AsynTask源码分析 AsynTask源码分析

这里写代码片AsyncTask是什么？
AsyncTask是Android为我们提供的方便编写异步任务的工具类
AsyncTask四个方法
1、onPreExecute() 主要做一些初始化的工作
2、doInBackground(Params…) 耗时操作放在这里
3、onProgressUpdate(Progress…) 用于显示任务执行的进度。
4、onPostExecute(Result) 更新UI
我们知道要使用AsynTask首先要new一个他的实例

public AsyncTask() { mWorker = new WorkerRunnable() { public Result call() throws Exception { mTaskInvoked.set(true); Result result = null; try { Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked result = doInBackground(mParams); Binder.flushPendingCommands(); } catch (Throwable tr) { mCancelled.set(true); throw tr; } finally { postResult(result); } return result; } }; mFuture = new FutureTask(mWorker) { @Override protected void done() { try { postResultIfNotInvoked(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); } } };

这段代码中其实只初始化了两个对象mWorker ，mFuture

private static abstract class WorkerRunnable implements Callable { Params[] mParams; } public interface Callable { /** * Computes a result, or throws an exception if unable to do so. * * @return computed result * @throws Exception if unable to compute a result */ V call() throws Exception; }

其实只是实现了Callable接口
mFuture呢？也只是初始化了callable 和state

public FutureTask(Callable callable) { if (callable == null) throw new NullPointerException(); this.callable = callable; this.state = NEW; // ensure visibility of callable }

接着在看execute（）；

@MainThread public final AsyncTask execute(Params... params) { return executeOnExecutor(sDefaultExecutor, params); } @MainThread public final AsyncTask 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; exec.execute(mFuture); return this; }

大家可以看到onPreExecute(); 这个时候还是在主线程中所以onPreExecute(); 可以更改UI，同时他也是第一个执行的方法
mWorker.mParams = params; 这句代码把参数赋值给了mParams

mWorker = new WorkerRunnable() { public Result call() throws Exception { mTaskInvoked.set(true); Result result = null; try { Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked result = doInBackground(mParams); Binder.flushPendingCommands(); } catch (Throwable tr) { mCancelled.set(true); throw tr; } finally { postResult(result); } return result; }

可以看到 result = doInBackground(mParams); 这质量执行了doInBackground（）方法，
在执行postResult(result);
private Result postResult(Result result) {
@SuppressWarnings(“unchecked”)
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult(this, result));
message.sendToTarget();
return result;
}
这里就可以看到 message.sendToTarget(); 这句话就是通过Handler发送了一个消息
public void sendToTarget() {
target.sendMessage(this);
}
在看Handler
private static class InternalHandler extends Handler {
public InternalHandler() {
super(Looper.getMainLooper());
}

@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;
}
通过what判断进入result.mTask.onProgressUpdate(result.mData);
还是onPostExecute(result); 方法
【AsynTask源码分析】总结：
AsynTask其实只是封装了Handler消息机制