How I can use callback in Kotlin?
JavaAndroidXmlKotlinListenerJava Problem Overview
I have View and one CircleShape , which should show toast in this View. And I use it in main Activity. This is my interface
interface OnClickListenerInterface {
fun onClick()
}
It is CircleShape( it is View in my xml) and listener in my View. I want to implement OnClick in my Activity.
var listener: OnClickListenerInterface? = null
mCircleShape.setOnClickListener(View.OnClickListener {
if (listener == null) return@OnClickListener
listener!!.onClick()
})
I know , that in Kotlin getters and setters generic automatics, but how I can set listener if it private. It is code from my Activity, but It doesn't work
CircleShape.listener = object :OnClickListenerInterface{
override fun onClick() {
ToastUtils.showSuccessMessage(getContext(),"pressed")
}
}
How I should to use Callback, onClickListenere in Kotlin?
Java Solutions
Solution 1 - Java
A more simpler solution by using lambda.
Inside CircleShape.kt, declare a lambda function.
var listener: (()->Unit)? = null
...
// When you want to invoke the listener
listener?.invoke()
Inside your Activity
mCircleShape.listener = {
// Do something when you observed a call
}
Solution 2 - Java
define a function like this:
fun performWork(param1: String, myCallback: (result: String?) -> Unit) {
// perform some network work
// on network finished
myCallback.invoke("result from network")
}
use like this:
performWork("http://..."){ result ->
//use result
}
Solution 3 - Java
On CircleShape.kt.
private listener OnClickListenerInterface? = null
...
fun setOnClickListener(listener: OnClickListenerInterface) {
this.listener = listener
}
On your Activity
mCircleShape.setOnClickListener(object: CircleShape.OnClickListenerInterface {
override fun onClick() {
// Do something here
}
})
If you're gonna use lambda expression, you can use a Function Type. Here how it looks like on CirclesShapt.kt
fun setOnClickListener(listener: () -> Unit){
listener() // or you could use optional if the lister is nullable "listener?.invoke()"
}
So in activity looks like.
mCircleShape.setOnClickListener {
// Do something here
}
Solution 4 - Java
to use Kotlin callbacks , I use them in my api calls for success or failure use
> create enum class for state
enum class APIState(val result: Boolean) {
SUCCESS(true),
FAILURE(false)}
> use call back in fun
private fun fetchFeesList(studentID:String,call:(APIState)->Unit){
... do stuff here , and use call(APIState.SUCCESS) or call(APIState.FAILURE) }
> when calling function fetchFeesList , call it like
fetchFeesList(studentID){
val res = it.result
if(res){
toast("success")
}else {
toast("failure")
}
}
> for toast("message") , use Anko Lib from GitHub : - https://github.com/Kotlin/anko
Solution 5 - Java
I was desperately looking for a Kotlin solution similar to what Java interfaces offer until I bumped into the suggestions offered thereabove. Having tried all of them, I was unable to arrive at a working solution that would suit my scenario.
This led to my own implementation which worked perfectly according to my use case, and therefore thought I could share the same here, though it may not be the perfect way of doing it, apologies in advance.
The steps:
- 1. Define your interface:
interface OnClickListenerInterface {
fun onClick()
}
- 2. Inside the class that will trigger the "onClick" callback i.e. "CircleShape" for your case:
Create a nullable variable.
var listener: OnClickListenerInterface? = null
Declare a function to initialise the variable above.
fun initOnClickInterface(listener: OnClickListenerInterface){
this.listener = listener
}
At the point where you want to trigger the "onClick" callback:
mCircleShape.setOnClickListener(View.OnClickListener {
if (listener == null) return@OnClickListener
listener?.onClick() // Trigger the call back
})
- 3. Inside the activity where you want to receive the "onClick" callback:
Make the activity implement the OnClickListenerInterface, then create an object of your CircleShape class.
class Activity : AppCompatActivity(), OnClickListenerInterface {
val mCircleShape = CircleShape()
// ...other stuff
Inside the onCreate function of this activity, initialise your interface using the initOnClickInterface function we created in the CircleShape class.
mCircleShape.initOnClickListenerInterface(this)
Then finish by overriding the onClick method of our interface by adding the code below in the activity.
override fun onClick() {
// Callback received successfully. Do your stuff here
}
The above steps worked for me.
As I said, in case of any issues with my coding, I'm a learner too .
Cheers!
Solution 6 - Java
Have you tried to use a lambda expression? For example in your case:
mCircleShape.setOnClickListener(
{ _ -> ToastUtils.showSuccessMessage(context,"pressed") }
)
Or if you want to make it more kotlin style:
mCircleShape.listener = (
{ _ -> ToastUtils.showSuccessMessage(context,"pressed") }
)
Solution 7 - Java
First of all you need to remove this code:
mCircleShape.setOnClickListener(View.OnClickListener {
if (listener == null) return@OnClickListener
listener!!.onClick()
})
Because listener is always null at first and your code just always returns.
var listener: OnClickListenerInterface? = null
is already public (this is a default access level in Kotlin). So you can just set it in your activity, when needed. Use listener?.onClick()
call to trigger it from your CircleShape.