前言

話說我真是怠惰啊，各種鴿蠢沿，寫jetpack又寫設(shè)計(jì)模式還寫個(gè)view恤磷，各種挖坑扫步，算了河胎，繼續(xù)慢慢填政敢，這次來聊下ViewModel

ViewModel

在Jetpcak里面，LifeCycles、ViewModel、LiveData這三個(gè)是相輔相成的灾馒，我個(gè)人的理解是ViewModel實(shí)現(xiàn)了LifeCycles的功能，也是有生命周期的感知的，而且它的生命周期更廣(比Activity還要廣)

導(dǎo)入

首先我們要依賴Google倉庫胎食，在project -> build.gradle里

  repositories {
        //一般來說自動(dòng)生成的項(xiàng)目都會(huì)默認(rèn)依賴Google和Jcenter倉庫的
        google()
        jcenter()
    }

然后是Module -> build.gradle

dependencies {
    def lifecycle_version = "2.0.0"

    // ViewModel and LiveData
    implementation "androidx.lifecycle:lifecycle-extensions:$lifecycle_version"
    // alternatively - just ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version" // For Kotlin use lifecycle-viewmodel-ktx
    // alternatively - just LiveData
    implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
    // alternatively - Lifecycles only (no ViewModel or LiveData). Some UI
    //     AndroidX libraries use this lightweight import for Lifecycle
    implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"

    annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version" // For Kotlin use kapt instead of annotationProcessor
    // alternately - if using Java8, use the following instead of lifecycle-compiler
    implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"

    // optional - ReactiveStreams support for LiveData
    implementation "androidx.lifecycle:lifecycle-reactivestreams:$lifecycle_version" // For Kotlin use lifecycle-reactivestreams-ktx

    // optional - Test helpers for LiveData
    testImplementation "androidx.arch.core:core-testing:$lifecycle_version"
}

看注釋按需導(dǎo)入

使用

簡單到爆炸的用法...

class MyViewModel : ViewModel(){
    var name: String = "xiao"
}

在Activity里面調(diào)用

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val model = ViewModelProviders.of(this)[MyViewMode::class.java]
        tv.text = model.name
    }
}

可能會(huì)遇到的問題厕怜， ViewModelProviders.of(this)爆紅，這個(gè)一般是AndroidX的包引起的蕾总，將項(xiàng)目遷移至AndroidX就好了粥航，Refactor -> Migrate to AndroidX

Fragment共享

這個(gè)也是ViewModel解決的痛點(diǎn)之一，of方法里面不僅可以傳Activity對(duì)象生百，也可以傳Fragment對(duì)象递雀，所以在同Activity下的不同F(xiàn)ragment，我們可以傳入相同的Activity對(duì)象來獲得實(shí)例蚀浆，以此解決Fragment之間的通訊問題缀程。比起直接弄個(gè)全局靜態(tài)對(duì)象來通信算是好太多了

//FirstFragment
class FirstFragment : Fragment() {
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.fragment_first, container, false)
    }
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val mode = activity?.let { ViewModelProviders.of(it).get(MyViewMode::class.java) }
        text.text = mode.toString()
    }
}
//SecondFragment
class SecondFragment : Fragment() {
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.fragment_second, container, false)
    }
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val mode = activity?.let { ViewModelProviders.of(it).get(MyViewMode::class.java) }
        text.text = mode.toString()
    }
}

可以看到2個(gè)Fragment拿到的是同一個(gè)對(duì)象

總結(jié)

ViewModel其實(shí)挺簡單的，在我的理解就是一個(gè)全局的靜態(tài)對(duì)象市俊，然后又有了生命周期杨凑，但其實(shí)它帶來的是一種新的思想，MVVM摆昧，也就是數(shù)據(jù)驅(qū)動(dòng)蠢甲，viewModel作為中樞，從數(shù)據(jù)庫獲取數(shù)據(jù)据忘，然后交由liveData通知UI更新，關(guān)于liveData搞糕，我們下期見