FirebaseRecyclerAdapter报错：迁移新适配器时遇CoordinatorLayout$Behavior重复定义

先搞定眼前的重复类错误 #

你遇到的Program type already present: android.support.design.widget.CoordinatorLayout$Behavior错误，本质是AndroidX和旧版Support库的依赖冲突。新版Firebase库已经全面迁移到AndroidX了，如果你还在项目里保留着旧的support依赖（比如com.android.support:design），就会出现这种重复类的情况。

解决步骤：

• 打开你的build.gradle（Module级别的），检查是否还有com.android.support开头的依赖，把它们替换成对应的AndroidX依赖。比如com.android.support:design要换成com.google.android.material:material:1.x.x（x选最新稳定版）。
• 确保gradle.properties里已经开启AndroidX迁移的配置：

  android.useAndroidX=true
  android.enableJetifier=true
  

• 执行Clean Project然后Rebuild Project，一般就能解决这个重复类问题了。

新版FirebaseAdapter结合FirebaseOptions和Query的正确姿势 #

旧版的FirebaseAdapter已经被废弃，现在官方推荐用FirebaseRecyclerAdapter（针对Firebase Realtime Database）或者FirebaseFirestoreRecyclerAdapter（针对Firestore）。结合FirebaseOptions的话，你需要先手动初始化自定义的FirebaseApp实例，再基于这个实例获取数据库引用并创建Query。

针对Firebase Realtime Database的示例 #

1. 先配置FirebaseOptions并初始化自定义FirebaseApp：

   FirebaseOptions options = new FirebaseOptions.Builder()
           .setApiKey("YOUR_API_KEY")
           .setApplicationId("YOUR_APP_ID")
           .setDatabaseUrl("YOUR_DATABASE_URL")
           .build();
   
   // 初始化自定义FirebaseApp，避免和默认App冲突
   FirebaseApp secondaryApp = FirebaseApp.initializeApp(context, options, "secondary");
   

2. 基于自定义App获取Database引用并创建Query：

   FirebaseDatabase database = FirebaseDatabase.getInstance(secondaryApp);
   Query query = database.getReference("your_collection").orderByChild("timestamp").limitToLast(50);
   

3. 初始化FirebaseRecyclerAdapter：

   // 先定义ViewHolder
   public class YourViewHolder extends RecyclerView.ViewHolder {
       TextView title;
       public YourViewHolder(View itemView) {
           super(itemView);
           title = itemView.findViewById(R.id.title);
       }
   }
   
   // 创建Adapter
   FirebaseRecyclerAdapter<YourModel, YourViewHolder> adapter = new FirebaseRecyclerAdapter<YourModel, YourViewHolder>(
           YourModel.class,
           R.layout.item_layout,
           YourViewHolder.class,
           query
   ) {
       @Override
       protected void populateViewHolder(YourViewHolder viewHolder, YourModel model, int position) {
           // 绑定数据到UI
           viewHolder.title.setText(model.getTitle());
       }
   };
   
   // 给RecyclerView设置Adapter
   recyclerView.setAdapter(adapter);
   

4. 别忘了在生命周期里启动/停止监听：

   @Override
   protected void onStart() {
       super.onStart();
       adapter.startListening();
   }
   
   @Override
   protected void onStop() {
       super.onStop();
       adapter.stopListening();
   }
   

针对Firestore的示例 #

用法逻辑类似，只是换成Firestore对应的API：

// 1. 初始化自定义FirebaseApp（同上）
FirebaseOptions options = new FirebaseOptions.Builder()
        .setApiKey("YOUR_API_KEY")
        .setApplicationId("YOUR_APP_ID")
        .setFirestoreUrl("YOUR_FIRESTORE_URL")
        .build();
FirebaseApp secondaryApp = FirebaseApp.initializeApp(context, options, "secondary");

// 2. 获取Firestore实例并创建Query
FirebaseFirestore db = FirebaseFirestore.getInstance(secondaryApp);
Query query = db.collection("your_collection").orderBy("timestamp", Query.Direction.DESCENDING).limit(50);

// 3. 配置FirestoreRecyclerOptions
FirestoreRecyclerOptions<YourModel> firestoreOptions = new FirestoreRecyclerOptions.Builder<YourModel>()
        .setQuery(query, YourModel.class)
        .build();

// 4. 创建并配置Adapter
FirebaseFirestoreRecyclerAdapter<YourModel, YourViewHolder> adapter = new FirebaseFirestoreRecyclerAdapter<YourModel, YourViewHolder>(firestoreOptions) {
    @Override
    protected void onBindViewHolder(@NonNull YourViewHolder holder, int position, @NonNull YourModel model) {
        // 绑定数据到UI
        holder.title.setText(model.getTitle());
    }

    @NonNull
    @Override
    public YourViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_layout, parent, false);
        return new YourViewHolder(view);
    }
};

// 5. 设置Adapter并管理监听
recyclerView.setAdapter(adapter);
// 生命周期监听同上，调用adapter.startListening()和adapter.stopListening()

内容的提问来源于stack exchange，提问作者depp