这个问题在这里已经有了答案:How to get the result of OnPostExecute() to main activity because AsyncTask is a separate class? (17 个回答) 5 年前关闭。
一个简单的问题:是否可以在 AsyncTask
中返回一个值?
//AsyncTask is a member class
private class MyTask extends AsyncTask<Void, Void, Void>{
protected Void doInBackground(Void... params) {
//do stuff
return null;
}
@Override
protected void onPostExecute(Void result) {
//do stuff
//how to return a value to the calling method?
}
}
然后在我的 Activity
/Fragment
中:
// The task is started from activity
myTask.execute()
// something like this?
myvalue = myTask.getvalue()
编辑:很久以前有人问过我不熟悉Java的地方,现在我对它更好,我将做一个快速总结:
异步任务的重点是任务是 asynchronous
,这意味着在任务上调用 execute()
后,任务开始在自己的线程上运行。从 asynctask 返回一个值是没有意义的,因为原来的调用线程已经在做其他事情(因此任务是异步的)。
想想时间:在某个时间点,您启动了一项将与主线程并行运行的任务。当并行运行的任务完成时,主线程上的时间也过去了。并行任务无法及时返回向主线程返回值。
我来自C,所以我对此知之甚少。但似乎很多人都有同样的问题,所以我想我会澄清一下。
WeakReference
或类似的东西
这就是 onPostExecute()
的用途。它在 UI 线程上运行,您可以将结果从那里传送到屏幕(或您需要的任何其他地方)。在最终结果可用之前不会调用它。如果您想提供中间结果,请查看 onProgressUpdate()
为什么不调用处理值的方法?
public class MyClass extends Activity {
private class myTask extends AsyncTask<Void, Void, Void> {
//initiate vars
public myTask() {
super();
//my params here
}
protected Void doInBackground(Void... params) {
//do stuff
return null;
}
@Override
protected void onPostExecute(Void result) {
//do stuff
myMethod(myValue);
}
}
private myHandledValueType myMethod(Value myValue) {
//handle value
return myHandledValueType;
}
}
最简单的方法是将调用对象传递给异步任务(如果您愿意,可以在构造它时):
public class AsyncGetUserImagesTask extends AsyncTask<Void, Void, Void> {
private MyImagesPagerFragment mimagesPagerFragment;
private ArrayList<ImageData> mImages = new ArrayList<ImageData>();
public AsyncGetUserImagesTask(MyImagesPagerFragment imagesPagerFragment) {
this.mimagesPagerFragment = imagesPagerFragment;
}
@Override
public Void doInBackground(Void... records) {
// do work here
return null;
}
@Override
protected void onPostExecute(Void result) {
mimagesPagerFragment.updateAdapter(mImages);
}
}
并且在调用类(您的活动或片段)中执行任务:
public class MyImagesPagerFragment extends Fragment {
@Override
public void onCreate(Bundle savedInstanceState) {
AsyncGetUserImagesTask mGetImagesTask = new AsyncGetUserImagesTask(this);
mGetImagesTask.execute();
}
然后 onPostExecuteMethod 将调用您喜欢的原始类的任何方法,例如:
public void updateAdapter(List<ImageData> images) {
mImageAdapter.setImages(images);
mImageAdapter.notifyDataSetChanged();
}
}
代码示例:Activity使用AsyncTask在后台线程中获取值,然后AsyncTask通过调用processValue将结果返回给Activity:
public class MyClass extends Activity {
private void getValue() {
new MyTask().execute();
}
void processValue(Value myValue) {
//handle value
//Update GUI, show toast, etc..
}
private class MyTask extends AsyncTask<Void, Void, Value> {
@Override
protected Value doInBackground(Void... params) {
//do stuff and return the value you want
return Value;
}
@Override
protected void onPostExecute(Value result) {
// Call activity method with results
processValue(result);
}
}
}
你可以试试这个:myvalue = new myTask().execute().get();
减号是它会冻结进程,直到异步不会完成;
您需要使用 "protocols" 向 AsynTask
委派或提供数据。
代表和数据源
委托是一个对象,当该对象遇到程序中的事件时,该对象代表另一个对象或与该对象协作。 (Apple definition)
协议是定义一些方法来委托某些行为的接口。
DELEGATE:在后台线程中从对象中捕获事件
异步任务:
public final class TaskWithDelegate extends AsyncTask<..., ..., ...> {
//declare a delegate with type of protocol declared in this task
private TaskDelegate delegate;
//here is the task protocol to can delegate on other object
public interface TaskDelegate {
//define you method headers to override
void onTaskEndWithResult(int success);
void onTaskFinishGettingData(Data result);
}
@Override
protected Integer doInBackground(Object... params) {
//do something in background and get result
if (delegate != null) {
//return result to activity
delegate.onTaskFinishGettingData(result);
}
}
@Override
protected void onPostExecute(Integer result) {
if (delegate != null) {
//return success or fail to activity
delegate.onTaskEndWithResult(result);
}
}
}
活动:
public class DelegateActivity extends Activity implements TaskDelegate {
void callTask () {
TaskWithDelegate task = new TaskWithDelegate;
//set the delegate of the task as this activity
task.setDelegate(this);
}
//handle success or fail to show an alert...
@Override
void onTaskEndWithResult(int success) {
}
//handle data to show them in activity...
@Override
void onTaskFinishGettingData(Data result) {
}
}
编辑:如果您在 doInBackground 中调用委托,并且委托尝试编辑某些视图,则会崩溃,因为视图只能由主线程操作。
// this refers to Activity
this.runOnUiThread(new Runnable() {
@Override
public void run() {
// Here you can edit views when task notify some changes from background thread
textView.setText(someValue);
}
});
额外的
DATASOURCE:在后台线程中向对象提供数据
异步任务:
public final class TaskWithDataSource extends AsyncTask<..., ..., ...> {
//declare a datasource with type of protocol declared in this task
private TaskDataSource dataSource;
private Object data;
//here is the task protocol to can provide data from other object
public interface TaskDataSource {
//define you method headers to override
int indexOfObject(Object object);
Object objectAtIndex(int index);
}
@Override
protected void onPreExecute(Integer result) {
if (dataSource != null) {
//ask for some data
this.data = dataSource.objectAtIndex(0);
}
}
@Override
protected Integer doInBackground(Object... params) {
//do something in background and get result
int index;
if (dataSource != null) {
//ask for something more
index = dataSource.indexOfObject(this.data);
}
}
}
活动:
public class DataSourceActivity extends Activity implements TaskDataSource {
void callTask () {
TaskWithDataSource task = new TaskWithDataSource;
//set the datasource of the task as this activity
task.setDataSource(this);
}
//send some data to the async task when it is needed...
@Override
Object objectAtIndex(int index) {
return new Data(index);
}
//send more information...
@Override
int indexOfObject(Object object) {
return new object.getIndex();
}
}
使用泛型参数
AsyncTask<Params, Progress, Result>
Params — 任务的输入数据类型
进展——如何让世界了解进展
Result — 任务的输出数据类型
想像
Result = task(Params)
例子
按字符串 URL 加载 YourObject
:
new AsyncTask<String, Void, YourObject>()
{
@Override
protected void onPreExecute()
{
/* Called before task execution; from UI thread, so you can access your widgets */
// Optionally do some stuff like showing progress bar
};
@Override
protected YourObject doInBackground(String... url)
{
/* Called from background thread, so you're NOT allowed to interact with UI */
// Perform heavy task to get YourObject by string
// Stay clear & functional, just convert input to output and return it
YourObject result = callTheServer(url);
return result;
}
@Override
protected void onPostExecute(YourObject result)
{
/* Called once task is done; from UI thread, so you can access your widgets */
// Process result as you like
}
}.execute("http://www.example.com/");
对于您从 AsyncTask 获得的结果,它需要在 super.onPostExcecute(result)
之后执行;因为这意味着后台和AsyncTask也已经完成了。例如:
... into your async task
@Override
protected void onPostExecute(MyBeanResult result) {
if (dialog.isShowing()) {
dialog.dismiss();
}
if (mWakeLock.isHeld()) {
mWakeLock.release();
}
super.onPostExecute(result);
MyClassName.this.checkResponseForIntent(result);
}
方法器 checkResponseForIntent 可能是这样的:
private void checkResponseForIntent(MyBeanResult response) {
if (response == null || response.fault != null) {
noServiceAvailable();
return;
}
... or do what ever you want, even call an other async task...
我在使用 AsyncTask 中的 .get()
时遇到了这个问题,而 ProgressBar 根本不适用于 get()
,实际上,它仅在 doInBackground 完成后才有效。
我希望它可以帮助你。
将 MainActivity 传递给 Async 类,因此您将在内部类中访问 MainActivity 函数,这对我来说很好:
public class MainActivity extends ActionBarActivity {
void callAsync()
{
Async as = new Async(this,12,"Test");
as.execute();
}
public void ReturnThreadResult(YourObject result)
{
// TO DO:
// print(result.toString());
}
}
public class Async extends AsyncTask<String, String, Boolean> {
MainActivity parent;
int param1;
String param2;
public Async(MainActivity parent,int param1,String param2){
this.parent = parent;
this.param1 = param1;
this.param2 = param2;
}
@Override
protected void onPreExecute(){};
@Override
protected YourObject doInBackground(String... url)
{
return result;
}
@Override
protected void onPostExecute(YourObject result)
{
// call an external function as a result
parent.ReturnThreadResult(result);
}
}
不定期副业成功案例分享
onPostExecute()
即可将结果存储在您选择的变量中。请注意,您的原始代码没有意义,因为(假设的)方法myTask.getValue()
将在之前调用结果可用。您也可以调用 AsyncTask 的get()
方法来获取结果,但在您确定结果可用之前,您不应从 UI 线程执行此操作。否则,它会阻塞 UI 线程,首先破坏了使用 AsyncTask 的目的。onPostExecute()
中选择的变量中,但是如何将该变量返回到您的活动中?例如,如果您希望您的任务连接到 Internet 并下载一些信息,然后您想对这些信息执行一些操作...您如何.execute()
AsyncTask
然后在接下来的情况下使用该信息执行某些操作在AsyncTask
完成之前运行的代码行?onPostExecute()
)。我使用这个类比:您不会编写等待用户输入的代码(例如,按下按钮);相反,您编写的事件处理程序会在用户提供一些输入时做出反应。将onPostExecute()
视为 AsyncTask 的结果可用时的事件处理程序。这就是您放置代码(或调用方法)的地方,除非结果确实可用,否则这些代码将不起作用。AsyncTask
真的不应该被认为是一个简单地获取和返回一些信息的实用函数,而是一个更大的东西,它可以获取信息并在获取该信息后操纵 UI(或其他)?