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.

實作：

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

apply為高階函數，它接受一個參數block，類型為 T.() -> Unit ( function-with-receiver)

在apply的函數體内，調用了傳入的block這個函數，然後傳回調用apply函數的對象執行個體。

//調用方法, 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.

它的實作代碼也隻有一行

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

with為高階函數，接收兩個參數：receiver，類型為T，block 類型為 T.() -> R,為 function-with-receiver type，隻能被T類型的對象調用.

同樣，在block方法體内，可以通過this來調用到receiver。

with傳回的類型為R，和block的傳回類型相同

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

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