Firebase 数据的 Android Studio 值事件侦听器

Question

总的来说，我是 Android 和 Firebase 的新手，但我搜索了很长时间，但没有找到适合我的问题的答案。我想建立一个带有 firebase 连接的 Android 项目。为此，我成功同步了项目，并按照官方文档执行了所有步骤。

环境

我使用的是 Android Studio 3.5.1。

错误说明

我的错误是，当我运行主要活动时，我得到“错误：找不到符号类 DatabaseError”。将鼠标悬停在 ValueEventListener 上时出现错误“必须将类 '从 ValueEventListener 派生的匿名类'声明为抽象或在 'ValueEventListener' 中实现抽象方法 'onCancelled(DatabaseError)'

这是我的主要活动

package com.example.test2;

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;


public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
// Write a message to the database
        FirebaseDatabase database = FirebaseDatabase.getInstance();
        DatabaseReference myRef = database.getReference("message");

        myRef.setValue("Hello, World!");

// Read from the database
        myRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                // This method is called once with the initial value and again
                // whenever data at this location is updated.
                String value = dataSnapshot.getValue(String.class);
                Log.d(TAG, "Value is: " + value);
            }

            @Override
            public void onCancelled(DatabaseError error) {
                // Failed to read value
                Log.w(TAG, "Failed to read value.", error.toException());
            }
        });

    }
}

这是我的 build.gradle 模块应用

apply plugin: 'com.android.application'
apply plugin: 'com.google.gms.google-services'

android {
compileSdkVersion 29
buildToolsVersion "29.0.2"
defaultConfig {
    applicationId "com.example.test2"
    minSdkVersion 16
    targetSdkVersion 29
    versionCode 1
    versionName "1.0"
    testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
    }
}
}

dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'androidx.appcompat:appcompat:1.1.0'
implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
implementation 'com.google.firebase:firebase-analytics:17.4.3'
implementation 'com.google.firebase:firebase-database:19.3.1'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'androidx.test.ext:junit:1.1.1'
androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
}

Answer 1

public class MainActivity extends AppCompatActivity {

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

// Write a message to the database
        FirebaseDatabase database = FirebaseDatabase.getInstance();
        DatabaseReference myRef = database.getReference("message");

        myRef.setValue("Hello, World!");

// Read from the database
        myRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                // This method is called once with the initial value and again
                // whenever data at this location is updated.
                String value = dataSnapshot.getValue(String.class);
                Log.d("TAG", "Value is: " + value);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {
                Log.w("TAG", "Failed to read value.", error.toException());
            }
        });
   }

}