AsyncTask简要分析
经典异步任务：AsyncTask,使用场景有：批量下载，批量拷贝等。官方文档就直接给出了一个批量下载的示例。
private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
protected Long doInBackground(URL... urls) {
int count = urls.length;
long totalSize = 0;
for (int i = 0; i < count; i++) {
totalSize += Downloader.downloadFile(urls[i]);
publishProgress((int) ((i / (float) count) * 100));
// Escape early if cancel() is called
if (isCancelled()) break;
}
return totalSize;
}
protected void onProgressUpdate(Integer... progress) {
setProgressPercent(progress[0]);
}
protected void onPostExecute(Long result) {
showDialog("Downloaded " + result + " bytes");
}
}
// Once created, a task is executed very simply:
new DownloadFilesTask().execute(url1, url2, url3);
这里简单看一下，AsyncTask中的几个经典问题：
AsyncTask开了几个线程？AsyncTask怎么实现的线程调度？
好吧，就想到了这两个问题。第一个问题，明显是坑。答案是未知的。看手机 cpu个数了。
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
// We want at least 2 threads and at most 4 threads in the core pool,
// preferring to have 1 less than the CPU count to avoid saturating
// the CPU with background work
private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
然后是第二个问题。简单回答就是一句话，通过开启线程执行具体逻辑，然后通过handler将结果发送到主线程。
进一步追踪的话，就会发现，AsyncTask = ThreadPoolExecutor + Handler 。
通过源码可以看到，AsyncTask在构造方法里面，已经完成了全部的准备工作。
MainHandler已经创建。FutureTask已经准备了。（说实话，FutureTask难过的让人吐血）public AsyncTask(@Nullable Looper callbackLooper) {
mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
? getMainHandler()
: new Handler(callbackLooper); mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams); // mParams 核心逻辑
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result); // 核心逻辑
}
return result;
}
}; mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get()); // get()
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
系消息
@MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(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; // 核心逻辑
exec.execute(mFuture); // 核心逻辑 return this;
} // 核心逻辑：handler，将结果发送到主线程
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}