Question

这是我的代码

package com.example.rollsystems;

import java.util.ArrayList;
import org.dto.Student;

import android.os.AsyncTask;
import android.os.Bundle;
import android.widget.ListView;
import android.widget.TextView;
import android.app.Activity;
import android.content.Context;
import com.example.listview.*;

import com.example.Utils.AllBO;
import com.example.rollsystems.R;

public class RollCallActivity extends Activity {

    ArrayList<Student> array;
    ListStudentAdapter arrayAdapter;
    ListView list;

    TextView txtClassAt;
    TextView txtSubjectAt;
    TextView txtInstructorAt;
    TextView txtTimeAt;
    TextView txtDateAt;
    Context context;

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

        public RollCallActivity activity;

        public RollCallTask(RollCallActivity a)
        {
            activity = a;
        }
        @Override
        protected Void doInBackground(Void... params) {
            //SharedPreferences sharedPref = getApplicationContext().getSharedPreferences("rs", Context.MODE_PRIVATE);
            //String RollCallID = sharedPref.getString("RollCallID", "14");

                String RollCallID = "14";
                list = (ListView)findViewById(R.id.listAtStudent);
                ArrayList<Student> rollCalls = new AllBO().getRollCallInfo(RollCallID);
                array = rollCalls;

            return  null;
        }
        @Override
        protected void onPostExecute(Void result) {
            // TODO Auto-generated method stub
            super.onPostExecute(result);
            arrayAdapter = new ListStudentAdapter(activity, R.layout.list_atstudent, array);
            list.setAdapter(arrayAdapter);
            txtClassAt = (TextView) findViewById(R.id.txtClassAt);
        }

    }

    /** Called when the activity is first created. */

      @Override
      public void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.tab_rollcall);

            new RollCallTask(this).execute();
            txtClassAt = (TextView) findViewById(R.id.txtClassAt);

        }


}

然后，当我跑：

E/AndroidRuntime(883): FATAL EXCEPTION: AsyncTask #1
E/AndroidRuntime(883): java.lang.RuntimeException: An error occured while executing
doInBackground()
E/AndroidRuntime(883):  at android.os.AsyncTask$3.done(AsyncTask.java:278)

每一个帮助都是宝贵的

Answer 1

您正尝试从后台线程访问UI。您需要在doInBackground之外执行此操作。

您还必须从UI线程中调用AsyncTask。不这样做是导致运行时错误的原因。

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

    public RollCallActivity activity;

    public RollCallTask(RollCallActivity a)
    {
        activity = a;
    }        

    @Override
    protected Void doInBackground(Void... params) {
        //SharedPreferences sharedPref = getApplicationContext().getSharedPreferences("rs", Context.MODE_PRIVATE);
        //String RollCallID = sharedPref.getString("RollCallID", "14");

        String RollCallID = "14";
        ArrayList<Student> rollCalls = new AllBO().getRollCallInfo(RollCallID);
        array = rollCalls;

        return  null;
    }
}

然后你的onCreate将

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.tab_rollcall);

        list = (ListView)findViewById(R.id.listAtStudent);
        txtClassAt = (TextView) findViewById(R.id.txtClassAt);
        new RollCallTask(this).execute();

    }

Answer 2

异步任务正在访问UI。不要在其中findViewById，使用活动/片段中的onCreate。

以下是一些常规问题：

String RollCallID = "14";
list = (ListView)findViewById(R.id.listAtStudent);
ArrayList<Student> rollCalls = new AllBO().getRollCallInfo(RollCallID);
array = rollCalls;

变量绝不应以大写rollCallID
xml id不应该是驼峰式R.id.list_as_student
数组未在适配器外部使用，为什么要在其外部使用变量？

Answer 3

使用MainActivity中的Handler对象并发布可运行的对象。要从背景中使用它，您需要将对象设置为可以在MainActivity之外调用的静态，或者您可以创建活动的静态实例来访问它。

活动内部

private static Handler handler;


handler = new Handler();

handler().post(new Runnable() {

    public void run() {
        //ui stuff here :)
    }
});

public static Handler getHandler() {
   return handler;
}

活动之外

MainActivity.getHandler().post(new Runnable() {

        public void run() {
            //ui stuff here :)
        }
    });

执行doInBackground时发生错误？

3 个答案: