kotlin源码分析-标准内置函数

1. TODO

方法声明：

@kotlin.internal.InlineOnlypublic inline fun TODO(): Nothing = throw NotImplementedError()

方法使用：

fun test(){    TODO("Not implemented")}

方法说明：有点类似于java的//todo,但是不同的是，因为它的方法告诉我们，它会抛出一个异常，也就是上面这个方法会导致程序崩溃。它的好处是：配合IDE自动声明的TODO, 会强制开发者去实现这个TODO或者删除它。

2.also

方法声明：

@kotlin.internal.InlineOnly@SinceKotlin("1.1")public inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }

方法使用：

var tvDisplay: TextView = ...tvDisplay.also{ textView->    textView.textSize = 13    textView.textColor = Color.WHITE}.setText("3333")

方法说明：also方法有点类似于Builder模式，如上它的好处是明显的，可以使得代码阅读性很强

3. apply

方法声明：

public inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this }

方法使用：

var tvDisplay: TextView = ...tvDisplay.apply{    textSize = 13    textColor = Color.WHITE}.setText("3333")

方法说明：和also方法一样

4. run

方法声明：

@kotlin.internal.InlineOnlypublic inline fun <T, R> T.run(block: T.() -> R): R = block()

方法使用：

    val items = listOf(                "America" to listOf("Brazil", "Canada", "United States"),                "Asia" to listOf("China", "India", "Japan"),                "Europe" to listOf("France", "Germany", "Spain", "United Kingdom"))    var b = items.run {            var m = get(0).first            var n = get(0).second            m + n        }    println(b)    //输出结果:America[Brazil, Canada, United States]

方法说明：对items对象，进行run方法，然后转换成另外一个对象R

5.with

方法声明：

@kotlin.internal.InlineOnlypublic inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()

方法使用：

    val items = listOf(                "America" to listOf("Brazil", "Canada", "United States"),                "Asia" to listOf("China", "India", "Japan"),                "Europe" to listOf("France", "Germany", "Spain", "United Kingdom"))    var b = with(items) {            var m = get(0).first            var n = get(0).second            m + n        }    println(b)

方法说明：和run方法一样，不一样的是: with是 (T), run是T.()

6. let

方法声明：

@kotlin.internal.InlineOnlypublic inline fun <T, R> T.let(block: (T) -> R): R = block(this)

方法使用：

    val items = listOf(                "America" to listOf("Brazil", "Canada", "United States"),                "Asia" to listOf("China", "India", "Japan"),                "Europe" to listOf("France", "Germany", "Spain", "United Kingdom"))    items.let {  list->          println(list[0].first)                       println(list[0].second)           }    //输出结果：America    //[Brazil, Canada, United States]

7.takeIf

方法声明：

@kotlin.internal.InlineOnly@SinceKotlin("1.1")public inline fun <T> T.takeIf(predicate: (T) -> Boolean): T? = if (predicate(this)) this else null

方法使用：

    val items = listOf(                "America" to listOf("Brazil", "Canada", "United States"),                "Asia" to listOf("China", "India", "Japan"),                "Europe" to listOf("France", "Germany", "Spain", "United Kingdom"))    var result = items.takeIf{        it[0].first.equals("Asia")    }    println(result)    //输出结果是null，因为it[0].first是"America"，所以不满足条件的情况下返回null

8.takeUnless

方法声明：

@kotlin.internal.InlineOnly@SinceKotlin("1.1")public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? = if (!predicate(this)) this else null

方法使用：

    val items = listOf(                "America" to listOf("Brazil", "Canada", "United States"),                "Asia" to listOf("China", "India", "Japan"),                "Europe" to listOf("France", "Germany", "Spain", "United Kingdom"))    var result = items.takeUnless{        it[0].first.equals("Asia")    }    println(result)    //输出结果：[(America, [Brazil, Canada, United States]), (Asia, [China, India, Japan]), (Europe, [France, Germany, Spain, United Kingdom])]

方法说明：刚好和takeIf方法相反

9.repeat

方法声明：

@kotlin.internal.InlineOnlypublic inline fun repeat(times: Int, action: (Int) -> Unit) {    for (index in 0..times - 1) {        action(index)    }}

方法使用：

repeat(9) {          println(it)}输出：0 1 2 3 4 5 6 7 8//重复9次，打印索引