Android Room：如何将来自多个SQL查询的数据合并到一个ViewModel中

Question

@Query("SELECT * from message_table WHERE date<:my_date ORDER BY date DESC LIMIT :offset")
LiveData<List<Message>> getOldMessages(long my_date, int offset);

@Query("SELECT * from message_table WHERE date>:my_date ORDER BY date DESC")
LiveData<List<Message>> getNewMessages(long my_date);

null

在messaging room活动中，我以以下方式观察LiveData：

    LiveData<List<Message>> newMessagesLiveData = mMessageViewModel.getNewMessages();
    newMessagesLiveData.observe(this, new Observer<List<Message>>() {
        @Override
        public void onChanged(@Nullable final List<Message> messages) {
            adapter.setMessages(messages);
            RecyclerView recyclerView = findViewById(R.id.recyclerview);
            recyclerView.scrollToPosition(adapter.getItemCount()-1);
        }
    });

然后适配器接收更改并自我更新：

public void setMessages(List<Message> messages){
    mMessages = messages;
    notifyDataSetChanged();
}
@Override
public void onBindViewHolder(MessageViewHolder holder, int position) {
    position = mMessages.size()-1-position;
    if (mMessages != null) {
        Message current = mMessages.get(position);
        holder.messageItemView.setText(current.getMessage());
    } else {
        holder.messageItemView.setText("No messages to display");
    }
}

Answer 1

我在帮助你选择1。你可以联合同一个表。

输入聊天时，将当前时间保存在本地变量中，如

long currentTime = System.currentTimeMillis();

现在需要合并两个查询。一个查询来自currentTime的最后偏移消息，一个查询大于currentTime的消息。

       @Query("SELECT * FROM (" +
        "SELECT * from message_table WHERE date<:my_date ORDER BY date DESC LIMIT :offset) UNION SELECT * from (SELECT * from message_table WHERE date>=:my_date ORDER BY date DESC)")
LiveData<List<Message>> getMessages(long my_date, int offset);