep = false;
try {
didIdle = true;
keep = ((IdleHandler)idler).queueIdle();
} catch (Throwable t) {
Log.wtf("MessageQueue", "IdleHandler threw exception", t);
}

if (!keep) {//如果返回false直接把这个从队列中去掉哦;
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
}
if (didIdle) {
tryIdle = false;
continue;
}

synchronized (this) {
try {
if (mMessages != null) {
if (mMessages.when-now > 0) {
Binder.flushPendingCommands();
this.wait(mMessages.when-now);
}
} else {
Binder.flushPendingCommands();
this.wait();
}
}
catch (InterruptedException e) {
}
}
}
}

final Message pullNextLocked(long now) {
Message msg = mMessages;
if (msg != null) {
if (now >= msg.when) {
mMessages = msg.next;
if (Config.LOGV) Log.v(
"MessageQueue", "Returning message: " + msg);
return msg;
}
}

return null;
}

/**
* 插入消息;
*/
final boolean enqueueMessage(Message msg, long when) {
if (msg.when != 0) {
throw new AndroidRuntimeException(msg
+ " This message is already in use.");
}
if (msg.target == null && !mQuitAllowed) {
throw new RuntimeException("Main thread not allowed to quit");
}
synchronized (this) {
if (mQuiting) {
//经常看到这条错误,意思就是handler此时已经为null了,也就是消息已经退出了哦;
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;
Message p = mMessages;
if (p == null || when == 0 || when < p.when) {
msg.next = p;
mMessages = msg;
this.notify();
} else {
Message prev = null;
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
msg.next = prev.next;
prev.next = msg;
this.notify();
}
}
return true;
}

//移除一条消息;
final boolean removeMessages(Handler h, int what, Object object,
boolean doRemove) {
synchronized (this) {
Message p = mMessages;
boolean found = false;
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;
}
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;
while (p != null && p.target == h && p.callback == r
&& (object == null || p.obj == object)) {
Message n = p.next;
mMessages = n;
p.recycle();
p = n;
}
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;
}
}
}

//删除所有消息包含handler h和Object object 的消息;
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;
}
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;
}
}
}

void poke()
{
synchronized (this) {
this.notify();
}
}
}