Android消息处理机制3——MessageQueue
16lz
2021-01-23
Android消息处理机制系列文章整体内容如下
Android消息处理机制1——Handler
Android消息处理机制2——Message
Android消息处理机制3——MessageQueue
Android消息处理机制4——Looper
MessageQueue比较重要的概念是管理消息队列,它的数据结构是一个单向链表。
管理消息队列
一 插入消息
通过boolean enqueueMessage(Message msg, long when)来插入消息
boolean enqueueMessage(Message msg, long when) { //省略代码 synchronized (this) { //省略代码 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; //新的message指向原来的链表 mMessages = msg; //含有新的message的单向链表,完成单向链表的插入操作 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; //指向的message if (p == null || when < p.when) { //p是尾部的message或者要插入的message的when小于指向的message的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); } } return true; }
有两种插入message的方式
- 从头部插入的条件:
- 当前的消息链表为空
- 当前的消息链表不为空 并且 要插入的message的when属性为0
- 当前的消息链表不为空 并且 要插入的message的when属性不为0 并且 处理message的时间小于当前链表头部message的时间
- 按照处理时间插入
- 不满足从头部插入的条件
二 取消息
只保留取数据的代码
Message next() { // 省略代码 for (;;) { synchronized (this) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); Message prevMsg = null; Message msg = mMessages; if (msg != null) { if (now < msg.when) { // Next message is not ready. Set a timeout to wake up when it is ready. nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); } else { //从链表中取出msg的when时间大于now的message mBlocked = false; if (prevMsg != null) { prevMsg.next = msg.next; } else { mMessages = msg.next; //将从第二个message开始的链表放入mMessages } msg.next = null; //断掉原来链表中的第一个message和其他message之间的联系,等于将其从链表中拿出来 if (DEBUG) Log.v(TAG, "Returning message: " + msg); msg.markInUse(); //取出一个messag然后退出死循环 return msg; } } else { // No more messages. nextPollTimeoutMillis = -1; } // Process the quit message now that all pending messages have been handled. if (mQuitting) { dispose(); return null; } } } }
三 删除消息
- 根据what删除message
void removeMessages(Handler h, int what, Object object) { if (h == null) { return; } synchronized (this) { Message p = mMessages; // 根据what删除头部message while (p != null && p.target == h && p.what == what && (object == null || p.obj == object)) { Message n = p.next; mMessages = n; p.recycleUnchecked(); p = n; } // 根据what删除其他位置的message while (p != null) { Message n = p.next; if (n != null) { if (n.target == h && n.what == what && (object == null || n.obj == object)) { Message nn = n.next; n.recycleUnchecked(); p.next = nn; continue; } } p = n; } } }
- 根据runnalbe删除指定的message
void removeMessages(Handler h, Runnable r, Object object) { if (h == null || 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.recycleUnchecked(); 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.recycleUnchecked(); p.next = nn; continue; } } p = n; } } }
- 根据obj删除message
void removeCallbacksAndMessages(Handler h, Object object) { if (h == null) { return; } 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.recycleUnchecked(); 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.recycleUnchecked(); p.next = nn; continue; } } p = n; } } }
退出消息队列
void quit(boolean safe) { //mQuitAllowed是在创建looper的时候设置的状态,是否允许该消息队列退出 if (!mQuitAllowed) { throw new IllegalStateException("Main thread not allowed to quit."); } synchronized (this) { if (mQuitting) { return; } mQuitting = true; if (safe) { //删除所有没有被处理的消息 removeAllFutureMessagesLocked(); } else { //删除所有的消息,不管该消息有没有被处理 removeAllMessagesLocked(); } // We can assume mPtr != 0 because mQuitting was previously false. nativeWake(mPtr); } }
删除所有的消息,不管该消息有没有被处理
private void removeAllFutureMessagesLocked() { final long now = SystemClock.uptimeMillis(); Message p = mMessages; if (p != null) { if (p.when > now) { //头部的message的when值是最小的,如果它都大于now说明该队列里面所有的message的when都大于now,即表示都没有执行 removeAllMessagesLocked(); } else { //头部message的when等于now或者小于now,但是还在messagequeue里面,说明该message可能正在被处理,可能是处于正在被取出的状态,为了安全则不对其进行写操作,去查看下一个message的when的值, Message n; for (;;) { n = p.next; if (n == null) { return; } if (n.when > now) { break; } p = n; } p.next = null; do { p = n; n = p.next; p.recycleUnchecked(); } while (n != null); } } }
调用私有方法,循环清空所有的message
private void removeAllMessagesLocked() { Message p = mMessages; while (p != null) { Message n = p.next; p.recycleUnchecked(); p = n; } mMessages = null; }
(完)
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