Android 如何在启动活动时检查（侦听）asynctask状态

我有一个活动“A”及其子片段，活动“B”和GetTask类。 用户启动应用程序并进入活动A。当用户查看ListView时，AsyncTask正在加载活动B的数据。但这可能需要一段时间，用户可以在没有ListView数据的情况下移动到活动B。所以他必须等待。我在这里创建了一个ProgressDialog，希望在异步任务完成时进行检查

现在，我将静态变量

instance

与静态方法

getInstance（）

一起使用，该方法在PostExecute方法中设置为“1”，然后在活动B中使用它

int instance = GetTask.getInstance();

然后我创建了一个ProgressDialog，但我无法获取AsyncTask状态，以了解何时取消对话框

task.getStatus（）

始终显示运行状态

我尝试使用OnComplete listener，下面是示例代码 公共类GetForeignToursTask扩展AsyncTask实现OnTaskCompleteListener{

ActivityA刚刚启动一个异步任务：

GetTask task = new GetTask(this, null);
task.execute(url);

GetTask类示例：

    private Context context;
    OnTaskCompleteListener listener;

    private static int instance = 0;

    public GetTask(Context context, OnTaskCompleteListener listener) {
        this.listener = listener;
        this.context = context;
    }

    @Override
    public void onTaskCompleted(int status) {
        Log.d("log", status); // I don't get here at all
    }

活动代码：

GetTask task = new GetTask();
task.getStatus(); // here is always RUNNING
int instance = GetTask.getInstance();

if (instance != 1) {
    final ProgressDialog dialog = new ProgressDialog(ToursListActivity.this);
    dialog.setMessage("Loading...");
    dialog.show();
    // I also need to pause here until AsyncTask is done.
}

listener.onTaskCompleted(1); // error here

OnTaskCompleteListener接口：

public interface OnTaskCompleteListener {
void onTaskCompleted(int status);
}

我不确定我在那里做得对。 据我所知，要使CompleteListener工作，它应该从一个我将要侦听的活动开始，但我的AsyncTask已经在运行，我不从ActivityB执行它。 如何使它更好？或者为什么我不使用onTaskCompleteListener方法

我将执行ActivityA中代码的GetTask更改为

listener.onTaskCompleted（1）；

，并在ActivityB中的

listener.onTaskCompleted（1）；

行获得相同的错误

NullPointerException

Upd ActivityB ListView依赖于用户将点击的ListItem。所以我认为在异步任务完成之前无法加载ListView

解决方案 我在AsyncTask中使用BroadcastReceiver和static方法来确定AsyncTask是否在ActivityB启动之前完成

这是我的代码。 在GetTask类中，我添加了方法：

static boolean taskStatus = false;
public static boolean GetTaskStatus() {
    return taskStatus;
}

在onPostExecute（）中

活动b

private ProgressDialog dialog;

protected void onCreate(Bundle savedInstanceState) {
    ...
    dialog = new ProgressDialog(ActivityB.this);
    taskStatus = GetTask.GetTaskStatus();
    ...
    if (!taskStatus) { // check if AsyncTask already completed
        dialog.setMessage("Loading...");
        dialog.show();
        dialog.setCancelable(false);
        dialog.setCanceledOnTouchOutside(false);
    } else {
        items = datasource.getSelectedItems(cid);
        setUpView(context, items);
        taskStatus = false; // in case of reloading data this should be returned to false I think
}

private BroadcastReceiver asynctaskcompleted = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        Log.d(TAG, "AsyncTask completed");
        datasource = new ItemsListDataSource(context);
        datasource.open();
        items = datasource.getSelectedItems(cid);
        setUpView(context, items);
        dialog.dismiss();
    }
};

public void onResume() {
    super.onResume();
    IntentFilter filter = new IntentFilter();
    filter.addAction("ru.myapp.asynctaskcompleted");
    filter.addCategory("android.intent.category.DEFAULT");
    registerReceiver(asynctaskcompleted, filter);
}

public void onPause() {
    super.onPause();
    unregisterReceiver(asynctaskcompleted);
}

这是我用过的；

---

就这样。谢谢您的帮助。

在后台任务中加载ListView 参考旧练习“从internet加载图标的ListView”，从internet加载位图是一项耗时的任务。因此，在本练习中修改了代码，实现了一个异步任务来处理ListView：位图加载在后台线程中，setListAdapter（）加载在onPostExecute（）中

row.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="wrap_content"
android:orientation="horizontal">
<ImageView
android:id="@+id/icon"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/icon"/>
<TextView
android:id="@+id/weekofday"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
</LinearLayout>

AndroidList.java

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;

import android.app.ListActivity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

public class AndroidList extends ListActivity {

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

 @Override
 protected void onPostExecute(Void result) {
  // TODO Auto-generated method stub
  setListAdapter(new MyCustomAdapter(AndroidList.this, R.layout.row, month));
  Toast.makeText(AndroidList.this,
    "onPostExecute \n: setListAdapter after bitmap preloaded",
    Toast.LENGTH_LONG).show();
 }

 @Override
 protected void onPreExecute() {
  // TODO Auto-generated method stub
  Toast.makeText(AndroidList.this,
    "onPreExecute \n: preload bitmap in AsyncTask",
    Toast.LENGTH_LONG).show();
 }

 @Override
 protected Void doInBackground(Void... params) {
  // TODO Auto-generated method stub
  preLoadSrcBitmap();
  return null;
 }

}

String image_URL=
 "http://4.bp.blogspot.com/_C5a2qH8Y_jk/StYXDpZ9-WI/AAAAAAAAAJQ/sCgPx6jfWPU/S1600-R/android.png";

public class MyCustomAdapter extends ArrayAdapter<String> {
 Bitmap bm;

 public MyCustomAdapter(Context context, int textViewResourceId,
   String[] objects) {
  super(context, textViewResourceId, objects);
  // TODO Auto-generated constructor stub

  bm = srcBitmap;
 }

 @Override
 public View getView(int position, View convertView, ViewGroup parent) {
  // TODO Auto-generated method stub
  //return super.getView(position, convertView, parent);

  View row = convertView;

  if(row==null){
   LayoutInflater inflater=getLayoutInflater();
   row=inflater.inflate(R.layout.row, parent, false);
  }

  TextView label=(TextView)row.findViewById(R.id.weekofday);
  label.setText(month[position]);
  ImageView icon=(ImageView)row.findViewById(R.id.icon);

  icon.setImageBitmap(bm);

  return row;
 }
}

Bitmap srcBitmap;
private void preLoadSrcBitmap(){
 BitmapFactory.Options bmOptions;
 bmOptions = new BitmapFactory.Options();
 bmOptions.inSampleSize = 1;
 srcBitmap = LoadImage(image_URL, bmOptions);
}

String[] month = {
  "January", "February", "March", "April",
  "May", "June", "July", "August",
  "September", "October", "November", "December"
  };

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

 /*setListAdapter(new ArrayAdapter<String>(this,
       R.layout.row, R.id.weekofday, DayOfWeek));*/
 new backgroundLoadListView().execute();
}

@Override
protected void onListItemClick(ListView l, View v, int position, long id) {
 // TODO Auto-generated method stub
 //super.onListItemClick(l, v, position, id);
 String selection = l.getItemAtPosition(position).toString();
 Toast.makeText(this, selection, Toast.LENGTH_LONG).show();
}

private Bitmap LoadImage(String URL, BitmapFactory.Options options)
{     
 Bitmap bitmap = null;
 InputStream in = null;     
 try {
  in = OpenHttpConnection(URL);
  bitmap = BitmapFactory.decodeStream(in, null, options);
  in.close();
 } catch (IOException e1) {
 }

 return bitmap;               
}

private InputStream OpenHttpConnection(String strURL) throws IOException{
 InputStream inputStream = null;
 URL url = new URL(strURL);
 URLConnection conn = url.openConnection();

 try{
  HttpURLConnection httpConn = (HttpURLConnection)conn;
  httpConn.setRequestMethod("GET");
  httpConn.connect();

  if (httpConn.getResponseCode() == HttpURLConnection.HTTP_OK) {
   inputStream = httpConn.getInputStream(); 
  } 
 }
 catch (Exception ex){
 }

 return inputStream;
}
}

import java.io.IOException；
导入java.io.InputStream；
导入java.net.HttpURLConnection；
导入java.net.URL；
导入java.net.URLConnection；
导入android.app.ListActivity；
导入android.content.Context；
导入android.graphics.Bitmap；
导入android.graphics.BitmapFactory；
导入android.os.AsyncTask；
导入android.os.Bundle；
导入android.view.LayoutInflater；
导入android.view.view；
导入android.view.ViewGroup；
导入android.widget.ArrayAdapter；
导入android.widget.ImageView；
导入android.widget.ListView；
导入android.widget.TextView；
导入android.widget.Toast；
公共类AndroidList扩展了ListActivity{
公共类backgroundLoadListView扩展
异步任务{
@凌驾
受保护的void onPostExecute（void结果）{
//TODO自动生成的方法存根
setListAdapter（新的MyCustomAdapter（AndroidList.this，R.layout.row，month））；
Toast.makeText（AndroidList.this，
“onPostExecute\n：位图预加载后的setListAdapter”，
Toast.LENGTH_LONG）.show（）；
}
@凌驾
受保护的void onPreExecute（）{
//TODO自动生成的方法存根
Toast.makeText（AndroidList.this，
“onPreExecute\n:在异步任务中预加载位图”，
Toast.LENGTH_LONG）.show（）；
}
@凌驾
受保护的Void doInBackground（Void…参数）{
//TODO自动生成的方法存根
预加载位图（）；
返回null；
}
}
字符串图像URL=
"http://4.bp.blogspot.com/_C5a2qH8Y_jk/StYXDpZ9-WI/AAAAAAAAAJQ/sCgPx6jfWPU/S1600-R/android.png";
公共类MyCustomAdapter扩展了ArrayAdapter{
位图bm；
公共MyCustomAdapter（上下文，int textViewResourceId，
字符串[]对象）{
超级（上下文、textViewResourceId、对象）；
//TODO自动生成的构造函数存根
bm=src位图；
}
@凌驾
公共视图getView（int位置、视图转换视图、视图组父视图）{
//TODO自动生成的方法存根
//返回super.getView（position、convertView、parent）；
视图行=转换视图；
if（行==null）{
LayoutInflater充气机=getLayoutInflater（）；
行=充气机。充气（R.layout.row，父级，false）；
}
TextView标签=（TextView）row.findViewById（R.id.weekofday）；
label.setText（月[职位]）；
ImageView图标=（ImageView）row.findViewById（R.id.icon）；
icon.setImageBitmap（bm）；
返回行；
}
}
位图；
私有void预加载srcbtmap（）{
选项bmOptions；
bmOptions=新的BitmapFactory.Options（）；
bmOptions.inSampleSize=1；
srcbtmap=LoadImage（image\u URL，bmOptions）；
}
字符串[]月={
“一月”、“二月”、“三月”、“四月”，
“五月”、“六月”、“七月”、“八月”，
“九月”、“十月”、“十一月”、“十二月”
};
/**在首次创建活动时调用*/
@凌驾
创建时的公共void（Bundle savedInstanceState）{
super.onCreate（savedInstanceState）；
//setContentView（R.layout.main）；
/*setListAdapter（新阵列适配器）（此，
R.layout.row，R.id.weekofday，DayOfWeek））*/
新建backgroundLoadListView（）.execute（）；
}
@凌驾
受保护的void onListItemClick（列表视图l、视图v、整数位置、长id）{
//TODO自动生成的方法存根
//super.onListItemClick（左、右、位置、id）；
字符串选择=l.getItemAtPosition（position）.toString（）；
Toast.makeText（这个，选择，Toast.LENGTH_LONG）.show（）；
}
私有位图加载图像（字符串URL，BitmapFactory.Options）
{     
位图=空；
InputStream in=null；
试一试{
in=OpenHttpConnection（URL）；
位图=BitmapFactory.decodeStream（in，null，options）；
in.close（）；
}捕获（IOE1异常）{
}
返回位
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;

import android.app.ListActivity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

public class AndroidList extends ListActivity {

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

 @Override
 protected void onPostExecute(Void result) {
  // TODO Auto-generated method stub
  setListAdapter(new MyCustomAdapter(AndroidList.this, R.layout.row, month));
  Toast.makeText(AndroidList.this,
    "onPostExecute \n: setListAdapter after bitmap preloaded",
    Toast.LENGTH_LONG).show();
 }

 @Override
 protected void onPreExecute() {
  // TODO Auto-generated method stub
  Toast.makeText(AndroidList.this,
    "onPreExecute \n: preload bitmap in AsyncTask",
    Toast.LENGTH_LONG).show();
 }

 @Override
 protected Void doInBackground(Void... params) {
  // TODO Auto-generated method stub
  preLoadSrcBitmap();
  return null;
 }

}

String image_URL=
 "http://4.bp.blogspot.com/_C5a2qH8Y_jk/StYXDpZ9-WI/AAAAAAAAAJQ/sCgPx6jfWPU/S1600-R/android.png";

public class MyCustomAdapter extends ArrayAdapter<String> {
 Bitmap bm;

 public MyCustomAdapter(Context context, int textViewResourceId,
   String[] objects) {
  super(context, textViewResourceId, objects);
  // TODO Auto-generated constructor stub

  bm = srcBitmap;
 }

 @Override
 public View getView(int position, View convertView, ViewGroup parent) {
  // TODO Auto-generated method stub
  //return super.getView(position, convertView, parent);

  View row = convertView;

  if(row==null){
   LayoutInflater inflater=getLayoutInflater();
   row=inflater.inflate(R.layout.row, parent, false);
  }

  TextView label=(TextView)row.findViewById(R.id.weekofday);
  label.setText(month[position]);
  ImageView icon=(ImageView)row.findViewById(R.id.icon);

  icon.setImageBitmap(bm);

  return row;
 }
}

Bitmap srcBitmap;
private void preLoadSrcBitmap(){
 BitmapFactory.Options bmOptions;
 bmOptions = new BitmapFactory.Options();
 bmOptions.inSampleSize = 1;
 srcBitmap = LoadImage(image_URL, bmOptions);
}

String[] month = {
  "January", "February", "March", "April",
  "May", "June", "July", "August",
  "September", "October", "November", "December"
  };

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

 /*setListAdapter(new ArrayAdapter<String>(this,
       R.layout.row, R.id.weekofday, DayOfWeek));*/
 new backgroundLoadListView().execute();
}

@Override
protected void onListItemClick(ListView l, View v, int position, long id) {
 // TODO Auto-generated method stub
 //super.onListItemClick(l, v, position, id);
 String selection = l.getItemAtPosition(position).toString();
 Toast.makeText(this, selection, Toast.LENGTH_LONG).show();
}

private Bitmap LoadImage(String URL, BitmapFactory.Options options)
{     
 Bitmap bitmap = null;
 InputStream in = null;     
 try {
  in = OpenHttpConnection(URL);
  bitmap = BitmapFactory.decodeStream(in, null, options);
  in.close();
 } catch (IOException e1) {
 }

 return bitmap;               
}

private InputStream OpenHttpConnection(String strURL) throws IOException{
 InputStream inputStream = null;
 URL url = new URL(strURL);
 URLConnection conn = url.openConnection();

 try{
  HttpURLConnection httpConn = (HttpURLConnection)conn;
  httpConn.setRequestMethod("GET");
  httpConn.connect();

  if (httpConn.getResponseCode() == HttpURLConnection.HTTP_OK) {
   inputStream = httpConn.getInputStream(); 
  } 
 }
 catch (Exception ex){
 }

 return inputStream;
}
}

mProgressDialog.show();
new GetTask()
{
    @Override
    protected void onPostExecute(Void result)
    {
        mProgressDialog.dismiss();

        //this means the task is done.
    }
}.execute(this, null);