Lazy与Delegated Property

https://kotlinlang.org/docs/delegated-properties.html
Kotlin官网提到，by后面的代理对象必须提供getValue()函数，用来获取相应的代理值。

我们经常用的by lazy，其实lazy就是一个函数，返回了一个SynchronizedLazyImpl对象，该对象(类)实现了getValue()方法:

/**
 * Creates a new instance of the [Lazy] that uses the specified initialization function [initializer]
 * and the default thread-safety mode [LazyThreadSafetyMode.SYNCHRONIZED].
 *
 * If the initialization of a value throws an exception, it will attempt to reinitialize the value at next access.
 *
 * Note that the returned instance uses itself to synchronize on. Do not synchronize from external code on
 * the returned instance as it may cause accidental deadlock. Also this behavior can be changed in the future.
 */
public actual fun  lazy(initializer: () -> T): Lazy = SynchronizedLazyImpl(initializer)

private class SynchronizedLazyImpl(initializer: () -> T, lock: Any? = null) : Lazy, Serializable {
    private var initializer: (() -> T)? = initializer
    @Volatile private var _value: Any? = UNINITIALIZED_VALUE
    // final field is required to enable safe publication of constructed instance
    private val lock = lock ?: this

    override val value: T
        get() {
            val _v1 = _value
            if (_v1 !== UNINITIALIZED_VALUE) {
                @Suppress("UNCHECKED_CAST")
                return _v1 as T
            }

            return synchronized(lock) {
                val _v2 = _value
                if (_v2 !== UNINITIALIZED_VALUE) {
                    @Suppress("UNCHECKED_CAST") (_v2 as T)
                } else {
                    val typedValue = initializer!!()
                    _value = typedValue
                    initializer = null
                    typedValue
                }
            }
        }

    override fun isInitialized(): Boolean = _value !== UNINITIALIZED_VALUE

    override fun toString(): String = if (isInitialized()) value.toString() else "Lazy value not initialized yet."

    private fun writeReplace(): Any = InitializedLazyImpl(value)
}

https://github.com/JetBrains/kotlin/blob/6a670dc5f38fc73eb01d754d8f7c158ae0176ceb/libraries/stdlib/jvm/src/kotlin/util/LazyJVM.kt

其中Lazy()是一个接口:

/**
 * Represents a value with lazy initialization.
 *
 * To create an instance of [Lazy] use the [lazy] function.
 */
public interface Lazy {
    /**
     * Gets the lazily initialized value of the current Lazy instance.
     * Once the value was initialized it must not change during the rest of lifetime of this Lazy instance.
     */
    public val value: T

    /**
     * Returns `true` if a value for this Lazy instance has been already initialized, and `false` otherwise.
     * Once this function has returned `true` it stays `true` for the rest of lifetime of this Lazy instance.
     */
    public fun isInitialized(): Boolean
}

https://github.com/JetBrains/kotlin/blob/master/libraries/stdlib/src/kotlin/util/Lazy.kt