Given the following lambda:
val lambda: () -> Unit = null
Which of the following calls is idomatic to Kotlin for calling a nullable lambda?
lambda?.let { it() }
vs
lambda?.invoke()
Let's ask Kotlin compiler:
val lambda: (() -> Unit)? = null
lambda()
Compilers says:
Reference has a nullable type '(() -> Unit)?', use explicit '?.invoke()' to make a function-like call instead
So yeah, seems that ?.invoke() is the way to go.
Although even this seems fine by me (and by compiler too):
if (lambda != null) {
lambda()
}
Here is a simple example:
fun takeThatFunction(nullableFun: (() -> Unit)?) {
nullableFun?.let { it() }
}
takeThatFunction { print("yo!") }
An alias on this case could be a more expressive approach
typealias TheNameOFYourCallback = ()-> Unit
class Something {
private var callback: TheNameOFYourCallback? = null
fun setTheCallback(callback: TheNameOFYourCallback) {
this.callback = callback
}
private fun usageOfCallback() {
callack?.invoke()
}
}
And then for using
val something = Something()
something.setTheCallback {
//Do your thing
}
Off course, the var could have been not private but in this way setting the callback can take advantage of the syntactic sugar of lambdas for methods, otherwise, the assignation form seems odd to me, judge by your self:
something.callback = {
//Do your thing
}
Related
We have a relatively simple builder pattern we use for test data generator in Kotlin.
The builders follow the pattern:
class ThingBuilder private constructor(
var param1: Int = 1,
var param2: Boolean = true
) {
private constructor(vararg inits: ThingBuilder.(ThingBuilder) -> Unit) : this() {
inits.forEach { it(this) }
}
fun build(): Thing {
return Thing(
param1,
param2
)
}
companion object {
fun asDefaultCase(init: ThingBuilder.(ThingBuilder) -> Unit = {}): ThingBuilder {
return ThingBuilder(init)
}
fun asSomethingElseCase(init: ThingBuilder.(ThingBuilder) -> Unit = {}): ThingBuilder {
return ThingBuilder({ b -> b.param2 = false }, init)
}
}
}
Here the Kotlin compiler reports a warning:
The expression is unused
which references the line:
inits.forEach { it(this) }
I've tried turning that into an Array<T> rather than varags but same warning occurs.
What would be the more correct way to make this structure where the consumers can pass in lambdas to configure the builder data?
(for reference, the code works correctly and the loop functions as expected)
This seems to be a rather old bug KT-21282 False positive UNUSED_EXPRESSION compiler warning with object and lambda with receiver / extension function type.
The fix is simple - just specify the explicit receiver and do this.it(this). I also don't see why you would need to pass this as both the receiver and the formal parameter to the block. I would just do this instead:
private constructor(vararg inits: ThingBuilder.() -> Unit) : this() {
inits.forEach { this.it() }
}
or:
private constructor(vararg inits: ThingBuilder.() -> Unit) : this() {
inits.forEach { it(this) }
}
Then you don't even need to write the b parameter in asSomethingElseCase:
fun asSomethingElseCase(init: ThingBuilder.() -> Unit = {}): ThingBuilder {
return ThingBuilder({ param2 = false }, init)
}
With all the well-known single-function listeners we can use a simpler lambda notation
view.setOnClickListener { do() }
instead of the original, longer Java way of
view.setOnClickListener(object : View.OnClickListener {
override fun onClick(v: View?) {
do()
}
})
But what exactly makes this work? I tried to do the same with my own listener:
private var listener: OnCopyPasteClickListener? = null
interface OnCopyPasteClickListener {
fun onPasteClick(text: String)
}
fun setOnCopyPasteClickListener(onCopyPasteClickListener: OnCopyPasteClickListener) {
listener = onCopyPasteClickListener
}
and while the long approach works just fine:
copypaste.setOnCopyPasteClickListener(object : CopyPasteMenu.OnCopyPasteClickListener {
override fun onPasteClick(text: String) {
do(text)
}
})
I can't make it accept the short one:
copypaste.setOnCopyPasteClickListener {
do(it)
}
The IDE gives a type mismatch error.
Actually, if you have only one function to be invoked, I recommend you use Kotlin Callback.
typealias OnDoWorkListener = ((String) -> Unit)
class Work {
var doWork: OnDoWorkListener? = null
fun doSomething() {
doWork?.invoke("Message Here")
}
}
And in your function, you just set the callback to it
fun main() {
val work = Work()
work.doWork = {
Log.d("WORK", "This gets called from the `work` object. Message: $it")
}
work.doSomething();
}
We can also use function to set the listener as well.
class Work {
var doWork: OnDoWorkListener? = null
fun doSomething() {
doWork?.invoke("Message Here")
}
fun setOnWorkListener(listener: OnDoWorkListener) {
doWork = listener
}
}
fun main() {
val work = Work()
work.setOnWorkListener {
Log.d("WORK", "This gets called from the `work` object. Message: $it")
}
work.doSomething()
}
Higher order functions make this work:
Kotlin functions are first-class, which means that they can be stored
in variables and data structures, passed as arguments to and returned
from other higher-order functions. You can operate with functions in
any way that is possible for other non-function values.
From the same page:
Passing a lambda to the last parameter
In Kotlin, there is a convention that if the last parameter of a
function accepts a function, a lambda expression that is passed as the
corresponding argument can be placed outside the parentheses:
val product = items.fold(1) { acc, e -> acc * e }
If the lambda is the only argument to that call, the parentheses can
be omitted entirely:
run { println("...") }
Knowing this, a possible update on your class would look like:
class CopyPaste {
private var listener: (String) -> Unit = {}
fun setOnCopyPasteClickListener(onCopyPasteClickListener: (String) -> Unit) {
listener = onCopyPasteClickListener
}
fun doCopyPaste(value: String) {
listener.invoke(value)
}
}
fun main() {
val copyPaste = CopyPaste()
copyPaste.setOnCopyPasteClickListener { println(it) }
copyPaste.doCopyPaste("ClipboardContent!")
}
The class CopyPaste stores the listener, which is a function that takes a String parameter and does not return anything. Its function setOnCopyPasteClickListener accepts a function with the same signature as the listener property and at the end doCopyPaste accepts a String parameter and passes it to the stored function.
Actually, just after I posted, I searched for more thoughts and found this thread: https://youtrack.jetbrains.com/issue/KT-7770 This is indeed a debated limitation as it currently only applies to Java, not Kotlin itself. There is also a suggestion there that gives almost the required simplicity:
interface OnCopyPasteClickListener {
fun onPasteClick(text: String)
companion object {
inline operator fun invoke(crossinline op: (text: String) -> Unit) =
object : OnCopyPasteClickListener {
override fun onPasteClick(text: String) = op(text)
}
}
}
and then, thanks to this overloaded operator, it can be called as:
copypaste.setOnCopyPasteClickListener(CopyPasteMenu.OnCopyPasteClickListener { text ->
do(text)
})
But as the suggested answers offer a more idiomatic solution, I'll accept one of those, I only wanted to include this approach here for reference.
I have the following functions to simulate the ternary operator for kotlin
fun Boolean.then(action: () -> Unit): Boolean {
if (this)
action.invoke()
return this
}
fun Boolean.otherwise(action: () -> Unit) {
if (!this)
action.invoke()
}
fun <T> Boolean.then(func: () -> T): T? {
if (this)
return func.invoke()
return null
}
fun <T> T?.otherwise(action: () -> T): T {
return this ?: action.invoke()
}
they are supposed to be used like this :
(check).then { doHello() }.otherwise { doWorld() }
val answer = (check).then { "hello" }.otherwise { "world" }
however when I try to assign a value using the above operators like this:
val visibility: Int = (show).then { View.VISIBLE }.alt { View.GONE }
I get an error saying that the required reply was Int but it actually got Unit which means that it called the first version of the methods instead of the second
Other than renaming the methods (when I changed the first two to thenDo and otherwiseDo it worked), can I write the above code in some way so that the compiler will know to call the second version?
I don't think you need both overloads. If you remove the ones that return Unit, then both your lines of code work:
(check).then { doHello() }.otherwise { doWorld() }
val answer = (check).then { "hello" }.otherwise { "world" }
That's because the first line, where the lambdas return Unit, e.g. doHello(), can still use the generic versions of then and otherwise, as they are still considered functions with a return value, namely Unit.
Although I agree with some the comments above: do you really need this? Why not just use if, which is an expression which returns a value (like the ternary operator). See discussion here for more info.
I'd like to have an applyif to work like:
builder.applyif(<condition expression>) {
builder.set...
}
to be equal with:
builder.apply {
if (<condition expression>) {
builder.set...
}
}
Is that possible?
Yes, of course. You can nearly program anything, but don't reinvent the wheel. Look at the bottom of the answer to see a standard Kotlin approach without own extension function(s) which may already suffice your needs (not exactly applyIf though).
Now, however, lets see how an applyIf might be implemented:
inline fun <T> T.applyIf(predicate: T.() -> Boolean, block: T.() -> Unit): T = apply {
if (predicate(this))
block(this)
}
Don't forget the inline if you are implementing extension functions with lambdas.
Here is an example usage of the above.
// sample class
class ADemo {
fun isTrue() = true
}
// sample usage using method references
ADemo().applyIf(ADemo::isTrue, ::println)
// or if you prefer or require it, here without
ADemo().applyIf( { isTrue() } ) {
println(this)
}
If you just want to supply a boolean instead, you can use the following extension function:
inline fun <T> T.applyIf(condition : Boolean, block : T.() -> Unit) : T = apply {
if(condition) block(this)
}
and call it with:
val someCondition = true
ADemo().applyIf(someCondition) {
println(this)
}
And now a possible Kotlin standard way with which more people could be familiar:
ADemo().takeIf(ADemo::isTrue)
?.apply(::println)
// or
ADemo().takeIf { it.isTrue() }
?.apply { println(this) }
If they do remember (I actually didn't until I saw Marko Topolniks comment) they should immediately know what's going on.
However, if you require the given value (i.e. ADemo()) after calling takeIf this approach might not work for you as the following will set the variable to null then:
val x = ADemo().takeIf { false }
?.apply { println(this) /* never called */ }
// now x = null
whereas the following will rather set the variable to the ADemo-instance:
val x = ADemo().applyIf(false) { println(this) /* also not called */ }
// now x contains the ADemo()-instance
Chaining the builder calls might not be so nice then. Still you can also accomplish this via standard Kotlin functions by combining the takeIf with apply or also (or with, let, run, depending on whether you want to return something or not or you prefer working with it or this):
val x = builder.apply {
takeIf { false }
?.apply(::println) // not called
takeIf { true }
?.apply(::println) // called
}
// x contains the builder
But then again we are nearly there where you were already in your question. The same definitely looks better with applyIf-usage:
val x = builder.applyIf(false, ::println) // not called
.applyIf(true) {
println(this) // called
}
// x contains the builder
Sure you can, you just need an extension function so you can call it on the builder, and you need it to take a Boolean parameter and the lambda to execute.
If you look at the source of the apply function itself, it will help with most of the implementation:
public inline fun <T> T.apply(block: T.() -> Unit): T {
block()
return this
}
Based on this, applyIf can be as simple as:
inline fun <T> T.applyIf(condition: Boolean, block: T.() -> Unit): T {
return if (condition) this.apply(block) else this
}
Usage looks like this:
builder.applyIf(x > 200) {
setSomething()
}
fun <T> T.applyIf(condition: Boolean, block: T.() -> T) = if (condition) block() else this
fun main() {
println("a".applyIf(true) { uppercase() }) // A
println("a".applyIf(false) { uppercase() }) // a
}
This declaration works, but is not the most beautiful code. Is there a way to return functions less ugly? I tried (s: String) -> writer.println(s) but this didn't work.
val writeStuff: (PrintWriter) -> (String) -> Unit = {
val writer = it
val f: (String) -> Unit = {
writer.println(it)
}
f
}
PrintWriter("test").use { writeStuff(it)("TEST") }
EDIT: a bit more concrete example:
val writeStuff: (PrintWriter) -> (String) -> Unit = { writer ->
{ writer.println(it) }
}
val sendStuff: (Any) -> (String) -> Unit = { sender ->
{ sender.equals(it) }
}
#Test fun test1() {
val li = listOf("a", "b", "c")
val process: List<(String) -> Unit> =
listOf(writeStuff(PrintWriter("a")), sendStuff(Object()))
process.map { li.map(it) }
}
First, you can simplify your code using lambda syntax with explicit parameter and inlining val f:
val writeStuff: (PrintWriter) -> (String) -> Unit = { writer ->
{ writer.println(it) }
}
But since Kotlin supports local function declarations, you can even make writeStuff a local fun instead of a val.
This would lead to the following code:
fun writeStuff(writer: PrintWriter): (String) -> Unit {
return { writer.println(it) }
}
Or, using the single expression syntax,
fun writeStuff(writer: PrintWriter): (String) -> Unit = { writer.println(it) }
The usage, however, will be the same:
PrintWriter("...").use { writeStuff(it)("...") }
I stumbled across this question while trying to figure out how to return a Function (the java interface) in Kotlin. While this doesn't directly answer the question, hopefully it'll help someone else who has the same query:
override fun myFun(param1: Object): Function<in Object, out String?> {
if (!param1.meetsCriteria())
return Function { obj -> null }
return Function { obj ->
"success"
}
}
In this case, I was overriding a method in a java interface that required me to return a Function instance. (Note that since the param is not used in my particular implementation above, I could remove it and just have the return result. eg return Function { null })
Edit: After some research, it turns out Kotlin covers this subject with their discussion on "SAM (single abstract method) conversions" here and here, though it may not be the most intuitive thing to look up when figuring out how to return Functions.