Kotlin grammar let, run, with, apply, also, takeIf difference with takeUnless

The official explanation: https://www.kotlincn.net/docs/reference/scope-functions.html

Select function

Features	Object Reference	return value	It is an extension function
let	it	Lambda results	Yes
run	this	Lambda results	Yes
run	-	Lambda results	No: there is no context object to call
with	this	Lambda results	No: context object as a parameter.
apply	this	Context object	Yes
also	it	Context object	Yes

The following is a brief guide accordance with the intended function of choice:

• Lambda execution on a non-null object: let
• The expressions as variables into partial ranges: let
• Object Configuration: apply
• Object configuration and results: run
• Needed for the operation of the expression statement: non-extension run
• Additional effects: also
• Group objects function call: with

Overlapping with different functions, so you can select specific functions according to the agreed project or team use.

Although the scope of the function is a way to make the code more concise, but avoid overuse them: it will reduce the readability of the code and cause errors. Avoid nesting range of functions and link them to be careful: it is easy for the current context object and thisconfused values or of it.

 

takeIf versus takeUnless

In addition to the range of the function, further comprising a standard library function takeIfand takeUnless. These functions allow you to check the state of the object is embedded in the call chain.

When invoked on an object having provided predicates and predicate if the object takeIfmatches the object is returned. Otherwise, it returns null. Thus, takeIfa single object filtering function. In turn, takeUnlessthe object if the object does not match the predicate is returned if the match the object is returned null. This object may be used as the parameter lambda it( ).

val str = "Hello"
val caps = str.takeIf { it.isNotEmpty() }?.toUpperCase()
//val caps = str.takeIf { it.isNotEmpty() }.toUpperCase() //compilation error
println(caps)

In the takeIflink to other functions and then takeUnless, do not forget to check the implementation of air safety or call ( ?.), because their return value can be empty.

fun displaySubstringPosition(input: String, sub: String) {
    input.indexOf(sub).takeIf { it >= 0 }?.let {
        println("The substring $sub is found in $input.")
        println("Its start position is $it.")
    }
}

displaySubstringPosition("010000011", "11")
displaySubstringPosition("010000011", "12")

to sum up:

the difference:

Since the range of the function is very similar nature, and therefore the difference between them is important to understand. Each range function has two main differences:

• Context object reference to the way
• return value.

apply：Context object may be used as Receiver this( ). The return value is the object itself

also：Context object used as a parameter it( ). The return value is the object itself

The above are two ways to return the object itself may call chain to perform more complex processing.

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runAnd withinvoke the same as the letspread function of the context object

let：Context object used as a parameter it( ). The return value is the result of lambda.

with：Non-expanded functions: context object passed as a parameter, but the lambda, which can be used as Receiver ( this). The return value is the result of lambda.

run：Context object may be used as Receiver this( ). The return value is the result of lambda.

The above three ways to return the result value of lambda.