1、AsyncTask基本使用：
AsyncTask asyncTask=new AsyncTask<Void,Void,Void>() {
@Override
protected void onPostExecute(Void aVoid) {
super.onPostExecute(aVoid);
//执行完成返回的方法 UI 主线程
}
@Override
protected Void doInBackground(Void... voids) {
//执行一些耗时操作 链接网络 读取大型的数据库 运行在Thread
//在此方法中调用publishProgress();，会将进度值更新在onProgressUpdate(Void... values)方法中
publishProgress();
return null;
}
@Override
protected void onPreExecute() {
//一调用就会执行的方法 更新UI 主线程
super.onPreExecute();
}
@Override
protected void onProgressUpdate(Void... values) {
super.onProgressUpdate(values);
//更新在doInBackground()方法中下载的进度;
}
};
asyncTask.execute("www.baidu.com");
2、AsyncTask源码阅读，点击asyncTask.execute("www.baidu.com");中的execute("www.baidu.com")方法进入AsyncTask的源码...
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
调用AsyncTask内部方法executeOnExecutor(sDefaultExecutor, params);点击进入该方法：
@MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {//初始状态mStatus == Status.PENDING，所以第一次进入该方法时，不会进入if()方法中。再次调用时，根据状态抛出异常。AsyncTask（同一对象）只能运行一次
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;//更新mStatus 的状态为Running
onPreExecute();//首先调用了这个方法
mWorker.mParams = params;//将传进来的参数赋值给 mWorker.mParams，在子线程中药用到
exec.execute(mFuture);//然后调用接口方法，应为这是个接口，没法往下看，所以在代码中查找mFuture这个变量用到的地方
return this;
}
mFuture = new FutureTask<Result>(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);
}
}
};
new FutureTask<Result>(mWorker) {}；得到mFuture对象，传入一个mWorker对象：
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);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
下面我们先来看看FutureTask是个什么东西：
public class FutureTask<V> implements RunnableFuture<V> {...}
首先它实现了RunnableFuture<V>接口,RunnableFuture<V>继承Runnable接口，实现run()方法
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}
所以在FutureTask<V>查找run()方法
public void run() {
if (state != NEW ||
!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
在run()方法中，callable就是通过构造方法传进来的mWorker这个参数，result = c.call();
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
现在回到mWorker，调用了call()方法：
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);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
调用了result = doInBackground(mParams);方法，在finally{}中，调用了postResult(result);将结果发送到主线程。
在AsyncTask中查找MESSAGE_POST_RESULT，发现：调用了finish()方法
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
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;
}
}
}
那么finish()又是什么方法呢？
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
最后通过onPostExecute(result);将结果传到主线程中去，同时将mStatus = Status.FINISHED;变成Status.FINISHED。完成此AsyncTask对象的异步任务。
