Create Handler 消息延时实现.md

blogs/Android/Framework/Interview/线程通信相关/Handler 消息延时实现.md

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+---
+Handler 的消息延时是怎么实现的？
+---
+
+1. 消息延时是做了什么特殊处理？
+2. 是发送延时了，还是消息延时处理了？
+3. 延时精度怎么样？
+
+```java
+boolean sendMessageDelayed(Message msg, long delayMillis){
+    if(delayMillis<0){
+        delayMillis=0;
+    }
+    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
+}
+public boolean sendMessageAtTime(Message msg, long uptimeMillis){
+    MessageQueue queue = mQueue;
+    return enqueueMessage(queue, msg, uptimeMillis);
+}
+boolean enqueueMessage(Message msg, long when){
+    msg.when = when;
+    Message p = mMessage, prew;
+    if(p==null||when==0||when<p.when){
+        msg.mext = p;
+        mMessage = msg;
+    }else{
+        for(;;){
+            prew = p;
+            p = p.next;
+            if(p==null||when<p.when){
+                break;
+            }
+        }
+        msg.next = p;
+        prev.next = msg;
+    }
+    nativeWake(mPtr);
+}
+Message next(){
+    for(int nextPollTimeoutMillis = 0;;){
+        nativePollOnce(ptr, nextPollTimeoutMillis);
+        final long now = SystemClock.uptimeMillis();
+        Message msg = mMessages;
+        if(msg != null){
+            if(now<msg.when){
+                nextPollTimeoutMillis = (int)Math.min(msg.when-now, Integer.MAX_VALUE);
+            }else{
+                mMessage = msg.next;
+                msg.next = null;
+                return msg;
+            }
+        }else{
+            nextPollTimeoutMillis = -1;
+        }
+    }
+}
+```
+
+#### 总结
+
+1. 消息队列按消息触发时间排序
+2. 设置 epoll_wait 的超时时间，使其在特定时间唤醒
+3. 关于延时精度