Kotlin高阶函数

run():执行调用传入方法

//方法定义
public inline fun <R> run(block: () -> R): R { 
contract { 
  callsInPlace(block, InvocationKind.EXACTLY_ONCE) 
  }
return block() 
}
method()
run({method()})
run{method()}
//以上三个方法等价

apply():执行调用传入方法并返回调用者本身

//方法定义
public inline fun <T> T.apply(block: T.() -> Unit): T { 
  contract { 
  callsInPlace(block, InvocationKind.EXACTLY_ONCE) 
  }
block() 
return this 
}
list<String>.apply({
  it.add("a")
  it.add("b")
})
list = {"a","b"}

let():将调用者作为传入方法的参数执行

//方法定义
public inline fun <T, R> T.let(block: (T) -> R): R { 
  contract { 
  callsInPlace(block, InvocationKind.EXACTLY_ONCE) 
  }
return block(this) 
}
"abc".let{println(it)}
//输出"abc"

also():将调用者作为参数传入方法执行并返回调用者

//方法定义
public inline fun <T> T.also(block: (T) -> Unit): T { 
  contract { 
  callsInPlace(block, InvocationKind.EXACTLY_ONCE) 
  }
block(this) 
return this 
}
val a = "ABC".also { 
println(it) //输出：ABC 
}
println(a)//输出：ABC 

with():使用传入的参数调用传入的方法

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