Android 异步执行在 onCreate 之后

Question

我真的是 android 应用程序创建的新手，我正在使用 Android studio 来处理我的项目。我正在从我的数据库中获取 json 结果并且获取的结果是我想要的，但我不知道如何让 asynctask 在 onCreate 之前完成。我的 ListView 是空的，但我通过调试器确认我的异步任务结果值存在。

更新：我跟踪错误到这里：

  public void populatedata(){
            /**get back result from pref**/
            SharedPreferences pref = getApplicationContext().getSharedPreferences("MyPref", MODE_PRIVATE);
            String result =pref.getString("result", "null");
        try{
            Log.d("ADebugTag", "Value: "+result);

            JSONObject person = new JSONObject(result);
            JSONArray jsonMainNode = person.getJSONArray("result");
            final int n = jsonMainNode.length();

            ListView lv= (ListView)findViewById(R.id.listView1);
            String[] from = new String[] {"id", "time"};
            int[] to = new int[] {android.R.id.text1};

            List<Map<String,String>> classList = new ArrayList<Map<String,String>>();

         //   for(int i = 0; i< n; i++){
                  //HashMap<String, String> map = new HashMap<String, String>();
         //       JSONObject jsonChildNode = jsonMainNode.getJSONObject(i);

          //      String id = jsonChildNode.getString("id");
           //     String schedule = jsonChildNode.getString("schedule");
           //     String time = jsonChildNode.getString("time");
            //    String output = schedule + time;
               // classList.add(createchild(id,output));
              //  Log.d("ADebugTag", "Value before loop: "+classList);
          //  }
          //  Log.d("ADebugTag", "Value after loop: "+classList);
            Log.d("ADebugTag", "Hi, Im here5");


            for(int i = 0; i < 10; i++){
                HashMap<String, String> map = new HashMap<String, String>();

                map.put("id", "time" + i);
                map.put("col_1", "col_1_item_" + i);
                map.put("col_2", "col_2_item_" + i);
                map.put("col_3", "col_3_item_" + i);
                classList.add(map);
            }

            SimpleAdapter simpleAdapter = new SimpleAdapter(this,classList, android.R.layout.simple_list_item_1, from, to);
            lv.setAdapter(simpleAdapter);

        }
        catch(JSONException e){
            Toast.makeText(getApplicationContext(), "Error"+e.toString(), Toast.LENGTH_SHORT).show();
        }

    }

    private HashMap<String, String>createchild(String id,String output){
        HashMap<String,String> childNo = new HashMap<String,String>();

        childNo.put(id,output);
        Log.d("ADebugTag", "Hi, Im here4");
        return childNo;

    }

the // for loop is the one i wanna work it out, and my string result `is{"result":[{"id":"00000000001","schedule":"000001","time":"10:21:37"}]}`


updated: It's my mistake, I did not create a R.layout and textview for it, I learned to use custom adapter and create an external layout.

public class ViewClassActivity extends AppCompatActivity{


    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_view_class);

        /**prepare list**/
        SharedPreferences pref = getApplicationContext().getSharedPreferences("MyPref", MODE_PRIVATE);
        String id =pref.getString("userid", "null");



        Log.d("ADebugTag", "Hi, Im here");
        new fetchclass(this,id).execute(id);


        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);

        getSupportActionBar().setDisplayHomeAsUpEnabled(true);


    }

    public void getdata(String result){
        //save preferences
        SharedPreferences pref = getApplicationContext().getSharedPreferences("MyPref", MODE_PRIVATE);
        SharedPreferences.Editor editor = pref.edit();

        editor.putString("result", result);  // Saving result string

        Log.d("ADebugTag", "Hi, Im here3");
        // Save the changes in SharedPreferences
        editor.apply(); // apply changes
        populatedata();

        Log.d("ADebugTag", "Hi, Im here6");
    }

    public void populatedata(){
        /**get back result from pref**/
        SharedPreferences pref = getApplicationContext().getSharedPreferences("MyPref", MODE_PRIVATE);
        String result =pref.getString("result", "null");
        Log.d("ADebugTag",result);

        try{
            Log.d("ADebugTag", "Value: "+result);

            JSONObject person = new JSONObject(result);
            JSONArray jsonMainNode = person.getJSONArray("result");
            final int n = jsonMainNode.length();

            ListView lv= (ListView)findViewById(R.id.listView1);
            String[] from = new String[] {"id", "schedule"};
            int[] to = new int[] {R.id.textView20,R.id.textView21};

            //List<Map<String,String>> classList = new ArrayList<Map<String,String>>();
            ArrayList<HashMap<String,String>> classList = new ArrayList<>();

            for(int i = 0; i< n; i++){
                HashMap<String, String> hashMap = new HashMap<>();
                JSONObject jsonChildNode = jsonMainNode.getJSONObject(i);

                String id = jsonChildNode.getString("id");
                String schedule = jsonChildNode.getString("schedule");
                String time = jsonChildNode.getString("time");
                String output = schedule + time;

                hashMap.put("id", id);
                hashMap.put("schedule", output);

                classList.add(hashMap);
                //classList.add(createchild(id,output));
                Log.d("ADebugTag", "Value before loop: "+classList);
            }

           CustomAdapter simpleAdapter = new CustomAdapter(this,classList, R.layout.list_view_item, from, to);
            lv.setAdapter(simpleAdapter);

        }
        catch(JSONException e){
            Toast.makeText(getApplicationContext(), "Error"+e.toString(), Toast.LENGTH_SHORT).show();
        }

    }

    private HashMap<String, String>createchild(String id,String output){
        HashMap<String,String> childNo = new HashMap<String,String>();

        childNo.put(id,output);
        Log.d("ADebugTag", "Hi, Im here4");
        return childNo;

    }

list_view_item.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin">

        <TextView
            android:id="@+id/textView20"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:padding="@dimen/activity_horizontal_margin"
            android:text="Demo1"
            android:textColor="#000" />

        <TextView
            android:id="@+id/textView21"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:padding="@dimen/activity_horizontal_margin"
            android:text="Demo2"
            android:textColor="#000"
            android:layout_alignParentTop="true"
            android:layout_toRightOf="@+id/textView20"
            android:layout_toEndOf="@+id/textView20" />

</RelativeLayout>

自定义适配器

public class CustomAdapter extends SimpleAdapter {
    LayoutInflater inflater;
    Context context;
    ArrayList<HashMap<String, String>> arrayList;

    public CustomAdapter(Context context, ArrayList<HashMap<String, String>> data, int resource, String[] from, int[] to) {
        super(context, data, resource, from, to);
        this.context = context;
        this.arrayList = data;
        inflater.from(context);
    }



}

使用此站点解决了我的问题，感谢大家的帮助：

custom-listview

Answer 1

您不能让异步任务在 onCreate 之前完成。这就是异步任务的全部要点——它并行执行。相反，您应该设置一个加载屏幕（即使只是像不确定的进度条一样简单的东西）向用户显示您正在等待数据，然后将数据加载到任务的 onPostExecute 列表中。

Answer 2

试试下面的代码：

 public class ViewClassActivity extends AppCompatActivity{


List<Map<String,String>> classList = new ArrayList<Map<String,String>>();

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_view_class);

    /**prepare list**/
    SharedPreferences pref = getApplicationContext().getSharedPreferences("MyPref", MODE_PRIVATE);
    String id =pref.getString("userid", "null");

    ListView listView = (ListView) findViewById(R.id.listView1);
    SimpleAdapter simpleAdapter = new SimpleAdapter(this,classList, android.R.layout.simple_list_item_1, new String[] {"result"}, new int[] {android.R.id.text1});
    listView.setAdapter(simpleAdapter);

    Log.d("ADebugTag", "Hi, Im here5");

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);

    getSupportActionBar().setDisplayHomeAsUpEnabled(true);


    Log.d("ADebugTag", "Hi, Im here");
    new fetchclass(this,id).execute(id);
}

private HashMap<String, String>createchild(String id,String output){
    HashMap<String,String> childNo = new HashMap<String,String>();
    childNo.put(id,output);
    Log.d("ADebugTag", "Hi, Im here4");
    return childNo;

}

public class fetchclass  extends AsyncTask<String,String,String>{

ProgressDialog proDialog;
String userid;

public ViewClassActivity activity;


public fetchclass(ViewClassActivity a, String id) {

    this.activity = a;
    this.userid = id;
}

protected void onPreExecute(){
    //pre-execute procedure
    // Showing progress loading dialog
    proDialog = new ProgressDialog(activity);
    proDialog.setMessage("Loading...");
    proDialog.setCancelable(false);
    proDialog.show();
}

@Override
protected String doInBackground(String... arg0) {
    try{
        String id = arg0[0];

        String link="http://www.commcreative.me/mnas/attendance.php";
        String data  = URLEncoder.encode("id", "UTF-8") + "=" + URLEncoder.encode(id, "UTF-8");

        URL url = new URL(link);
        URLConnection conn = url.openConnection();

        conn.setDoOutput(true);
        OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());

        wr.write( data );
        wr.flush();

        BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));

        StringBuilder sb = new StringBuilder();
        String line = null;

        // Read Server Response
        while((line = reader.readLine()) != null)
        {
            sb.append(line);
            break;
        }

        try{
            Log.d("ADebugTag", "Value: "+sb.toString(););

            JSONObject person = new JSONObject(sb.toString(););
            JSONArray jsonMainNode = person.getJSONArray("result");
            final int n = jsonMainNode.length();

            for(int i = 0; i<n;i++){
                JSONObject jsonChildNode = jsonMainNode.getJSONObject(i);

                String id = jsonChildNode.getString("id");
                String schedule = jsonChildNode.getString("schedule");
                String time = jsonChildNode.getString("time");
                String output = schedule + time;
                classList.add(createchild(id,output));
                Log.d("ADebugTag", "Value: "+classList);
            }

            simpleAdapter .notifyDataSetChange();

        }
        catch(JSONException e){
            Toast.makeText(getApplicationContext(), "Error"+e.toString(), Toast.LENGTH_SHORT).show();
        }

        return sb.toString();
    }
    catch(Exception e){
        return new String("Exception: " + e.getMessage());
    }
}


@Override
protected void onPostExecute(String result){

    // Dismiss the progress dialog
    if (proDialog.isShowing())
        proDialog.dismiss();
    //dismiss end

   // Log.d("ADebugTag", "Hi, Im here2");
    // activity.getdata(result);

}
}

private HashMap<String, String>createchild(String id,String output){
    HashMap<String,String> childNo = new HashMap<String,String>();
    childNo.put(id,output);
    Log.d("ADebugTag", "Hi, Im here4");
    return childNo;

}



}

Android 异步执行在 onCreate 之后

Android Async Execution comes after onCreate

android

listview

android-asynctask

simpleadapter