1、Loop.prepare()：創(chuàng)建Loop對象，初始化Loop中的MessageQueue跟當前的Thread；
private static void prepare(boolean quitAllowed) {
????if (sThreadLocal.get() != null) {
????????throw new RuntimeException("Only one Looper may be created per thread");
????}
????sThreadLocal.set(new Looper(quitAllowed));
}
2、Loop.loop()：拿到當前線程Loop對象，在拿到Loop中的消息隊列，for循環(huán)輪詢遍歷消息隊列，不斷調(diào)用MessageQueue的next方法，取出消息；
final Looper me = myLooper();
if (me == null) {
????throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
for (;;) {
????Message msg = queue.next(); // might block
????if (msg == null) {
????????// No message indicates that the message queue is quitting. return;
????}
????msg.target.dispatchMessage(msg);
}
3、創(chuàng)建handler對象：創(chuàng)建handler對象時，會拿到當前線程的Loop對象，除了Main線程，其他線程都得初始化Loop對象，否則會報錯。通過Loop對象，把handler跟所屬線程的消息隊列關(guān)聯(lián)起來；
public Handler(Callback callback, boolean async) {
? ??mQueue = mLooper.mQueue;
}
4、發(fā)送消息：實際上是把消息加到消息隊列中，但在這一步之前，會把handler對象賦值給msg的target屬性；
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
????msg.target = this;
????if (mAsynchronous) {
????????msg.setAsynchronous(true);
????}
????return queue.enqueueMessage(msg, uptimeMillis);
}
5、接收消息：將消息加入到了消息隊列，通過Loop的輪詢，就能取出當前加入到消息隊列的消息。最終把消息分發(fā)給消息體的target屬性（調(diào)用了Handler的dispatchMessage方法），這個target實際上就是發(fā)出消息的handler對象；
public void dispatchMessage(Message msg) {
????if (msg.callback != null) {
????????handleCallback(msg);
????} else {
????????if (mCallback != null) {
????????????if (mCallback.handleMessage(msg)) {
????????????????return;
? ? ? ? ? ? }
????????}
????????handleMessage(msg);
????}
}
6、處理消息：最終來到了Handler的dispatchMessage方法，方法里會判斷callback對象是否為空，如果為空，就調(diào)用handleMessage方法，就是我們平時創(chuàng)建handler時重寫的handleMessage方法。
handleMessage(msg)
