Question

我对AsyncTask有问题，onPostExecute方法在doInBackground方法完成工作之前运行。

我正在尝试从Firebase数据库中获取一些数据，将其保存到“ Didattica”对象的数组列表中，以传递给可扩展列表视图，我在数据库上有4个对象。

问题是onPostExecute在doInBackground之后不久运行，然后doInBackground完成了它的工作，填充了arrayList。

public class DidatticaActivity extends AppCompatActivity {

ArrayList<Didattica> reishiki1 = null;
private ExpandableListView listView1;
private ExpandableListAdapter listAdapter1;
private List<String> listDataHeader1;
private HashMap<String, List<String>> listHash1;
AsyncTaskDidattica atDidattica;

final FirebaseDatabase database = FirebaseDatabase.getInstance();
private DatabaseReference refDidattica1;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_didattica);

    listHash1 = new HashMap<>();
    listDataHeader1 = new ArrayList<>();
    listView1 = findViewById(R.id.lvReishiki);
    atDidattica = new AsyncTaskDidattica();
    atDidattica.execute();
    listAdapter1 = new ExpandableListAdapter(this, listDataHeader1,listHash1);
    listView1.setAdapter(listAdapter1);
}

private class AsyncTaskDidattica extends AsyncTask<String, String, HashMap<String, ArrayList<Didattica>>>{

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }
    @Override
    protected HashMap<String, ArrayList<Didattica>> doInBackground(String... strings) {
        HashMap result = new HashMap<>();
        reishiki1 = new ArrayList<>();
        try {
            refDidattica1 = database.getReference("didattica/1-reishiki");

            refDidattica1.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot1) {
                    Log.i("shinseiLog", "onDataChange" );
                    for (DataSnapshot child1 : dataSnapshot1.getChildren()) {
                        Didattica d1 = child1.getValue(Didattica.class);
                        reishiki1.add(d1);
                    }
                    Log.i("shinseiLog", "rei size1: " + reishiki1.size());
                }
                @Override
                public void onCancelled(DatabaseError databaseError) {}
            });
            result.put("reishiki", reishiki1);
            Log.i("shinseiLog", "map size 1: " + result.size());
            return result;
        }catch(Exception e){
            Log.i("shinseiLog", "eccezione in doInBackground");
        }
        return null;
    }
    @Override
    protected void onPostExecute(HashMap<String, ArrayList<Didattica>> result) {
        super.onPostExecute(result);
        ArrayList<Didattica> rei = result.get("reishiki");
        Log.i("shinseiLog", "rei size2: " + rei.size());
        for(Didattica r: rei){
            listDataHeader1.add(r.getTitolo());
            listHash1.put(r.getTitolo(),r.getList());
        }
        Log.i("shinseiLog", "header: " + listDataHeader1.size());
        Log.i("shinseiLog", "hash: " + listHash1.size());

        listAdapter1.notifyDataSetChanged();
        }
    }
}

我添加了一些日志以帮助理解顺序：

I/shinseiLog: map size 1: 1
I/shinseiLog: rei size2: 0
I/shinseiLog: header: 0
I/shinseiLog: hash: 0
I/shinseiLog: onDataChange
I/shinseiLog: rei size1: 4

最后一行正确显示arrayList包含我的所有数据，但是为时已晚。为什么以这种方式执行？我在哪里错了？

谢谢

Answer 1

Firebase已经是异步的，您无需使用AsyncTask即可获取或上传数据。

Answer 2

请记住，firebase是异步的。所以你的方法...

public void onDataChange(DataSnapshot dataSnapshot1) {
}

...正在另一个线程中执行。这就是onPostExecute首先执行的原因。

尝试此操作，将onPostexecute代码中的代码放入方法onDataChange，see this picture中

Android-asynctask执行顺序

2 个答案: