Message

• Messeage 是安卓消息机制中，消息的实体，表示需要处理的消息内容；

本文介绍两部分：
1. 消息的构成
2.消息的缓冲池

---

首先看下类的注释文档：

 * Defines a message containing a description and arbitrary data object that can be
 * sent to a {@link Handler}.  This object contains two extra int fields and an
 * extra object field that allow you to not do allocations in many cases.
 *
 * <p class="note">While the constructor of Message is public, the best way to get
 * one of these is to call {@link #obtain Message.obtain()} or one of the
 * {@link Handler#obtainMessage Handler.obtainMessage()} methods, which will pull
 * them from a pool of recycled objects.</p>

Message是发送到handler的消息对象，包括两个int变量和一个对象等
建议获取Message时采用缓冲池的方法，即Message.obtain() or Handler.obtainMessage()

• Message的构成
  消息标识： （每个handler的命名空间独立，无需担心冲突问题）

public int what;

存储int类型

public int arg1;
public int arg2;

存储对象

public Object obj;

存储数据

Bundle data
标明指定处理的handler
Handler target

runnable

Runnable callback;

---

• 缓存池

/package/ Message next;
private static final Object sPoolSync = new Object();
private static Message sPool; // 当前缓冲池的头结点
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
private static boolean gCheckRecycle = true;

采取链表的方式获取到当前可用缓冲池的头结点，若缓冲池为空，则新建一个(注意新建的对象怎么加入缓冲池？后续会有)

/**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    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();
    }

回收消息到缓冲池

    /**
     * Return a Message instance to the global pool.
     * <p>
     * You MUST NOT touch the Message after calling this function because it has
     * effectively been freed.  It is an error to recycle a message that is currently
     * enqueued or that is in the process of being delivered to a Handler.
     * </p>
     */
    public void recycle() {
        if (isInUse()) {
            if (gCheckRecycle) {
                throw new IllegalStateException("This message cannot be recycled because it "
                        + "is still in use.");
            }
            return;
        }
        recycleUnchecked();
    }

头插法插入结点

/**
     * Recycles a Message that may be in-use.
     * Used internally by the MessageQueue and Looper when disposing of queued Messages.
     */
    void recycleUnchecked() {
        // Mark the message as in use while it remains in the recycled object pool.
        // Clear out all other details.
        flags = FLAG_IN_USE;
        what = 0;
        arg1 = 0;
        arg2 = 0;
        obj = null;
        replyTo = null;
        sendingUid = -1;
        when = 0;
        target = null;
        callback = null;
        data = null;
        //生成一个缓冲池，大小为MAX_POOL_SIZE
        synchronized (sPoolSync) {
            if (sPoolSize < MAX_POOL_SIZE) {
                next = sPool;
                sPool = this;
                sPoolSize++;
            }
        }
    }

最后呢：看下Handler.obtainMessage怎么实现的

/**
     * Returns a new {@link android.os.Message Message} from the global message pool. More efficient than
     * creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this).
     *  If you don't want that facility, just call Message.obtain() instead.
     */
    public final Message obtainMessage()
    {
        return Message.obtain(this);
    }

/**
     * Same as {@link #obtain()}, but sets the value for the <em>target</em> member on the Message returned.
     * @param h  Handler to assign to the returned Message object's <em>target</em> member.
     * @return A Message object from the global pool.
     */
    public static Message obtain(Handler h) {
        Message m = obtain();
        m.target = h;

        return m;
    }

可见和Message.obtain()一样的，难怪建议直接用obtain获取~~

转载于:https://www.jianshu.com/p/6aa968e49ab9