by viewModels() 与by activityViewModels()你真的懂了么

kotlin中用by关键字来实现委托，委托有类委托，属性委托。标题的 by viewModels 与 by activityViewModels 是用来实现属性委托的。
简单举个类委托的例子：

interface Animal {
    fun do()
}

class Bird(val s: String) : Animal {
    override fun do() { do(s) }
}

class Dog(a: Animal) : Animal by a

fun main() {
    val bird = Bird("bird")
    Dog(bird).do()
}

bird

可以看出，Dog里面没有定义do方法，却可以打印出bird，证明dog把方法委托给了传递进来的bird
重点来了，属性委托。

class Animal {
    val name :String by AnimalDelegate()
}
class AnimalDelegate {
    operator fun getValue(animal: Animal, property: KProperty<*>): String {
        val s = "小猪"
        println("setValue ${property.name} is ${s}")
        return s
    }

    operator fun setValue(animal: Animal, property: KProperty<*>, s: String) {
        println("setValue ${property.name} set to be ${s}")
    }
}

上面的实现了对name的属性委托，注意，被委托的对象，也就是 by后面的，必须实现这两个方法！

    operator fun getValue(animal: Animal, property: KProperty<*>): String {
        val s = "小猪"
        println("setValue ${property.name} is ${s}")
        return s
    }

    operator fun setValue(animal: Animal, property: KProperty<*>, s: String) {
        println("setValue ${property.name} set to be ${s}")
    }

看方法签名，第一个参数为被委托对象所在的类，第二个参数为 KProperty

为什么会讲属性委托？因为 by lazy by viewModels by activityViewModes本质上都是属性委托。
lazy是函数，viewModels是函数，activityViewModels也是函数，他们返回了Delegate 对象。且这个对象必须要有 operator fun getValue operator fun setValue ，用val生命不需要 setValue.
点开viewModels源码，如下

public inline fun  ComponentActivity.viewModels(
    noinline factoryProducer: (() -> Factory)? = null
): Lazy {
    val factoryPromise = factoryProducer ?: {
        defaultViewModelProviderFactory
    }

    return ViewModelLazy(VM::class, { viewModelStore }, factoryPromise)
}

再点

public class ViewModelLazy (
    private val viewModelClass: KClass,
    private val storeProducer: () -> ViewModelStore,
    private val factoryProducer: () -> ViewModelProvider.Factory
) : Lazy {
    private var cached: VM? = null

    override val value: VM
        get() {
            val viewModel = cached
            return if (viewModel == null) {
                val factory = factoryProducer()
                val store = storeProducer()
                ViewModelProvider(store, factory).get(viewModelClass.java).also {
                    cached = it
                }
            } else {
                viewModel
            }
        }

    override fun isInitialized(): Boolean = cached != null
}

再点

public interface Lazy {
    
    public val value: T

    
    public fun isInitialized(): Boolean
}


public fun  lazyOf(value: T): Lazy = InitializedLazyImpl(value)


@kotlin.internal.InlineOnly
public inline operator fun  Lazy.getValue(thisRef: Any?, property: KProperty<*>): T = value

最后一行是关键！用扩展函数声明了 operator getValue 函数。
真相大白。