簡述HandlerThread原始碼
摘要：
public class HandlerThread extends Thread {
int mPriority;
int mTid = -1;
Looper mLooper;
private @Nullable Handler mHandler;
...
public class HandlerThread extends Thread {
int mPriority;
int mTid = -1;
Looper mLooper;
private @Nullable Handler mHandler;
public HandlerThread(String name) {
super(name);
mPriority = Process.THREAD_PRIORITY_DEFAULT;
}
/**
* Constructs a HandlerThread.
* @param name
* @param priority The priority to run the thread at. The value supplied must be from
* {@link android.os.Process} and not from java.lang.Thread.
*/
public HandlerThread(String name, int priority) {
super(name);
mPriority = priority;
}
/**
* Call back method that can be explicitly overridden if needed to execute some
* setup before Looper loops.
*/
protected void onLooperPrepared() {
}
@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}
/**
* This method returns the Looper associated with this thread. If this thread not been started
* or for any reason isAlive() returns false, this method will return null. If this thread
* has been started, this method will block until the looper has been initialized.
* @return The looper.
*/
public Looper getLooper() {
if (!isAlive()) {
return null;
}
// If the thread has been started, wait until the looper has been created.
synchronized (this) {
while (isAlive() && mLooper == null) {
try {
wait();
} catch (InterruptedException e) {
}
}
}
return mLooper;
}
/**
* @return a shared {@link Handler} associated with this thread
* @hide
*/
@NonNull
public Handler getThreadHandler() {
if (mHandler == null) {
mHandler = new Handler(getLooper());
}
return mHandler;
}
關鍵的就是這些，很明顯的可以看出，handlerThread在一個子執行緒裡面封裝好了looper和handler，所以這就是為什麼在子執行緒裡面使用handlerThread直接run就可以了。
在getLooper裡面，handlerThread對執行緒是否存在加了一個判斷，因為handler在主執行緒，而handlerThread建立的Looper在子執行緒，如果子執行緒沒有建立或者子執行緒的Looper沒有建立，那handler也不知道給誰發訊息。所以這裡handlerThread加了一層判斷，確保子執行緒的Looper建立。
關於handlerThread的使用場景，一般就是在子執行緒中使用，因為如果按照正常的handler使用的話，需要自己額外在子執行緒新建一個Looper（由於主執行緒已經預設有一個Looper了，所以在主執行緒可以直接用handler）。例如在子執行緒不斷地獲取資料更新UI的時候，就可以用到handlerThread。