用Kotlin-koans学Kotlin【五】 iv_properties

本节熟悉属性和委托属性

32. n32Properties

在Kotlin中，类不允许拥有字段，但是给了我们field关键字实现相同的功能

class PropertyExample() {
    var counter = 0
    var propertyWithCounter: Int? = 0
        set(value) {
            field = value
            counter++
        }
        get
}

33. n33LazyProperty

懒加载

class LazyProperty(val initializer: () -> Int) {
    var value: Int? = null
    val lazy:Int
    get() {
        if (value==null){
            value = initializer()
        }
        return value!!
    }
}

initializer是一个lambda表达式，这个表达式会在lazy属性第一次被访问的时候执行，且仅执行一次

34. n34DelegatesExamples

用委托实现懒加载

class LazyPropertyUsingDelegates(val initializer: () -> Int) {
    val lazyValue: Int by lazy(initializer)
}

35. n35HowDelegatesWork

该练习帮我们了解委托如何工作，D类把date属性的set和get方法委托给EffectiveDate

class EffectiveDate<R> : ReadWriteProperty<R, MyDate> {
    var timeInMillis: Long? = null

    operator override fun getValue(thisRef: R, property: KProperty<*>): MyDate = timeInMillis!!.toDate()
    operator override fun setValue(thisRef: R, property: KProperty<*>, value: MyDate) {
        timeInMillis = value.toMillis()
    }
}