Handler相關知識梳理
1、爲什麼要使用Handler?
因爲Android系統只能在主線更新UI,而我們的數據操作經常是由子線程來完成的。爲什麼子線程不能更新UI?因爲如果多線程操作UI會導致UI處於不可預知的狀態;那可不可以用加鎖的方式來解決這個問題呢?加鎖會讓訪問UI的邏輯變得很複雜,而且會降低訪問效率。
2、Handler消息機制原理
在程序中,我們一般的做法是調用Handler.SendMessage等方法來發送消息,最終會調用Handler.SendMessageAtTime方法將消息插入到消息隊列MessageQueue中
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;//消息隊列,以單鏈表的形式保存
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;//此處的target即爲Handler
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
在MessageQueue.enqueueMessage方法中,會根據消息要發送的時間來插入
boolean enqueueMessage(Message msg, long when) {
synchronized (this) {
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;
mMessages = msg;
needWake = mBlocked;
} else {
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.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;
}
Looper是消息機制中非常重要的一個東西,主線程會調用Looper.loop方法一直輪詢,發現有消息就將消息轉發給Handler
那麼主線程的Looper是怎麼創建的呢?
在應用程序啓動的過程中,會創建一個主線程,ActivityThread,看它的main方法(應用程序的真正入口)
public static void More ...main(String[] args) { //只貼了相關的代碼
Looper.prepareMainLooper();//爲主線程創建了Looper
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler =thread.getHandler();//主線程的Handler
}
Looper.loop();//進入消息循環
}
重點來看Looper.loop()方法:
public static void loop() {
final Looper me = myLooper();//調用了sThreadLocal.get()來獲取當前線程的looper,ThreadLocal以UI線程爲KEY,以Looper爲值這樣就把一個線程和一個looper關聯起來了
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
for (;;) {//死循環,會一直輪詢接收消息
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// This must be in a local variable, in case a UI event sets the logger
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);//此處即通過Message中保存的Handler對象來發送消息
```
}
}
再回到Handler的dispatchMessage:
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
public void handleMessage(Message msg) {
}
可以看到,最終調用的handleMessage方法是一個空方法,也就是我們在new一個handler時重寫的方法
我們再來看看Looper的構造方法:
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
可以看到,在Looper的構造方法中,new了一個消息隊列
而在我們的Handler的構造方法中:
public Handler(Callback callback, boolean async) {
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
所以,通過Handler發送的消息,插入的消息隊列是Looper new出來的
3、圖解