Android 消息处理源代码分析（1）

在Android中，通常被使用的消息队列的代码在文件夹\sources\android-22\android\os下，涉及到下面几个类文件

Handler.java

Looper.java

Message.java

MessageQueue.java

Message.java
public final class Message implements Parcelable {

    public int what;    //消息种类

    public int arg1;    //低开销的整型參数

    public int arg2;

    public Object obj;  //Object型数据

    public Messenger replyTo;  //消息处理完后通知给发送者

    /*package*/ int flags;   //消息标记：正在使用和异步等

    /*package*/ long when;   //消息创建时的时间
    
    /*package*/ Bundle data; //消息附带的额外数据
    
    /*package*/ Handler target; //消息接受者，处理者
    
    /*package*/ Runnable callback; //优先使用回调处理来处理消息
    
    /*package*/ Message next;   //下一个消息。形成链表

    private static Message sPool;    //消息池中的头消息
    

上面中的target,通常由又一次实现的Handler子类的handleMessage函数来处理消息

 public static Message obtain() {     //获取消息的函数，假设有消息的话则获取出来m，链表指针移动一位，否则则返回一条空消息
        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();
    }


 public void sendToTarget() {    //发送消息给处理者
        target.sendMessage(this);    //调用Handler.java中的函数
    }

}

MessageQueue.java
public final class MessageQueue {
	
	Message mMessages;    //当前要处理的消息
	
	//当须要从链表中获取一个消息时。就会调用next函数，若消息队列中没有消息，则会堵塞等待，通过调用nativePollOnce函数来完毕
	Message next() {...}
	
	boolean enqueueMessage(Message msg, long when) {     //按时间顺序加入消息
        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("MessageQueue", e.getMessage(), e);
                msg.recycle();
                return false;
            }

            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                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);  //调用底层唤醒函数，管道唤醒
            }
        }
        return true;
    }