kotlin源码分析-标准内置函数

1. TODO

方法声明：

@kotlin.internal.InlineOnly
public 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.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.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.InlineOnly
public inline fun  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.InlineOnly
public inline fun  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.InlineOnly
public inline fun  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.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.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.InlineOnly
public 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