Kotlin高阶函数之with,let,apply,run

    上一篇文章中介绍了RecycleView的Kotlin编程，在创建ViewHolder的时候有这么一段代码：

    class MyViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        fun bind(forecast: Forecast) {
            itemView.tv_text1.text = forecast.date
            itemView.tv_text2.text = forecast.high
            itemView.tv_text3.text = forecast.low
            itemView.tv_text4.text = forecast.notice
        }
    }

    这篇文章将使用Kotlin高阶函数with,let,apply,run对其演化，来说明这几个函数的用法和区别。

with函数

    with函数传入对象可以直接调用对象的方法，返回是最后一行。

    public inline fun  with(receiver: T, block: T.() -> R): R {
        contract {
            callsInPlace(block, InvocationKind.EXACTLY_ONCE)
        }
        return receiver.block()
    }

    例如：with里面传入itemView对象，调用itemView的tv_text1，tv_text2……等方法，返回Unit类型数据。

    fun bind(forecast: Forecast) = with(itemView) {
                tv_text1.text = forecast.date
                tv_text2.text = forecast.high
                tv_text3.text = forecast.low
                tv_text4.text = forecast.notice
                setOnClickListener { itemClick(forecast) }
    }

apply函数

    调用某对象的apply函数，在函数范围内，可以任意调用该对象的任意方法，并返回该对象。

    public inline fun  T.apply(block: T.() -> Unit): T {
        contract {
            callsInPlace(block, InvocationKind.EXACTLY_ONCE)
        }
        block()
        return this
    }

    例如：调用itemClick(forecast)对象的apply方法,并且返回itemClick(forecast)对象，他没有返回值，返回Unit。

    fun bind(forecast: Forecast) = itemClick(forecast).apply {
        itemView.tv_text1.text = forecast.date
        itemView.tv_text2.text = forecast.high
        itemView.tv_text3.text = forecast.low
        itemView.tv_text4.text = forecast.notice
        itemView.setOnClickListener { itemClick(forecast) }
    }

let函数

    默认当前这个对象作为闭包的it参数，返回值是函数里面最后一行，或者指定return。

    public inline fun  T.run(block: T.() -> R): R {
        contract {
            callsInPlace(block, InvocationKind.EXACTLY_ONCE)
        }
        return block()
    }

    例如：调用itemView的let方法，把itemView对象作为it使用，返回Unit。

    fun bind(forecast: Forecast) = itemView.let {
        it.tv_text1.text = forecast.date
        it.tv_text2.text = forecast.high
        it.tv_text3.text = forecast.low
        it.tv_text4.text = forecast.notice
        it.setOnClickListener { itemClick(forecast) }
    }

run函数

    调用指定的函数块，返回最后一行或者return相关内容。

    public inline fun  T.run(block: T.() -> R): R {
        contract {
            callsInPlace(block, InvocationKind.EXACTLY_ONCE)
        }
        return block()
    }

    例如：

    fun bind(forecast: Forecast) = run {
        itemView.tv_text1.text = forecast.date
        itemView.tv_text2.text = forecast.high
        itemView.tv_text3.text = forecast.low
        itemView.tv_text4.text = forecast.notice
        itemView.setOnClickListener { itemClick(forecast) }
    }