Android根据ArrayList长度切换布局可见性功能失效如何解决？

核心问题 #

1. Firebase Firestore的addSnapshotListener是异步操作，你在onCreate中调用监听注册方法后立刻判断列表长度，此时网络请求还未返回，列表为空，所以永远会走显示按钮的逻辑
2. 异步回调中把数据添加到列表后，仅通知了适配器刷新，没有重新执行控件显隐逻辑，数据加载完成后UI状态不会同步更新
3. onCreate中多次调用EventChangeListener()会重复注册监听器，导致后续数据变化时回调多次，列表重复添加数据

修复步骤 #

1. 抽取UI状态更新公共方法 #

新增独立的显隐逻辑方法，方便数据变更时重复调用：

// 需先将addMed、addHM、recyclerView、myArrayList设为Activity成员变量
private void updateUIVisibility() {
    if (myArrayList.isEmpty()) {
        recyclerView.setVisibility(View.GONE);
        addMed.setVisibility(View.VISIBLE);
        addHM.setVisibility(View.VISIBLE);
    } else {
        recyclerView.setVisibility(View.VISIBLE);
        addMed.setVisibility(View.GONE);
        addHM.setVisibility(View.GONE);
    }
}

2. 精简onCreate逻辑 #

删除冗余的重复监听调用和初始化时的长度判断：

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_home_page);
    // 按钮声明移到成员变量位置，不要做局部变量
    addMed = (Button) findViewById(R.id.add_medications_btn);
    addHM = (Button) findViewById(R.id.add_measurements_btn);
    rootAuthen = FirebaseAuth.getInstance();

    recyclerView = findViewById(R.id.recyclerViewHome);
    recyclerView.setHasFixedSize(true);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));
    myArrayList = new ArrayList<medication_info>();
    myAdapter = new myHomeAdpater(home_page.this, myArrayList);
    recyclerView.setAdapter(myAdapter);
    
    // 初始化默认无数据，先更新一次UI
    updateUIVisibility();
    // 仅注册一次监听即可
    EventChangeListener();
}

3. 修改异步回调逻辑 #

每次数据加载完成后触发UI更新，同时优化重复数据问题：

private void EventChangeListener() {
    FirebaseUser currentFirebaseUser = FirebaseAuth.getInstance().getCurrentUser();
    fstore.document("users/"+currentFirebaseUser.getUid()).collection("New Medications")
            .orderBy("Medication", Query.Direction.ASCENDING).addSnapshotListener(new EventListener<QuerySnapshot>() {
        @Override
        public void onEvent(@Nullable QuerySnapshot queryDocumentSnapshots, @Nullable FirebaseFirestoreException e) {
            if(e != null)
            {
                if(progressDialog.isShowing())
                    progressDialog.dismiss();
                Log.e("Firestore error", e.getMessage());
                return;
            }
            // 先清空旧数据，避免列表重复添加
            myArrayList.clear();
            for(DocumentChange dc : queryDocumentSnapshots.getDocumentChanges())
            {
                if(dc.getType() == DocumentChange.Type.ADDED) {
                    medication_info m = dc.getDocument().toObject(medication_info.class);
                    m.setId(dc.getDocument().getId());
                    myArrayList.add(m);
                }
            }
            // 所有数据处理完成后再统一刷新和更新UI
            myAdapter.notifyDataSetChanged();
            updateUIVisibility();
            if(progressDialog.isShowing()) {
                progressDialog.dismiss();
            }
        }
    });
}

内容的提问来源于stack exchange，提问作者Lance Quinto