關(guān)于為什么AsyncTask一定要在主線程中創(chuàng)建？
我們知道AsyncTask中UI的更新是通過InternalHandler實(shí)現(xiàn)的，這就說明InternalHandler必須是通過主線程的Loop來創(chuàng)建的。
看源碼
private static class InternalHandler extends Handler {
public InternalHandler(Looper looper) {
super(looper);
}
@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult result = (AsyncTaskResult) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);
break;
}
}
}
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
}else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
InternalHandler的創(chuàng)建
private static HandlergetMainHandler() {
synchronized (AsyncTask.class) {
if (sHandler ==null) {
sHandler =new InternalHandler(Looper.getMainLooper());
}
return sHandler;
}
}
在API26中，InternalHandler 是通過Looper.getMainLooper()創(chuàng)建的，所以AsyncTask不論是在子線程還是主線程中創(chuàng)建都無所謂。在Android開發(fā)藝術(shù)探索一書中之所以強(qiáng)調(diào)AsyncTask必須在主線程中創(chuàng)建是因?yàn)橹霸创a中InternalHandler的創(chuàng)建方式如下：
private static final InternalHandler sHandler = new InternalHandler();
是通過當(dāng)前線程的Loop創(chuàng)建的，如果當(dāng)前線程是子線程，那自然是行不通的。
execute方法一定要在主線程中調(diào)用？
execute的源碼實(shí)現(xiàn)如下：
@MainThread
public final AsyncTaskexecute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
@MainThread
public final AsyncTaskexecuteOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+" the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+" the task has already been executed "
+"(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
@MainThread
protected void onPreExecute() {
}
通過源碼可以看出，execute方法最先調(diào)用的是onPreExecute方法，而onPreExecute方法必須運(yùn)行在主線程，如果一定要在子線程中調(diào)execute方法那就表示你不能在onPreExecute方法中做更新UI的操作，因?yàn)榇藭r(shí)onPreExecute方法為子線程。onProgressUpdate， onPostExecute ， onCancelled 方法仍然運(yùn)行在主線程（通過InternalHandler回調(diào)到主線程）。
