Android 列表适配器未使用 Live Data、ViewHolder 和 ViewModelProvider 显示所有数据

Question

我正在学习本教程并在旅途中根据我的项目调整内容：https://codelabs.developers.google.com/codelabs/android-training-livedata-viewmodel/index.html?index=..%2F..android-training#0

我有一个包含我想在活动中显示的对话列表的数据库，单击它们应该可以让我进入对话屏幕。例如，3 个对话应该与 whatsapp 主屏幕完全一样，问题是即使很难，我可以在对话变量中看到更多对话历史条目，而回收视图中只显示一个。

活动代码：

public class activity_conversations_list extends AppCompatActivity {

    private ChatViewModel chatViewModel;

    public void startConversation(View view) {
        //click on conversation
        Intent intent = new Intent(this, activity_conversation.class);
        startActivity(intent);
    }

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

        RecyclerView recyclerView = findViewById(R.id.recyclerViewConversations);
        final ConversationListAdapter adapter = new ConversationListAdapter(this);
        recyclerView.setAdapter(adapter);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));

        chatViewModel = ViewModelProviders.of(this).get(ChatViewModel.class);

        chatViewModel.getConversations().observe(this, new Observer<List<Conversation>>() {
            @Override
            public void onChanged(@Nullable final List<Conversation> conversations) {
                // Update the cached copy of the words in the adapter.
                adapter.setConversations(conversations);
            }
        });
    }
}

对话列表活动的xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".activity_conversations_list">

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/recyclerViewConversations"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:scrollbars="vertical" />
</LinearLayout>

列出适配器类：

public class ConversationListAdapter
        extends RecyclerView.Adapter<ConversationListAdapter.ConversationItemViewHolder> {

    //conversations list
    private List<Conversation> conversations;
    private final LayoutInflater layoutInflater;

    public ConversationListAdapter(Context context) { layoutInflater = LayoutInflater.from(context); }

    @Override
    public ConversationItemViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View itemView = layoutInflater.inflate(R.layout.view_conversation_list_item, parent, false);
        return new ConversationItemViewHolder(itemView);
    }

    @Override
    public void onBindViewHolder(ConversationItemViewHolder holder, int position) {
        if (conversations != null) {
            Conversation current = conversations.get(position);
            holder.textViewConversationName.setText(current.getConversation_name());
        } else {
            // Covers the case of data not being ready yet.
            holder.textViewConversationName.setText("No conversations");
        }
    }

    public void setConversations(List<Conversation> conversations){
        this.conversations = conversations;
        notifyDataSetChanged();
    }

    @Override
    public int getItemCount() {
        if (conversations != null)
            return conversations.size();
        else return 0;
    }

    class ConversationItemViewHolder extends RecyclerView.ViewHolder {
        private final TextView textViewConversationName;

        private ConversationItemViewHolder(View itemView) {
            super(itemView);
            textViewConversationName = itemView.findViewById(R.id.textViewConversationName);
        }
    }
}

列表项的xml：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/linearLayoutConversationListItem"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/textViewConversationName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:clickable="true"
        android:focusable="auto"
        android:onClick="startConversation"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

更改列表项的 xml 到下面的一个

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/linearLayoutConversationListItem"
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/textViewConversationName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:clickable="true"
        android:focusable="auto"
        android:onClick="startConversation"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

你的项目在那里，只是你的项目填满 1 个全屏，如果你开始滚动你会看到它在那里。