More detail, the usage of let, also, apply, run, with. When you first writing Kotlin, you must have questions on what is the difference between all these lambda functions and how to use them. This is a notes on how to properly use these lambda functions.

Let

Let accepts the object as a parameter and returns the result of the lambda.

1	val Height = Plant("Orange", 20).let { it.height * 2 }

it is Plant, Height is Int = 40

Also

Also accepts the object as a parameter and returns the object itself.

1 2	val plant = Plant("Orange", 40) val result = plant.also { it.age = 50 }

it is “Plant*, “result* is Plant. In this case, plant and result points to the same object.

Apply

Apply is used for post-construction configuration. Object is not passed as a parameter, but rather as this. It return the object itself.

(An object passed in such way is called receiver. It is a function literal with receiver)

val plant = Plant().apply {
    setTitle("Orange")
    setHeight(40)
}

setTitle and setHeight are both functions for object Plant

Run

Run is similar to Apply and is another function literal with receiver. Object is not passed as a parameter but rather as this. The difference is it it return the value of expression of the lambda function.

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2
3

plant?.run {
    if (height < 20) SmallPlantView else TallPlantView
}.show()

(height is a property of Plant)

If you want to run a block and do not want to use the return value, use Run. Normally it is assumed that the return value of expression is not used. So it is perfectly use to safe unwrap optional variable.

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2
3

plant.run {
    grow()
}

(grow is a function of Plant)

With

With is similar to Run. The only difference is With do not accept optional variables.

(Assume plant is not optional variable)

with(plant) {
    height = 20
	  grow()
}

plant.run {
    height = 20
    grow()
}

(These are similar)

(Now, we assume plant is optional variable)

with(plant) {
    this?.height = 20
    this?.grow()
}

plant?.run {
    this?.height = 20
    this?.grow()
}

Summary

Here is a table of summary.

Parameter | Same Object Return | Different Object Return

It | also | let
this | apply | run, with

Kotlin Lambda Deep Look

Let

Also

Apply

Run

With

Summary