在 activity android 中传递对 ViewModel 的引用的好方法

Question

我有一个从基础 class 继承的 ViewModel，我希望相应的 Activity 也从基础 class 继承。 activity 每次都会调用派生的 ViewModel 的相同方法。所以它会是这样的：

BaseViewModel:

abstract class BaseViewModel(application: Application) : AndroidViewModel(application) {
    protected val context = getApplication<Application>().applicationContext

    protected var speechManager: SpeechRecognizerManager? = null

    var _actionToTake : MutableLiveData<AnalyseVoiceResults.Actions> = MutableLiveData()
    var actionToTake : LiveData<AnalyseVoiceResults.Actions> = _actionToTake

    open fun stopListening() {
        if (speechManager != null) {
        speechManager?.destroy()
        speechManager = null
    }

    open fun startListening() {
        val isListening = speechManager?.ismIsListening() ?: false
        if (speechManager == null) {
           SetSpeechListener()
        } else if (!isListening) {
           speechManager?.destroy()
           SetSpeechListener()
        }

    }
}

基础Activity

class BaseActivity : AppCompatActivity() {

    private lateinit var baseViewModel: BaseViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
    }

    fun goback() {
        super.onBackPressed()
        baseViewModel.stopListening()
        finish()
    }

    fun startListening() { 
        baseViewModel.startListening()
    }

    override fun onDestroy() {
        super.onDestroy()
        baseViewModel.stopListening()
    }
}

派生Activity:

class DerivedActivity : BaseActivity() {

    private val nextActivityViewModel: NextActivityViewModel by inject()
                                    ///^^inherits from BaseViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)


        /*** pass reference ***/
        baseViewModel = nexActivityViewModel

        nextActivityViewModel.actionToTake.observe(this, object : Observer<AnalyseVoiceResults.Actions?> {
            override fun onChanged(t: AnalyseVoiceResults.Actions?) {
                if (t?.equals(AnalyseVoiceResults.Actions.GO_BACK) ?: false) {
                    goback()
                }
            }
         })

        startListening()
    }
}

这是否会导致内存泄漏，为此 activity 有两个视图模型实例？有一个更好的方法吗？我不想为我的所有活动重复相同的代码。（如果我用一个碱基片段做这件事，我也会有同样的问题）。

Answer 1

使这个 var baseViewModel: BaseViewModel 成为一个抽象变量，所有子 class 都必须覆盖它。因此，当您调用 startListening 和 stopListening 时，这些方法将从子实现中调用。

编辑：

将 BaseActivity 设为抽象 class，将 baseViewModel 设为抽象变量

abstract class BaseActivity : AppCompatActivity() {

    private abstract var baseViewModel: BaseViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
    }

    fun goback() {
        super.onBackPressed()
        baseViewModel.stopListening()
        finish()
    }

    fun startListening() { 
        baseViewModel.startListening()
    }

    override fun onDestroy() {
        super.onDestroy()
        baseViewModel.stopListening()
    }
}

因此，您的 DerivedActivity 必须覆盖 baseViewModel，每次调用父亲的 class 都会触发子

class DerivedActivity : BaseActivity() {

    override val baseViewModel: NextActivityViewModel by inject()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

Answer 2

Will this cause memory leaks to have two instances of a view model for this activity?

不，这种方法没有内存泄漏。您也没有相同 activity 的 2 个 ViewModel 实例。它是 ViewModel 的单个实例，在 BaseActivity 和 BaseViewModel 中被不同的变量引用。

Is there a better way to do this?

我看到的第一个问题是您在 ViewModels 中有 Android 特定代码，这不是一个好的做法。您应该将语音管理器代码移动到基础 activity 本身，并且 ViewModel 应该只保存您希望在方向更改时保留的 "state" 数据。这将确保所有语音管理方法（创建、恢复、销毁）都在基础 activity 中。具体 activity 只有在状态改变时才会有观察者。

如果您遵循任何架构模式（如 MVP），一旦您将 Speech Manager 代码移出到 activity，将其进一步移出到 Presenter 将变得显而易见。

编辑：我没有在生产中使用 MVVM 模式，但这是您可能想要的轻型变体：

基本思想是将语音管理代码移动到生命周期感知组件中。 view/activity 中的所有 UI 代码和视图模型中的业务逻辑/非 android 状态。根据您目前分享的要求，我认为使用基础 activity 或视图模型没有意义。

/**
 * All the speech related code is encapsulated here, so any new activity/fragment can use it by registering it's lifecycle
 */

class SpeechManager(private val context: Context): LifecycleObserver {

    val TAG = "SpeechManager"

    private var speechRecognizer: SpeechRecognizer? = null

    fun registerWithLifecycle(lifecycle: Lifecycle) {
        Log.e(TAG, "registerWithLifecycle")
        lifecycle.addObserver(this)
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun start() {
        Log.e(TAG, "start")
        speechRecognizer = (speechRecognizer ?: SpeechRecognizer.createSpeechRecognizer(context)).apply {

//            setRecognitionListener(object : RecognitionListener {
//                //implement methods
//            })
        }
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun stop() {
        Log.e(TAG, "stop")
        speechRecognizer?.run {
            stopListening()
            destroy()
        }
    }
}

视图模型：

class SpeechViewModel: ViewModel() {

    val TAG = "SpeechViewModel"

    //List all your "data/state" that needs to be restores across activity restarts
    private val actions: MutableLiveData<Actions> = MutableLiveData<Actions>().apply { value = Actions.ActionA }

    //Public API for getting observables and all use-cases
    fun getActions() = actions
    fun doActionA(){
        //validations, biz logic
        Log.e(TAG, "doActionA")
        actions.value = Actions.ActionA
    }
    fun doActionB(){
        Log.e(TAG, "doActionB")
        actions.value = Actions.ActionB
    }
}

sealed class Actions{
    object ActionA: Actions()
    object ActionB: Actions()
}

Activity/View:

class SpeechActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_speech)
        setSupportActionBar(toolbar)
        initSpeechManager()
    }

    private lateinit var speechManager: SpeechManager
    private lateinit var speechViewModel: SpeechViewModel

    /**
     * Register lifecycle aware components and start observing state changes from ViewModel.
     * All UI related code should ideally be here (or your view equivalent in MVVM)
     */
    private fun initSpeechManager() {
        speechManager = SpeechManager(this).apply {registerWithLifecycle(lifecycle)}
        speechViewModel = ViewModelProviders.of(this).get(SpeechViewModel::class.java).apply {
            getActions().observe(this@SpeechActivity, Observer<Actions>{
                when(it){
                    is Actions.ActionA -> {
                        Log.e(TAG, "Perform ActionA")
                        speechManager.start()
                    }
                    is Actions.ActionB -> {
                        Log.e(TAG, "Perform ActionB")
                        speechManager.stop()
                        super.onBackPressed()
                    }
                }
            })
        }
    }
}

在 activity android 中传递对 ViewModel 的引用的好方法

Good way to pass a reference to a ViewModel in an activity android

android

pass-by-reference

android-viewmodel