Kotlin 中的 appy和with方法

Kotlin 中的 appy和with方法

apply

apply:Calls the specified function block with this value as its receiver and returns this value.

实现：

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

apply为高阶函数，它接受一个参数block，类型为 T.() -> Unit ( function-with-receiver)
在apply的函数体内，调用了传入的block这个函数，然后返回调用apply函数的对象实例。

//调用方法, 省去了 this
fun getDeveloper(): Developer {
    return Developer().apply {
        developerName = "Amit Shekhar"
        developerAge = 22
    }
}

with

Calls the specified function block with the given receiver as its receiver and returns its result.

它的实现代码也只有一行

public inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()

with为高阶函数，接收两个参数：receiver，类型为T，block 类型为 T.() -> R,为 function-with-receiver type，只能被T类型的对象调用.

同样，在block方法体内，可以通过this来调用到receiver。

with返回的类型为R，和block的返回类型相同

fun getPersonFromDeveloper(developer: Developer): Person {
    return with(developer) {
        Person(developerName, developerAge)
    }
}

参考：
learn kotlin apply vs with
what is a receiver in kotlin
What is a purpose of Lambda's with Receiver?
https://www.jianshu.com/p/22c743dca2d4

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