从Firebase检索数据时的应用加载程序文本

Question

我正在尝试创建一个Android应用，该应用的数据在Firebase realtimeDatabase中可用。现在，我想添加一个预加载器文本，以使访问者不会出错（加载时没有任何数据）。我尝试使用线程，这是可以的，但是在加载数据时出现了问题。加载应用程序数据后，显示“不幸的是您的应用程序已停止”的应用程序（在隐藏加载程序文本之前，应用程序弯曲）。直到未加载应用程序数据，应用程序显示加载程序文本，但加载数据后，加载程序文本未隐藏并且应用程序崩溃。哪里错了？

package com.abdullah.md.allsimoffers;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Handler;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.LinearLayout;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.AdView;
import com.google.android.gms.ads.MobileAds;
import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import java.util.ArrayList;

public class SecondActivity extends AppCompatActivity {
DatabaseReference dref;
ListView listview;
ArrayList<String> list = new ArrayList<>();
ArrayList<String> keyCheck = new ArrayList<>();
Bundle bundle;
String title;
private AdView adView;
private AdRequest adRequest;
TextView preloader;
ArrayAdapter<String> adapter;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_second);
    MobileAds.initialize(this, "ca-app-pub-3940256099942544~3347511713");


    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    getSupportActionBar().setDisplayShowHomeEnabled(true);


    preloader = findViewById(R.id.preloaderId);

    adView = (AdView) findViewById(R.id.adViewId);
    adRequest = new AdRequest.Builder().build();
    adView.loadAd(adRequest);


    listview = findViewById(R.id.listViewId);
    adapter = new ArrayAdapter<String>(this, R.layout.custom_layout, R.id.headingTextViewId, list);
    listview.setAdapter(adapter);


    bundle = getIntent().getExtras();
    if (bundle != null) {

        String intentValue = bundle.getString("intentValue");
        if (intentValue.contains("banglalink")) {
            dref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://allsimoffers.firebaseio.com/banglalink");
            title = "Banglalink";
        } else if (intentValue.contains("grameenphone")) {
            dref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://allsimoffers.firebaseio.com/grameenphone");
            title = "grameenPhone";
        } else if (intentValue.contains("robi")) {
            dref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://allsimoffers.firebaseio.com/robi");
            title = "Robi";
        } else if (intentValue.contains("airtel")) {
            dref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://allsimoffers.firebaseio.com/airtel");
            title = "Airtel";
        } else if (intentValue.contains("teletalk")) {
            dref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://allsimoffers.firebaseio.com/teletalk");
            title = "Teletalk";
        }
    }




    getSupportActionBar().setTitle(title);


    new preloaderClass().execute();

//---------------------------onCreate method end--------------------------
}


class preloaderClass extends AsyncTask<Void,Void, ArrayList<String>> {



    @Override
    protected void onPreExecute() {
        listview.setVisibility(View.GONE);
        preloader.setVisibility(View.VISIBLE);
        super.onPreExecute();
    }

    @Override
    protected ArrayList<String> doInBackground(Void... voids) {
        dref.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                list.add(dataSnapshot.getValue(String.class));
                keyCheck.add(dataSnapshot.getKey());
                adapter.notifyDataSetChanged();
            }

            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {

                String value = dataSnapshot.getValue(String.class);
                String key = (dataSnapshot.getKey());
                int valueOfKey = keyCheck.indexOf(key);
                list.set(valueOfKey, value);
                adapter.notifyDataSetChanged();
            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
                list.remove(dataSnapshot.getValue(String.class));
                keyCheck.remove(dataSnapshot.getKey());
                adapter.notifyDataSetChanged();
            }

            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String s) {
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Toast.makeText(SecondActivity.this, databaseError.toException().toString(), Toast.LENGTH_SHORT).show();
            }
        });
        return list;
    }


    @Override
    protected void onPostExecute(ArrayList<String> strings) {
        if (!strings.isEmpty()){
            listview.setVisibility(View.VISIBLE);
            preloader.setVisibility(View.GONE);
        }
        else if (strings.isEmpty()){
            Toast.makeText(SecondActivity.this, "Failed", Toast.LENGTH_SHORT).show();
        }
        super.onPostExecute(strings);
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.options_menu, menu);
    return super.onCreateOptionsMenu(menu);
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    if (item.getItemId() == R.id.homeMenuId) {

        Intent intent = new Intent(this, FirstActivity.class);
        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);
        startActivity(intent);
        finish();

    } else if (item.getItemId() == R.id.aboutMenuId) {
        Intent intent = new Intent(this, ThirdActivity.class);
        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);
        startActivity(intent);
        finish();

    } else if (item.getItemId() == android.R.id.home) {
        Intent intent = new Intent(this, FirstActivity.class);
        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);
        startActivity(intent);
        finish();
    }
    return super.onOptionsItemSelected(item);
}

// ---------------------选项菜单结束--------------------- -----------------

}

Answer 1

您永远不要从后台线程访问UI元素（例如预加载器）。您的应用程序由于尝试访问UI元素而不是main thread而崩溃。 要修复您的代码，您可以致电

runOnUiThread(new Runnable(){
   preloader.setVisibility(View.GONE);
})

但我真的鼓励您检查在后台加载数据的适当方法，以从后台线程更新UI。选中AsyncTask，IntentService，ThreadPools，RxJava，Coroutines。

同样从您的代码中，为什么每次都需要检查list，为什么在创建/更新list后就不能删除预加载器

已更新： 我并不是说您需要后台线程。 Firebase侦听器已在主线程中调用。检查一下：

public class SecondActivity extends AppCompatActivity implements ChildEventListener {
DatabaseReference dref;
ListView listview;
ArrayList<String> list = new ArrayList<>();
ArrayList<String> keyCheck = new ArrayList<>();
Bundle bundle;
String title;
private AdView adView;
private AdRequest adRequest;
TextView preloader;
ArrayAdapter<String> adapter;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_second);
    MobileAds.initialize(this, "ca-app-pub-3940256099942544~3347511713");


    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    getSupportActionBar().setDisplayShowHomeEnabled(true);


    preloader = findViewById(R.id.preloaderId);

    adView = (AdView) findViewById(R.id.adViewId);
    adRequest = new AdRequest.Builder().build();
    adView.loadAd(adRequest);


    listview = findViewById(R.id.listViewId);
    adapter = new ArrayAdapter<String>(this, R.layout.custom_layout, R.id.headingTextViewId, list);
    listview.setAdapter(adapter);


    bundle = getIntent().getExtras();
    if (bundle != null) {

        String intentValue = bundle.getString("intentValue");
        if (intentValue.contains("banglalink")) {
            dref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://allsimoffers.firebaseio.com/banglalink");
            title = "Banglalink";
        } else if (intentValue.contains("grameenphone")) {
            dref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://allsimoffers.firebaseio.com/grameenphone");
            title = "grameenPhone";
        } else if (intentValue.contains("robi")) {
            dref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://allsimoffers.firebaseio.com/robi");
            title = "Robi";
        } else if (intentValue.contains("airtel")) {
            dref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://allsimoffers.firebaseio.com/airtel");
            title = "Airtel";
        } else if (intentValue.contains("teletalk")) {
            dref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://allsimoffers.firebaseio.com/teletalk");
            title = "Teletalk";
        }
    }


    getSupportActionBar().setTitle(title);


}

@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
    list.add(dataSnapshot.getValue(String.class));
    keyCheck.add(dataSnapshot.getKey());
    adapter.notifyDataSetChanged();


}

@Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {

    String value = dataSnapshot.getValue(String.class);
    String key = (dataSnapshot.getKey());
    int valueOfKey = keyCheck.indexOf(key);
    list.set(valueOfKey, value);
    adapter.notifyDataSetChanged();
}

@Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
    list.remove(dataSnapshot.getValue(String.class));
    keyCheck.remove(dataSnapshot.getKey());
    adapter.notifyDataSetChanged();
}

@Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}

@Override
public void onCancelled(DatabaseError databaseError) {
    Toast.makeText(SecondActivity.this, databaseError.toException().toString(), Toast.LENGTH_SHORT).show();
}

private void updateUI() {
    if (!list.isEmpty()) {
        listview.setVisibility(View.VISIBLE);
        preloader.setVisibility(View.GONE);
    } else {
        listview.setVisibility(View.GONE);
        preloader.setVisibility(View.VISIBLE);
    }
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.options_menu, menu);
    return super.onCreateOptionsMenu(menu);
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    if (item.getItemId() == R.id.homeMenuId) {

        Intent intent = new Intent(this, FirstActivity.class);
        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);
        startActivity(intent);
        finish();

    } else if (item.getItemId() == R.id.aboutMenuId) {
        Intent intent = new Intent(this, ThirdActivity.class);
        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);
        startActivity(intent);
        finish();

    } else if (item.getItemId() == android.R.id.home) {
        Intent intent = new Intent(this, FirstActivity.class);
        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);
        startActivity(intent);
        finish();
    }
    return super.onOptionsItemSelected(item);
}

}