How do I call an extension function that's nested inside an object from outside that object?
Here is an example:
package test
object Example {
fun String.exampleExtension() {
}
}
fun test(str: String) {
str.exampleExtension() // Doesn't work. How can I use that extension function?
}
As far as I can tell this isn't possible, but I'm not sure, so I figured I'd ask.
There are 2 ways you can do this.
The simplest is:
fun test(str: String) {
with (Example) {
str.exampleExtension()
}
}
The other thing you can do is manually import the extension function. As of Kotlin 1.3.41, the compiler doesn't offer this via code completion, and you have to manually request it.
import test.Example.exampleExtension
fun test(str: String) {
str.exampleExtension() // Works now because of the import
}
I created a Kotlin bug to try and fix this, and have the auto-complete suggest this as an import: https://youtrack.jetbrains.com/issue/KT-33221
Related
I'm using kotlin compiler to analyze kotlin source file and I need to get where the method is defined using KtCallExpression.
KtCallExpression in question is retrieved from method call.
For better explanation, here's example, if there's source file named Main.kt with contents:
fun main() {
f1()
Test.f2()
}
fun f1() {
}
class Test {
companion object {
fun f2() {
}
}
}
when analyzing function main, it will produce 2 KtCallExpression, one for f1() and other for f2()
However I'm not sure how to get file Main.kt from KtCallExpression for f1(), or class Test from KtCallExpression for f2()
I tried to look for its documentations, however Kotlin compiler doesn't contain enough docs. I've also tried bunch of auto completion suggested by Intellij Idea but none of those doesn't seem to lead me to original declaration.
I have Kotlin some code that works as a class but when I try and run it as a Kotlin script I am getting " error: 'getNameAndVersion' is a member and an extension at the same time. References to such elements are not allowed"
enum class Env { Test, Sit }
data class ImageVersions(val apiName: String, val versions: Map<Env, String?>)
fun String.getNameAndVersion() = substringBefore(':') to substringAfter(':')
val testList = listOf("api-car-v1:0.0.118", "api-dog-v1:0.0.11", "api-plane-v1:0.0.36")
val sitList = listOf("api-car-v1:0.0.119", "api-plane-v1:0.0.37", "api-dog-v1:0.0.12")
getVersions(
mapOf(
Env.Test to testList,
Env.Sit to sitList
)
).forEach(::println)
fun getVersions(envMap: Map<Env, List<String>>): List<ImageVersions> {
val envApiNameMap = envMap.mapValues { it.value.associate(String::getNameAndVersion) }
val allApiNames = envApiNameMap.flatMap { it.value.keys }.distinct()
return allApiNames.map { apiName ->
ImageVersions(apiName, envApiNameMap.mapValues { it.value[apiName] })
}
}
I don't think I'm doing anything wrong with the way I'm using the method reference but according to my compiler I'm wrong. Appreciate some help with this. thanks
kotlinc -script .\myscript.kts
error: 'getNameAndVersion' is a member and an extension at the same time. References to such elements are not allowed
I don't have any experience with scripts but this error occurs when you try to reference a function inside a class that is also an extension function. (Here it is pointing to String::getNameAndVersion). Maybe when you run a script, the entire code is wrapped inside a class and then executed.
To fix this you can do one of the following:
Convert the function to a normal function which accepts a String parameter.
fun getNameAndVersion(s: String) = s.substringBefore(':') to s.substringAfter(':')
And replace String::getNameAndVersion with just ::getNameAndVersion in associate function
Other option is to directly call the function in the associate's lambda instead of passing a reference of this function.
.associate { it.getNameAndVersion() }
The question is in the comment. I want to refer to the outer function append, and not the one that's defined in the StringBuilder, how do I do this?
fun append(c: Char) {
println("TEST")
}
fun sbTest() = with(StringBuilder()) {
for(c in 'A'..'Z') {
append(c) // how do I refer to the append function declared above?
}
toString()
}
I know I can introduce a function reference variable, like this:
val f = ::append
and call f instead of append, but is there another way?
The problem is that anything called within with shadows the outer functions, because this is introduced. The same problem appears if you have a class and a top-level function with a function with the same signature.
The obvious option would just be re-naming it. Also, the function you have there isn't really descriptive compared to what it actually does. But if you for some reason can't rename, there are still other options.
Top-level methods can be referenced by package in Kotlin, for an instance like com.some.package.method. It can also be imported as such, which is the most common way to do it. There are very few methods that are called as com.some.package.method().
Kotlin, like Python, allows as in imports. Which means, you can do this:
package com.example;
// This is the important line; it declares the import with a custom name.
import com.example.append as appendChar; // Just an example name; this could be anything ***except append***. If it's append, it defeats the purpose
fun append(c: Char) {
println("TEST")
}
fun sbTest() = with(StringBuilder()) {
for(c in 'A'..'Z') {
appendChar(c)
}
toString()
}
Alternatively, as I mentioned, you can add the package:
for(c in 'A'..'Z') {
com.example.append(c)
}
val f = ::append is of course an option too, either way, it is still easier to rename the function than create imports with as or constants, assuming you have access to the function (and that it doesn't belong to a dependency).
If your file is outside a package, which I do not recommend you do, you can just declare the import as:
import append as appendChar
You could also use an extension function instead of with(), such as .let{...}.
This will send StringBuilder as an argument to the extension function as it (You can rename it to whatever you want btw):
fun sbTest() = StringBuilder().let{ it ->
for(c in 'A'..'Z') {
// using string builder
it.append(c)
// using your function
append(c)
}
it.toString()
}
The .let{...} function returns your last statement, aka the String from toString(), so your original function would still return it properly. Other functions can return this instead, such as .also{...}
I tend to use extension functions rather than with() as they're more flexible.
See this post to master extension functions: https://medium.com/#elye.project/mastering-kotlin-standard-functions-run-with-let-also-and-apply-9cd334b0ef84
EDIT: Got also{} and let{} mixed up. I switched them
I have top-level function like
fun sendNotification(context:Context, data:Data) {
...//a lot of code here
}
That function creates notifications, sometimes notification can contain image, so I have to download it. I`m using Glide which is wrapped over interface ImageManager, so I have to inject it. I use Koin for DI and the problem is that I cannot write
val imageManager: ImageManager by inject()
somewhere in my code, because there is no something that implements KoinComponent interface.
The most obvious solution is to pass already injected somewhere else imageManager as parameter of function but I dont want to do it, because in most cases I dont need imageManager: it depends on type of Data parameter.
Easiest way is to create KoinComponent object as wrapper and then to get variable from it:
val imageManager = object:KoinComponent {val im: ImageManager by inject()}.im
Btw its better to wrap it by some function, for example I use
inline fun <reified T> getKoinInstance(): T {
return object : KoinComponent {
val value: T by inject()
}.value
}
So if I need instance I just write
val imageManager:ImageManager = getKoinInstance()
or
val imageManager = getKoinInstance<ImageManager>()
I did it in this way
fun Route.general() {
val repo: OperationRepo by lazy { GlobalContext.get().koin.get() }
...
}
In Kotlin I have this function to wrap a transaction:
fun wrapInTransaction(code: () -> Unit) {
realmInstance.beginTransaction();
code.invoke()
realmInstance.commitTransaction();
}
How can I get access to realmInstance in the invoked code?
The easy solution here is to make code a function with receiver:
fun wrapInTransaction(code: Realm.() -> Unit) {
realmInstance.beginTransaction();
realmInstance.code()
realmInstance.commitTransaction();
}
Inside a lambda which you pass as code you will be able to use this to reference the RealmInstance and to use its members directly as if inside a member function.
Calling realmInstance.code() is just calling code with passing realmInstance as a receiver to it.
The other answers correctly demonstrate how to pass the RealmInstance object to the lambda. In addition, you can make the whole function an extension function which makes the call site a bit prettier:
fun Realm.wrapInTransaction(code: Realm.() -> Unit) {
//this is implicit
beginTransaction();
code()
commitTransaction();
}
The call site will look like this:
Realm.getInstance(this).wrapInTransaction {
createObject(User.class)
}
Change the wrapInTransaction function to accept an extensions method on realmInstance like so:
fun wrapInTransaction(code:Realm.() -> Unit){
realmInstance.beginTransaction();
realmInstance.code()
realmInstance.commitTransaction();
}
Then you can use it like:
wrapInTransaction {
println("realm instance is $this, instanceId: $instanceId")
}
Where for the sake of the example the Realm looks like:
class Realm {
val instanceId = 42
fun beginTransaction() {
}
fun commitTransaction() {
}
}
The above technique is possible thanks to Kotlin's Function Literals with Receiver that make it possible to set the this instance (receiver) within lambda function body. It makes it easy to build type safe builders that reassemble ones from Groovy or Ruby.
This answer provides more samples on the technique.