如何使用Data Binding在ListView中展示输入结果

核心问题点 #

• 点击按钮新增列表项时，错误读取新建空Model的null值赋值，没有获取EditText绑定的用户输入内容
• 每次点击都重新创建Adapter实例，且ViewModel的adapter字段未声明可观察，Data Binding无法感知适配器变更
• 自定义Adapter的getView方法存在空指针风险，且错误调用父类getView方法返回默认视图，没有使用Data Binding渲染的列表项布局
• Model类未实现数据观察接口，双向绑定的文本同步存在异常

分步修复方案 #

1. 修正MainActivity点击事件逻辑 #

初始化时一次性创建Adapter实例，点击事件中正确读取输入内容、新增列表项后通知数据刷新即可，无需重复创建Adapter：

private Model model = new Model();
private Handlers handlers = new Handlers();
private ViewModel viewModel = new ViewModel(model);
private ArrayList<Model> listItem = new ArrayList<>();
ActivityMainBinding mainBinding = DataBindingUtil.setContentView(this,R.layout.activity_main);

// 初始化阶段创建适配器，不要在点击事件中重复创建
viewModel.setAdapter(new CustomAdapter(MainActivity.this, listItem));

handlers.setOnClickListener(new MyView.onClickListener() {
    @Override
    public void onDamn(Model inputModel) {
        Model newItem = new Model();
        // 正确获取输入框绑定的用户输入内容
        newItem.setText(inputModel.getText());
        listItem.add(newItem);
        // 通知适配器数据刷新
        viewModel.getAdapter().notifyDataSetChanged();
    }
});

mainBinding.setModel(model);
mainBinding.setHandlers(handlers);
mainBinding.setViewModel(viewModel);

2. 修正ViewModel的adapter字段可观察配置 #

给adapter字段添加@Bindable注解，赋值时通知属性变更，保证Data Binding能识别到适配器的绑定：

package com.example.list;

import androidx.databinding.BaseObservable;
import androidx.databinding.Bindable;

public class ViewModel extends BaseObservable {
    private Model model;
    public ViewModel(Model model){
        this.model = model;
    }
    public void setModelText(String text){
        model.setText(text);
        notifyPropertyChanged(BR.modelText);
    }
    @Bindable
    public String getModelText(){
        return model.getText();
    }

    private CustomAdapter adapter;

    public void setAdapter(CustomAdapter adapter) {
        this.adapter = adapter;
        notifyPropertyChanged(BR.adapter);
    }

    @Bindable
    public CustomAdapter getAdapter() {
        return adapter;
    }
}

3. 修正CustomAdapter的getView渲染逻辑 #

复用列表项时通过Tag获取已有的Binding实例避免空指针，移除父类getView调用，直接返回Data Binding渲染的视图，添加executePendingBindings保证数据及时刷新：

package com.example.list;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.databinding.DataBindingUtil;

import com.example.list.databinding.ActivityItemBinding;

import java.util.ArrayList;

public class CustomAdapter extends ArrayAdapter<Model> {
    private ArrayList<Model> listItem;

    public CustomAdapter(Context context, ArrayList<Model> listItem){
        super(context,0,listItem);
        this.listItem = listItem;
    }

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
        ActivityItemBinding itemBinding;
        Model model = listItem.get(position);

        if (convertView == null){
            itemBinding = DataBindingUtil.inflate(LayoutInflater.from(parent.getContext()),R.layout.activity_item,parent,false);
            convertView = itemBinding.getRoot();
            convertView.setTag(itemBinding);
        } else {
            itemBinding = (ActivityItemBinding) convertView.getTag();
        }
        ViewModel itemViewModel = new ViewModel(model);
        itemViewModel.setModelText(model.getText());
        itemBinding.setViewModel(itemViewModel);
        // 立即执行绑定，避免列表滑动时数据错乱
        itemBinding.executePendingBindings();
        return convertView;
    }

    @Override
    public int getCount() {
        return listItem.size();
    }

    @Nullable
    @Override
    public Model getItem(int position) {
        return listItem.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }
}

4. 修正Model类的双向绑定支持 #

让Model继承BaseObservable，给text字段添加可观察注解，保证双向绑定数据同步正常：

package com.example.list;

import androidx.databinding.BaseObservable;
import androidx.databinding.Bindable;

public class Model extends BaseObservable {
    private String text;

    @Bindable
    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
        notifyPropertyChanged(BR.text);
    }
}

修复后EditText输入的内容会在点击按钮后正常添加到ListView中展示，不会出现空值、不刷新的问题。

内容的提问来源于stack exchange，提问作者Aloyrd Vey