- 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~~
