ViewModel与LiveData的简单使用

使用注意点

LiveData与ViewModel一般都是配合着一起使用。

第一步：创建ViewModel类

package com.example.jetpackdemo03;

import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;

public class NameViewModel extends ViewModel {

    private MutableLiveData<String> currentName;
    public int i;

    public MutableLiveData<String> getCurrentName() {
        if (currentName == null) {
            currentName = new MutableLiveData<>();
        }
        return currentName;
    }
}

第二步：实例化 ViewModel 、创建监听者、绑定监听者

package com.example.jetpackdemo03;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    private NameViewModel model;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //获取控件
        Button btn = findViewById(R.id.btn);
        TextView tv = findViewById(R.id.tv);

        // ViewModel 实例的创建
        ViewModelProvider.AndroidViewModelFactory instance = ViewModelProvider.AndroidViewModelFactory.getInstance(getApplication());
        model = new ViewModelProvider(this, instance).get(NameViewModel.class);

        //需要一个观察者来观察数据
        Observer observer = new Observer<String>() {
            @Override
            public void onChanged(String string) {
                tv.setText(string);
            }
        };

        //订阅
        model.getCurrentName().observe(this, observer);

        //设置点击事件
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                model.getCurrentName().setValue("wustyq" + (model.i++));
            }
        });

    }

}

优化 LiveData 的管理

• 使用单例模式

package com.example.jetpackdemo03;

import androidx.lifecycle.MutableLiveData;

import java.util.HashMap;
import java.util.Map;

public class LiveDataBus {

    //存放订阅者
    private Map<String, MutableLiveData<Object>> bus;

    private static LiveDataBus liveDataBus = new LiveDataBus();

    public LiveDataBus() {
        bus = new HashMap<>();
    }

    private static LiveDataBus getInstance() {
        return liveDataBus;
    }

    //注册订阅者
    public synchronized <T> MutableLiveData<T> with(String key, Class<T> type) {
        if (!bus.containsKey(key)) {
            bus.put(key, new MutableLiveData<Object>());
        }

        return (MutableLiveData) bus.get(key);
    }
}