我正在阅读有关AsyncTask的文章,并尝试了下面的简单程序。但这似乎并不奏效。我该怎么做呢?

public class AsyncTaskActivity extends Activity {

    Button btn;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        btn = (Button) findViewById(R.id.button1);
        btn.setOnClickListener((OnClickListener) this);
    }

    public void onClick(View view){
        new LongOperation().execute("");
    }

    private class LongOperation extends AsyncTask<String, Void, String> {
        @Override
        protected String doInBackground(String... params) {
            for(int i=0;i<5;i++) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText("Executed");
            return null;
        }

        @Override
        protected void onPostExecute(String result) {
        }

        @Override
        protected void onPreExecute() {
        }

        @Override
        protected void onProgressUpdate(Void... values) {
        }
    }
}

我只是试图在后台进程中5秒后更改标签。

这是我的main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="fill_parent"
              android:layout_height="fill_parent"
              android:orientation="vertical" >
    <ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:indeterminate="false"
        android:max="10"
        android:padding="10dip">
    </ProgressBar>
    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Start Progress" >
    </Button>
    <TextView android:id="@+id/output"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Replace"/>
</LinearLayout>

当前回答

异步操作的最短示例:

class MyAsyncTask extends android.os.AsyncTask {
    @Override
    protected Object doInBackground(Object[] objects) {
        // Do something asynchronously
        return null;
    }
}

运行它:

(new MyAsyncTask()).execute();

其他回答

当你在工作线程中,你不能直接操作Android上的UI元素。

当你使用AsyncTask时,请理解回调方法。

例如:

public class MyAyncTask extends AsyncTask<Void, Void, Void>{

    @Override
    protected void onPreExecute() {
        // Here you can show progress bar or something on the similar lines.
        // Since you are in a UI thread here.
        super.onPreExecute();
    }

    @Override
    protected void onPostExecute(Void aVoid) {
        super.onPostExecute(aVoid);
        // After completing execution of given task, control will return here.
        // Hence if you want to populate UI elements with fetched data, do it here.
    }

    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
        // You can track you progress update here
    }

    @Override
    protected Void doInBackground(Void... params) {
        // Here you are in the worker thread and you are not allowed to access UI thread from here.
        // Here you can perform network operations or any heavy operations you want.
        return null;
    }
}

仅供参考: 要从工作线程访问UI线程,您可以在视图上使用runOnUiThread()方法或post方法。

例如:

runOnUiThread(new Runnable() {
    textView.setText("something.");
});

or
    yourview.post(new Runnable() {
    yourview.setText("something");
});

这将帮助你更好地了解事情。因此在你的情况下,你需要在onPostExecute()方法中设置你的textview。

好的,您正在尝试通过另一个线程访问GUI。总的来说,这不是一种好的做法。

AsyncTask在另一个线程中执行doInBackground()中的所有内容,该线程不能访问视图所在的GUI。

preExecute()和postExecute()提供了在这个新线程中发生重大操作之前和之后访问GUI的权限,甚至可以将长时间操作的结果传递给postExecute(),然后显示处理的任何结果。

看到这些行,你稍后更新你的TextView:

TextView txt = findViewById(R.id.output);
txt.setText("Executed");

把它们放在onPostExecute()中。

然后你会看到你的TextView文本更新后,doInBackground完成。

我注意到你的onClick监听器没有检查哪个视图已被选中。我发现最简单的方法是通过switch语句。我有一个完整的类编辑下面所有的建议,以避免混乱。

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.provider.Settings.System;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.view.View.OnClickListener;

public class AsyncTaskActivity extends Activity implements OnClickListener {

    Button btn;
    AsyncTask<?, ?, ?> runningTask;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        btn = findViewById(R.id.button1);

        // Because we implement OnClickListener, we only
        // have to pass "this" (much easier)
        btn.setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        // Detect the view that was "clicked"
        switch (view.getId()) {
        case R.id.button1:
            if (runningTask != null)
                runningTask.cancel(true);
            runningTask = new LongOperation();
            runningTask.execute();
            break;
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // Cancel running task(s) to avoid memory leaks
        if (runningTask != null)
            runningTask.cancel(true);
    }

    private final class LongOperation extends AsyncTask<Void, Void, String> {

        @Override
        protected String doInBackground(Void... params) {
            for (int i = 0; i < 5; i++) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // We were cancelled; stop sleeping!
                }
            }
            return "Executed";
        }

        @Override
        protected void onPostExecute(String result) {
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText("Executed"); // txt.setText(result);
            // You might want to change "executed" for the returned string
            // passed into onPostExecute(), but that is up to you
        }
    }
}

简单:

LongOperation MyTask = new LongOperation();
MyTask.execute();

如果你打开AsyncTask类,你可以看到下面的代码。

public abstract class AsyncTask<Params, Progress, Result> {
    @WorkerThread
    protected abstract Result doInBackground(Params... params);
    @MainThread
    protected void onPreExecute() {
    }
    @SuppressWarnings({"UnusedDeclaration"})
    @MainThread
    protected void onPostExecute(Result result) {
    }
}

AsyncTask的特性

AsyncTask是一个抽象类 AsyncTask有3个泛型参数。 AsyncTask有doInBackground onPreExecute onPostExecute的抽象方法 doInBackground是WorkerThread(你不能更新UI) onPreExecute是主线程 onPostExecute是主线程(你可以更新UI)

例子

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login);
    mEmailView = (AutoCompleteTextView) findViewById(R.id.email);

    AsyncTask<Void, Void, Post> asyncTask = new AsyncTask<Void, Void, Post>() {
        @Override
        protected Post doInBackground(Void... params) {
            try {
                ApiClient defaultClient = Configuration.getDefaultApiClient();
                String authorization = "eyJhbGciOiJIUzI1NiJ9.eyJ1c2VyX2lkIjoxLCJleHAiOjE1ODIxMzM4MTB9.bA3Byc_SuB6jzqUGAY4Pyt4oBNg0VfDRctZ8-PcPlYg"; // String | JWT token for Authorization
                ApiKeyAuth Bearer = (ApiKeyAuth) defaultClient.getAuthentication("Bearer");
                Bearer.setApiKey(authorization);
                PostApi apiInstance = new PostApi();
                String id = "1"; // String | id
                Integer commentPage = 1; // Integer | Page number for Comment
                Integer commentPer = 10; // Integer | Per page number For Comment
                Post result;
                try {
                    result = apiInstance.apiV1PostsIdGet(id, authorization, commentPage, commentPer);
                } catch (ApiException e) {
                    e.printStackTrace();
                    result = new Post();
                }
                return result;
            } catch (Exception e) {
                e.printStackTrace();
                return new Post();
            }
        }

        @Override
        protected void onPostExecute(Post post) {
            super.onPostExecute(post);
            if (post != null) {
                mEmailView.setText(post.getBody());
                System.out.print(post);
            }
        }
    };
    asyncTask.execute();
}

我相信它是正确执行,但你试图改变UI元素在后台线程,这不会做。

修改你的调用和AsyncTask如下:

调用类

注意:我个人建议在任何执行AsyncTask线程的地方使用onPostExecute(),而不是在扩展AsyncTask本身的类中。我认为这使代码更容易阅读,特别是如果你需要AsyncTask在多个地方处理略有不同的结果。

new LongThread() {
    @Override public void onPostExecute(String result) {
        TextView txt = (TextView) findViewById(R.id.output);
        txt.setText(result);
    }
}.execute("");

LongThread类(extends AsyncTask):

@Override
protected String doInBackground(String... params) {
    for (int i = 0; i < 5; i++) {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    return "Executed";
}