1.基本概念

1.1组件介绍

ViewModel: 存放和管理UI界面就是View中的数据

LiveData: 给数据添加一个观察者，当数据发生变动时，自动刷新UI界面上的数据

DataBinding: 管理UI界面的控件，就不需要要findViewById之类，同时可以将数据和一定操作回绑到view中

通过上面这三个组件就能够使activity或fragment只需要专注业务逻辑本身

1.2Demo介绍

1.2.1UI设计

一个textView显示一个数值

两个Button，分别实现对数值进行加一和减一

1.2.2项目结构

2.修改build.gradle(Module:app)文件

启动dataBinding：dataBinding.enabled = true

添加依赖：implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'

修改之后代码如下：

apply plugin: 'com.android.application'

apply plugin: 'kotlin-android'

apply plugin: 'kotlin-android-extensions'

android {

compileSdkVersion 29

buildToolsVersion "29.0.2"

defaultConfig {

applicationId "com.example.kotlinviewmodeldemo"

minSdkVersion 15

targetSdkVersion 29

versionCode 1

versionName "1.0"

testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

//启动dataBinding

dataBinding.enabled = true

}

buildTypes {

release {

minifyEnabled false

proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'

}

}

}

dependencies {

implementation fileTree(dir: 'libs', include: ['*.jar'])

implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

implementation 'androidx.appcompat:appcompat:1.1.0'

implementation 'androidx.core:core-ktx:1.1.0'

implementation 'androidx.constraintlayout:constraintlayout:1.1.3'

testImplementation 'junit:junit:4.12'

androidTestImplementation 'androidx.test.ext:junit:1.1.1'

androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

//添加依赖

implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'

}

3.继承ViewModel创建自己的ViewModel

package com.example.kotlinviewmodeldemo

import androidx.lifecycle.MutableLiveData

import androidx.lifecycle.ViewModel

class SelfViewModel : ViewModel() {

//声明一个私有变量number

private val _number: MutableLiveData by lazy {

//MutableLiveData是LiveData的一个容器，Int是数据类型

MutableLiveData().also {

it.value = 0

}

}

val number: MutableLiveData get() = _number

fun modifyNumber(n: Int) {

_number.value = _number.value?.plus(n)

}

}

4.修改xml文件

4.1选中constraintlayout，会出现一个黄色的小灯泡，点击它

4.2点击，转成binging Layout文件

4.3在xml文件中添加代码

引用变量：android:text="@{String.valueOf(data.number)} //data.number是Int型，要先转化成字符串 "

引用方法：android:onClick="@{()->data.modifyNumber(-1)} //这是一个监听事件，引用方法"

4.4完整的activity_main.xml文件如下

xmlns:app="http://schemas.android.com/apk/res-auto"

xmlns:tools="http://schemas.android.com/tools">

name="data"

type="com.example.kotlinviewmodeldemo.SelfViewModel" />

android:layout_width="match_parent"

android:layout_height="match_parent"

tools:context=".MainActivity">

android:id="@+id/textView"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:textSize="60sp"

android:text="@{String.valueOf(data.number)}"

app:layout_constraintBottom_toBottomOf="parent"

app:layout_constraintEnd_toEndOf="parent"

app:layout_constraintHorizontal_bias="0.5"

app:layout_constraintLeft_toLeftOf="parent"

app:layout_constraintRight_toRightOf="parent"

app:layout_constraintStart_toStartOf="parent"

app:layout_constraintTop_toTopOf="parent"

app:layout_constraintVertical_bias="0.25" />

android:id="@+id/buttonLess"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:layout_marginTop="80dp"

android:text="-1"

android:onClick="@{()->data.modifyNumber(-1)}"

app:layout_constraintEnd_toEndOf="parent"

app:layout_constraintStart_toStartOf="@+id/guideline"

app:layout_constraintTop_toBottomOf="@+id/textView" />

android:id="@+id/buttonPlus"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:layout_marginTop="80dp"

android:text="+1"

android:onClick="@{()->data.modifyNumber(1)}"

app:layout_constraintEnd_toStartOf="@+id/guideline"

app:layout_constraintStart_toStartOf="parent"

app:layout_constraintTop_toBottomOf="@+id/textView" />

android:id="@+id/guideline"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:orientation="vertical"

app:layout_constraintGuide_percent="0.5" />

5.配置activity

package com.example.kotlinviewmodeldemo

import android.os.Bundle

import androidx.appcompat.app.AppCompatActivity

import androidx.databinding.DataBindingUtil

import androidx.lifecycle.ViewModelProvider

import com.example.kotlinviewmodeldemo.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {

override fun onCreate(savedInstanceState: Bundle?) {

super.onCreate(savedInstanceState)

// setContentView(R.layout.activity_main)

//ViewModel绑定

val selfViewModel: SelfViewModel = ViewModelProvider(this).get(SelfViewModel::class.java)

//dataBinding 将View上的控件绑定到binding

val binding: ActivityMainBinding = DataBindingUtil.setContentView(this, R.layout.activity_main)

binding.data = selfViewModel //给xml文件中的data赋值

binding.lifecycleOwner = this //绑定生命周期

}

}

6.github源码

https://github.com/YDDUONG/jetpack_ViewModel_LiveData_DataBinding