Android架構組件——簡單運用(Kotlin)

概述

前面介紹了架構組件中Lifcycle、LiveData、ViewModel以及Room的相關知識，在看了谷歌的例子後用kotlin簡單寫一下實際應用，直接上代碼

環境依賴

project的build.gradle文件：

buildscript {
    ext.kotlin_version = '1.1.51'
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

module的build.gradle文件：

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

android {
    compileSdkVersion 26
    defaultConfig {
        applicationId "example.rxx.kotlin"
        minSdkVersion 21
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation"org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"
    implementation 'com.android.support:appcompat-v7:26.1.0'
    implementation 'com.android.support.constraint:constraint-layout:1.0.2'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.1'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'

    // ViewModel and LiveData
    implementation "android.arch.lifecycle:extensions:1.1.0"
    // alternatively, just ViewModel
    implementation "android.arch.lifecycle:viewmodel:1.1.0"
    // alternatively, just LiveData
    implementation "android.arch.lifecycle:livedata:1.1.0"

    annotationProcessor "android.arch.lifecycle:compiler:1.1.0"

    // Room (use 1.1.0-alpha3 for latest alpha)
    implementation "android.arch.persistence.room:runtime:1.0.0"
    annotationProcessor "android.arch.persistence.room:compiler:1.0.0"

    // Paging
    implementation "android.arch.paging:runtime:1.0.0-alpha6"

    // Test helpers for LiveData
    testImplementation "android.arch.core:core-testing:1.1.0"

    // Test helpers for Room
    testImplementation "android.arch.persistence.room:testing:1.0.0"

    implementation "android.arch.lifecycle:common-java8:1.1.0"

    // RxJava support for Room (use 1.1.0-alpha3 for latest alpha)
    implementation "android.arch.persistence.room:rxjava2:1.0.0"

    // ReactiveStreams support for LiveData
    implementation "android.arch.lifecycle:reactivestreams:1.1.0"

}

創建數據源

新建一個user表：

@Entity(tableName = "user")
data class User(@PrimaryKey
                @ColumnInfo(name = "userId")
                val id: String = UUID.randomUUID().toString(),
                @ColumnInfo(name = "userName")
                val userName: String)

新建一個UserDao用來操作user表：

@Dao
interface UserDao {

    // 配合LiveData使用
    @Query("SELECT * FROM user WHERE userId = :id")
    fun getUserById(id: String): MutableLiveData<User>

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun insertUser(user: User)

    @Delete
    fun deleteAllUsers()

}

新建一個數據庫：

@Database(entities = arrayOf(User::class), version = 1)
abstract class UsersDatabase : RoomDatabase() {

    abstract fun userDao(): UserDao

    companion object {

        @Volatile private var INSTANCE: UsersDatabase? = null

        // 單例
        fun getInstance(context: Context) =
                INSTANCE ?: synchronized(this) {
                    INSTANCE ?: buildDatabase(context).also { INSTANCE = it }
                }

        // 新建數據庫
        private fun buildDatabase(context: Context) =
                Room.databaseBuilder(context.applicationContext,
                        UsersDatabase::class.java, "Sample.db")
                        .build()
    }
}

創建ViewModel

創建一個ViewModel保存UI數據：

class UserViewModel(private val dataSource: UserDao) : ViewModel() {

    companion object {
        const val USER_ID = "1"
    }

    private lateinit var user: MutableLiveData<User>

    fun getUser(): MutableLiveData<User> {
        return dataSource.getUserById(USER_ID).also { user = it }
    }

    fun get(): MutableLiveData<User> {
        if (user == null) {
            user = MutableLiveData()
        }
        return user
    }

    fun updateUserName(userName: String): Completable {
        return Completable.fromAction {
            val newUser = User(USER_ID, userName)
            dataSource.insertUser(newUser)
            user.value = newUser
        }
    }
}

自定義一個ViewModelFactory：

class UserViewModelFactory(private val dataSource: UserDao) : ViewModelProvider.Factory {

    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(UserViewModel::class.java)) {
            return UserViewModel(dataSource) as T
        }
        throw IllegalArgumentException("Unknown ViewModel Class")
    }

}

提供創建factory的初始化方法：

object Injection {

    fun provideUserDataSource(context: Context): UserDao {
        val database = UsersDatabase.getInstance(context)
        return database.userDao()
    }

    fun provideViewModelFactory(context: Context): UserViewModelFactory {
        val dataSource = provideUserDataSource(context)
        return UserViewModelFactory(dataSource)
    }
}

在UI層使用（即activity等）：

class MainActivity : AppCompatActivity() {

    private lateinit var viewModelFactory: UserViewModelFactory

    private lateinit var viewModel: UserViewModel

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

        viewModelFactory = Injection.provideViewModelFactory(this)
        viewModel = ViewModelProviders.of(this, viewModelFactory).get(UserViewModel::class.java)
        viewModel.getUser().observe(this, Observer { tv_test.text = it?.userName })
        tv_test.setOnClickListener { updateUserName() }
    }

    private fun updateUserName() {
        val userName = "change"
        tv_test.isEnabled = false
        viewModel.updateUserName(userName)
    }

    companion object {
        private val TAG = MainActivity::class.java.simpleName
    }
}

注意： ViewModel中LiveData保持的數據只要有變化，activity中該LiveData的observe就會回調

總結

這就是簡單的使用流程，後續會使用MvpClean架構來優化

合理化建議

我們藉助Goole組件開發的同時，也可以藉助其他三方庫來輔助我們開發出更好的應用, 開發者技術前線建議：

• LiveData在某些情況下可使用RxJava2代替。

• Lifecyle等生命週期管理我們可以藉助RxLifeCyle。

• 數據層官方推薦使用Room或者Realm，等其他ORM皆可。

• 網絡請求庫推薦使用Retrofit+Okhttp

• 多層之間解耦合，推薦使用服務發現(Service Locator) 或者依賴注入(DI)，推薦Dagger2。

• Modle和View綁定我們可以使用DataBinding進行快速實現

• 在使用組件架構時候，推薦使用MvpClean，切記不要Mvp，Mvp，Mvp！