Android ViewModel - ViewModel 생성 과정

안드로이드에서 Fragment가 서로 같은 ViewModel을 공유하기도 하며, Configure Change가 발생해도 ViewModel을 유지합니다. ViewModelProvider를 사용하기 때문에 가능한데 어떻게 동작하는지 확인할 것입니다.

 

2021.12.06 - [Android/Lifecycle] - Android ViewModel - Activity가 ViewModel을 유지하는 방법

2021.12.29 - [Android/Lifecycle] - Android ViewModel - Fragment가 ViewModel을 유지하는 방법

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ViewModelProvider 생성

    public constructor(
        owner: ViewModelStoreOwner
    ) : this(owner.viewModelStore, defaultFactory(owner))

    /**
     * Creates `ViewModelProvider`, which will create `ViewModels` via the given
     * `Factory` and retain them in a store of the given `ViewModelStoreOwner`.
     *
     * @param owner   a `ViewModelStoreOwner` whose [ViewModelStore] will be used to
     * retain `ViewModels`
     * @param factory a `Factory` which will be used to instantiate
     * new `ViewModels`
     */
    public constructor(owner: ViewModelStoreOwner, factory: Factory) : this(
        owner.viewModelStore,
        factory
    )

UI Controller(Activity, Fragment)에서 ViewModelProvider를 생성할 때 ViewModelStoreOwner와 Factory를 전달합니다.

 

ViewModelStoreOwner는 ViewModelStore를 참조하는 목적이고, Factory는 ViewModel을 생성하는 목적입니다.

public open class ViewModelProvider(
    private val store: ViewModelStore,
    private val factory: Factory
)

주 생성자로 직접 ViewModelStore와 Factory를 전달할 수 있으며 어떤 방법으로 생성해도 상관없습니다.

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ViewModel 요청

    @MainThread
    public open operator fun <T : ViewModel> get(modelClass: Class<T>): T {
        val canonicalName = modelClass.canonicalName
            ?: throw IllegalArgumentException("Local and anonymous classes can not be ViewModels")
        return get("$DEFAULT_KEY:$canonicalName", modelClass)
    }

    /**
     * Returns an existing ViewModel or creates a new one in the scope (usually, a fragment or
     * an activity), associated with this `ViewModelProvider`.
     *
     * The created ViewModel is associated with the given scope and will be retained
     * as long as the scope is alive (e.g. if it is an activity, until it is
     * finished or process is killed).
     *
     * @param key        The key to use to identify the ViewModel.
     * @param modelClass The class of the ViewModel to create an instance of it if it is not
     * present.
     * @return A ViewModel that is an instance of the given type `T`.
     */
    @Suppress("UNCHECKED_CAST")
    @MainThread
    public open operator fun <T : ViewModel> get(key: String, modelClass: Class<T>): T {
        var viewModel = store[key]
        if (modelClass.isInstance(viewModel)) {
            (factory as? OnRequeryFactory)?.onRequery(viewModel)
            return viewModel as T
        } else {
            @Suppress("ControlFlowWithEmptyBody")
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }
        viewModel = if (factory is KeyedFactory) {
            factory.create(key, modelClass)
        } else {
            factory.create(modelClass)
        }
        store.put(key, viewModel)
        return viewModel
    }

get() 함수를 통해 ViewModel을 요청합니다. ViewModelStore에서 ViewModel이 있는지 없는지 확인하여 있으면 재사용하고 없으면 Factory를 통해 생성 후 ViewModelStore에 저장합니다.

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마무리

1. ViewModelProvider 생성(ViewModelStoreOwner, Factory)

2. ViewModelStore 참조

3. ViewModel 유무 확인

4. Factory를 통한 ViewModel 생성

5. ViewModel 재사용