ViewModelFactory

ViewModelFactory.kt

/*
 *    Copyright 2019 Danny Lamarti
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *        http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
class ViewModelFactory(block: Builder.() -> Unit = {}) : ViewModelProvider.NewInstanceFactory() {

    private val viewModels = Builder()
        .apply(block)
        .viewModels
        .toMap()
    private var params = Params()

    fun withParams(vararg params: Any?): ViewModelFactory =
        also {
            it.params = params.toList().let(::Params)
        }

    @Suppress("UNCHECKED_CAST")
    override fun <T : ViewModel?> create(modelClass: Class<T>): T =
        try {
            viewModels[modelClass]
                ?.invoke(params) as T
                ?: super.create(modelClass)
        } finally {
            params = Params()
        }

    inner class Builder {

        val viewModels = mutableMapOf<Class<*>, (Params) -> ViewModel>()

        inline fun <reified T : ViewModel> viewModel(crossinline block: (params: Params) -> T) =
            viewModels.set(T::class.java, { block(it) })

    }

    @Suppress("UNCHECKED_CAST")
    data class Params(private val params: List<Any?> = emptyList()) {

        operator fun <T> component1() = get<T>(0)
        operator fun <T> component2() = get<T>(1)
        operator fun <T> component3() = get<T>(2)
        operator fun <T> component4() = get<T>(3)
        operator fun <T> component5() = get<T>(4)
        operator fun <T> component6() = get<T>(5)

        operator fun <T> get(index: Int) = params[index] as T

    }

    interface Owner {

        val factory: ViewModelFactory

    }

}

ViewModelFactoryExample.kt

/*
 *    Copyright 2019 Danny Lamarti
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *        http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */


import androidx.fragment.app.Fragment
import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProviders

class TestViewModel : ViewModel()
class ParameteredViewModel(val id: String) : ViewModel()

fun example(fragment: Fragment) {
    val factory = ViewModelFactory {
        viewModel { TestViewModel() }
        viewModel { (id: String) -> ParameteredViewModel(id) } // Gets the id from the UI
    }

    val testViewModel: TestViewModel = ViewModelProviders
            .of(fragment, factory)
            .get(TestViewModel::class.java)

    val parameteredViewModel: TestViewModel = ViewModelProviders
            .of(fragment, factory.withParams("IDENTIFIER")) // UI passes the id argument
            .get(TestViewModel::class.java)
}