Before
Lopper是跨進(jìn)程的關(guān)鍵類寞酿，閱讀本文之前最好先熟悉Looper,MessageQueue為好；
先看源碼
public class Handler {
final MessageQueue mQueue;
public final boolean sendMessage(Message msg) {
return sendMessageDelayed(msg, 0);
}
public final 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;
if (queue == null) {
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);//回調(diào)到熟悉的方法啦
}
}
}
分析
- 由熟悉的
sendMessage開始徽千，最后都是調(diào)用enqueueMessage方法忠藤，而最終還是調(diào)用MessageQueue類的enqueueMessage方法(點(diǎn)我),到此為止我們一般都是在新線程調(diào)用； - 接下來
Looper類的loop方法會(huì)在當(dāng)前線程（如果是更新UI則在主線程）從MessageQueue中獲取最新消息真竖，通過msg.target.dispatchMessage(msg);調(diào)用到Handler中來（上述代碼）儡蔓，然后調(diào)用到handlemessage(msg)我們就熟悉啦； - 由此疼邀，Handler跨線程最重要的是在當(dāng)前線程（初始化
Handler的線程）進(jìn)行loop輪詢，而變量是可以在不同線程訪問·的召锈，所以Handler可以在其他線程向MessageQueue中插入數(shù)據(jù)旁振，而loop則在當(dāng)前線程不斷去取數(shù)據(jù)，取得數(shù)據(jù)就回調(diào)涨岁，達(dá)到跨線程的目的拐袜；
