Andriod --- JetPack (四)：BaseObservable 与 ObservableField 双向绑定

1.Andriod — JetPack (一)：初识 JetPack

2.Andriod — JetPack (二)：LifeCycle 的诞生

3.Andriod — JetPack (三)：ViewModel 的诞生

4.Andriod — JetPack (四)：BaseObservable 与 ObservableField 双向绑定

5.Andriod — JetPack (五)：DataBinding + LiveData +ViewModel 简单实例

6.Andriod — JetPack (六)：Room 增删改查

7.Andriod — JetPack (七)：Room + ViewModel + LiveData 增删改查实例

一、什么叫单向绑定与双向绑定

单向绑定：对象变化的时候 TextView 自动更新
双向绑定：对象变化的时候 EditView 自动更新。EditText 变化的时候对象的值也发生变化。

二、BaseObservable 双向绑定代码实例

build.gradle

android {
    compileSdkVersion 29
    buildToolsVersion "30.0.3"
    dataBinding {
        enabled = true
    }
}

MainActivity.java

import androidx.appcompat.app.AppCompatActivity;
import androidx.databinding.DataBindingUtil;
import androidx.databinding.ViewDataBinding;

import android.os.Bundle;

import com.example.baseobservable.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ActivityMainBinding activityMainBinding = DataBindingUtil.setContentView(this, R.layout.activity_main);
        activityMainBinding.setUserViewModel(new UserViewModel());
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">

    <data>
        <variable
            name="userViewModel"
            type="com.example.baseobservable.UserViewModel" />
    </data>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

        <EditText
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@={userViewModel.userName}"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

User.java

package com.example.baseobservable;

public class User {
    public String userName;

    public User(String userName) {
        this.userName = userName;
    }
}

UserViewModel.java

package com.example.baseobservable;

import android.util.Log;

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

// 并不是真正的 ViewModel
public class UserViewModel extends BaseObservable {
    private User user;

    public UserViewModel() {
        this.user = new User("Jack");
    }

    /*
     * 用 Bindable 进行绑定
     * 只要对象发生变化 EditText 就会发生变化
     */
    @Bindable
    public String getUserName() {
        return user.userName;
    }

    // EditText 中的内容发生变化就会调用 setUserName
    public void setUserName(String userName) {
        if(userName != null && !userName.equals(user.userName)) {
            user.userName = userName;
            Log.d("liang", userName);
            notifyPropertyChanged(BR.userName); // 通知属性发生变化 , build 之后自动生成
        }
    }
}

结果：

三、ObservableField 双向绑定代码实例

需要修改的文件：
UserViewModel.java

package com.example.baseobservable;

import android.util.Log;

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

// 并不是真正的 ViewModel
public class UserViewModel {
    private ObservableField<User> userObservableField;

    public UserViewModel() {
        User user = new User("Jack");
        userObservableField = new ObservableField<>();
        userObservableField.set(user); // userObservableField 与 user 关联起来
    }

    public String getUserName() {
        return userObservableField.get().userName;
    }
    public void setUserName(String userName) {
        Log.d("liang", userName);
        userObservableField.get().userName = userName;
    }

}