variable argument ViewModel

前言
vararg
Array
one more thing
another one more thing

前言

implementation 'androidx.fragment:fragment-ktx:1.5.4'
implementation 'androidx.activity:activity-ktx:1.5.1'

Android 官方 Activity/Fragment 扩展库里对 ViewModel 的扩展构造方法无法携带参数，使用不太方便，轮一个支持可变长参数的 ViewModel 扩展方法。

本轮子绝大多数时候都可替代官方的扩展方法（不传构造参数即可），唯一例外的是 SaveState 重建场景，官方默认使用的 SavedStateViewModelFactory 支持 SaveState 重建，由于这类场景过于小众且支持起来比较复杂，故本轮子暂未考虑。

vararg

ViewModelExt.kt

import android.os.Build
import androidx.activity.ComponentActivity
import androidx.activity.viewModels
import androidx.annotation.MainThread
import androidx.fragment.app.Fragment
import androidx.fragment.app.activityViewModels
import androidx.fragment.app.viewModels
import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider

fun argViewModelFactory(vararg args: Any): ViewModelProvider.Factory =
    object : ViewModelProvider.NewInstanceFactory() {
        @Suppress("UNCHECKED_CAST")
        override fun <VM : ViewModel> create(modelClass: Class<VM>): VM {
            modelClass.constructors.filter {
                args.size == if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
                    it.parameterCount
                } else {
                    it.parameterTypes.size
                }
            }.forEach {
                try {
                    return it.newInstance(*args) as VM
                } catch (ignore: Exception) {
                }
            }
            throw InstantiationException("Cannot create an instance of $modelClass")
        }
    }

@MainThread
inline fun <reified VM : ViewModel> Fragment.argViewModels(vararg args: Any) =
    viewModels<VM> { argViewModelFactory(*args) }

@MainThread
inline fun <reified VM : ViewModel> Fragment.activityArgViewModels(vararg args: Any) =
    activityViewModels<VM> { argViewModelFactory(*args) }

@MainThread
inline fun <reified VM : ViewModel> ComponentActivity.argViewModels(vararg args: Any) =
    viewModels<VM> { argViewModelFactory(*args) }

如果扩展函数显式指定了返回类型，则 viewModels 可以省去类型指定（由编译器智能推导）:

@MainThread
inline fun <reified VM : ViewModel> Fragment.argViewModels(vararg args: Any): Lazy<VM> =
    viewModels { argViewModelFactory(*args) }

使用方式：

// 1
private val viewModel: FooViewModel by argViewModels(2, true, "haha")

// 2
private val viewModel by argViewModels<FooViewModel>(2, true, "haha")


// 3
private val args: Array<Any> = arrayOf(2, true, "haha")
private val viewModel: FooViewModel by argViewModels(*args)

// 4
private val args: Array<Any> = arrayOf(2, true, "haha")
private val viewModel by argViewModels<FooViewModel>(*args)


class FooViewModel(a: Int, b: Boolean, c: String) : ViewModel()

Array

使用 vararg 可变参数的函数不能用作高阶函数的参数，将 vararg 替换成 Array 才可以用于高阶函数，当然本文这里不存在高阶函数问题，仅记录该用法：

import android.os.Build
import androidx.activity.ComponentActivity
import androidx.activity.viewModels
import androidx.annotation.MainThread
import androidx.fragment.app.Fragment
import androidx.fragment.app.activityViewModels
import androidx.fragment.app.viewModels
import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider

fun argViewModelFactory(args: Array<out Any>): ViewModelProvider.Factory =
    object : ViewModelProvider.NewInstanceFactory() {
        @Suppress("UNCHECKED_CAST")
        override fun <VM : ViewModel> create(modelClass: Class<VM>): VM {
            modelClass.constructors.filter {
                args.size == if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
                    it.parameterCount
                } else {
                    it.parameterTypes.size
                }
            }.forEach {
                try {
                    return it.newInstance(*args) as VM
                } catch (ignore: Exception) {
                }
            }
            throw InstantiationException("Cannot create an instance of $modelClass")
        }
    }

@MainThread
inline fun <reified VM : ViewModel> Fragment.argViewModels(vararg args: Any) =
    viewModels<VM> { argViewModelFactory(args) }

@MainThread
inline fun <reified VM : ViewModel> Fragment.activityArgViewModels(vararg args: Any) =
    activityViewModels<VM> { argViewModelFactory(args) }

@MainThread
inline fun <reified VM : ViewModel> ComponentActivity.argViewModels(vararg args: Any) =
    viewModels<VM> { argViewModelFactory(args) }

one more thing

由于传入的参数都是 Any 类型，故无法通过参数确定对应的 constructor，只能匹配 constructor 参数个数，然后挨个尝试，这样效率不够高，而且感觉不是很优雅。一种解决方式便是在传入参数的时候同时传入参数对应的 class type，通过参数的 class type 可以直接获取对应的 constructor，缺点就是调用方会相对麻烦一些。

import androidx.activity.ComponentActivity
import androidx.activity.viewModels
import androidx.annotation.MainThread
import androidx.fragment.app.Fragment
import androidx.fragment.app.activityViewModels
import androidx.fragment.app.viewModels
import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider
import java.lang.reflect.InvocationTargetException

@Throws(
    NoSuchMethodException::class,
    SecurityException::class,
    IllegalAccessException::class,
    IllegalArgumentException::class,
    InstantiationException::class,
    InvocationTargetException::class,
)
fun argViewModelFactory(vararg args: Pair<Any, Class<*>>): ViewModelProvider.Factory =
    object : ViewModelProvider.NewInstanceFactory() {
        override fun <VM : ViewModel> create(modelClass: Class<VM>): VM {
            val params: Array<Any> = args.map { it.first }.toTypedArray()
            val signature: Array<Class<*>> = args.map { it.second }.toTypedArray()
            val constructor = modelClass.getConstructor(*signature)
            return constructor.newInstance(*params)
        }
    }

@MainThread
inline fun <reified VM : ViewModel> Fragment.argViewModels(
    vararg args: Pair<Any, Class<*>>
) = viewModels<VM> { argViewModelFactory(*args) }

@MainThread
inline fun <reified VM : ViewModel> Fragment.activityArgViewModels(
    vararg args: Pair<Any, Class<*>>
) = activityViewModels<VM> { argViewModelFactory(*args) }

@MainThread
inline fun <reified VM : ViewModel> ComponentActivity.argViewModels(
    vararg args: Pair<Any, Class<*>>
) = viewModels<VM> { argViewModelFactory(*args) }

相应的使用方式：

// 1
private val viewModel: FooViewModel by argViewModels(
    2 to Int::class.java,
    true to Boolean::class.java,
    "haha" to String::class.java
)

// 2
private val viewModel by argViewModels<FooViewModel>(
    2 to Int::class.java,
    true to Boolean::class.java,
    "haha" to String::class.java
)


// 3
private val pairs: Array<Pair<Any, Class<*>>> = arrayOf(
    2 to Int::class.java,
    true to Boolean::class.java,
    "haha" to String::class.java
)
private val viewModel: FooViewModel by argViewModels(*pairs)

// 4
private val pairs: Array<Pair<Any, Class<*>>> = arrayOf(
    2 to Int::class.java,
    true to Boolean::class.java,
    "haha" to String::class.java
)
private val viewModel by argViewModels<FooViewModel>(*pairs)


class FooViewModel(a: Int, b: Boolean, c: String) : ViewModel()

another one more thing

传入参数对应的 class type 是为了优雅高效地寻找到对应的 constructor，那么问题来了：我们为何不直接把 constructor 传进去呢？

here we go:

import androidx.activity.ComponentActivity
import androidx.activity.viewModels
import androidx.annotation.MainThread
import androidx.fragment.app.Fragment
import androidx.fragment.app.activityViewModels
import androidx.fragment.app.viewModels
import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider

fun <VM : ViewModel> argViewModelFactory(
    constructor: (Array<out Any>) -> VM
): (Array<out Any>) -> ViewModelProvider.Factory = {
    object : ViewModelProvider.NewInstanceFactory() {
        @Suppress("UNCHECKED_CAST")
        override fun <VM : ViewModel> create(modelClass: Class<VM>): VM {
            return constructor(it) as VM
        }
    }
}

@MainThread
inline fun <reified VM : ViewModel> Fragment.argViewModels(
    noinline constructor: (Array<out Any>) -> VM,
    vararg args: Any
): Lazy<VM> = viewModels { argViewModelFactory(constructor)(args) }

@MainThread
inline fun <reified VM : ViewModel> Fragment.activityArgViewModels(
    noinline constructor: (Array<out Any>) -> VM,
    vararg args: Any
): Lazy<VM> = activityViewModels { argViewModelFactory(constructor)(args) }

@MainThread
inline fun <reified VM : ViewModel> ComponentActivity.argViewModels(
    noinline constructor: (Array<out Any>) -> VM,
    vararg args: Any
): Lazy<VM> = viewModels { argViewModelFactory(constructor)(args) }

相应的使用方式：

private val viewModel by argViewModels(
    { FooViewModel(it[0] as Int, it[1] as Boolean, it[2] as String) },
    2, true, "haha"
)

class FooViewModel(a: Int, b: Boolean, c: String) : ViewModel()

可以看到，调用方其实并没有省事儿，反而还更麻烦了。不过，也不失为一种思路。

目录

前言

vararg

Array

one more thing

another one more thing