How to set up a listener for a variable in Kotlin

Question

How to I set up an interface listener for detecting a variable change in Kotlin. I successful implemented the following in Java, but am running into issues doing it in Kotlin:

Interface:

public interface InterfaceRefreshList
    {
    public void refreshListRequest();
    }

Class containing listener:

public class SignalChange
    {
    private static boolean refreshListSwitch;
    private static List<InterfaceRefreshList> refreshListListeners = new ArrayList<>();
    public static void setRefreshList(boolean value)
    {
    refreshListSwitch = value;
    for(InterfaceRefreshList l : refreshListListeners)
        l.refreshListRequest();
    }
    public static void addRefreshListListener(InterfaceRefreshList l)
        {
        refreshListListeners.add(l);
        }
    }

Class where listener is listening:

public class FragmentBrowse extends Fragment
{
    public FragmentBrowse() /// Constructor
        {
        SignalChange.addRefreshListListener(() -> refreshList());
        }
    refreshList()
    {
    // do something
    }
}

To signal a change:

SignalChange.setRefreshList(true);

I can set up the interface and the signal class:

class SignalChange
{
    private var refreshListSwitch: Boolean = false
    var setSwitch: Boolean
        get() = refreshListSwitch
        set(value)
        {
            refreshListSwitch = value
        }

    private var refreshListListeners = ArrayList<InterfaceRefreshPersonsList>()
    fun sendRefreshSignal()
    {
        for(l in refreshListListeners) l.refreshPersonsList()
    }

    fun addRefreshListListener(l: InterfaceRefreshPersonsList)
    {
        refreshListListeners.add(l)
    }

}

But I cannot setup the listener in the FragmentBrowse class. The fragment class doesn't allow constructors.

Which issues are you running into? Why don't you simply run the Java to Kotlin converter on your Java code? — yole, Jan 22 '19 at 17:52
The converter doesn't simply work like that. For example, the static setter for the private variable I had to do: private var refreshListSwitch: Boolean = false var setSwitch: Boolean get() = refreshListSwitch set(value) { refreshListSwitch = value } — Samuel, Jan 22 '19 at 18:18
Why did you have to do that? This looks entirely unnecessary. Anyway, you didn't answer what issues specifically you ran into with your Kotlin code. — yole, Jan 22 '19 at 18:21
The code for adding a listener needs to be placed in an [initializer block](http://kotlinlang.org/docs/reference/classes.html#constructors). The built-in Java to Kotlin converter in IntelliJ IDEA is capable of performing this conversion automatically. — yole, Jan 22 '19 at 19:30
I already tried in init block and added instance of SignalChange: signalChange = SignalChange(), but the following does not convert: signalChange.addRefreshListListener(() -> refreshList()); — Samuel, Jan 22 '19 at 19:43

Sergio · Accepted Answer · 2019-01-22T20:37:22.233

19

You can use built-in Kotlin delegates, for example:

object SignalChange {
    var refreshListListeners = ArrayList<InterfaceRefreshList>()

    // fires off every time value of the property changes
    var property1: String by Delegates.observable("initial value") { property, oldValue, newValue ->
        // do your stuff here
        refreshListListeners.forEach { 
            it.refreshListRequest()
        }
    }
}

interface InterfaceRefreshList {
    fun refreshListRequest()
}

Add listeners like this:

SignalChange.refreshListListeners.add(object : InterfaceRefreshList {
    override fun refreshListRequest() {
        refreshList()
    }
})

OR

Intead of interface you can use lambda:

object SignalChange {
    var refreshListListeners = ArrayList<() -> Unit>()

    // fires off every time value of the property changes
    var property1: String by Delegates.observable("initial value") { property, oldValue, newValue ->
        // do your stuff here
        refreshListListeners.forEach {
            it()
        }
    }
}

And to add listener just call:

SignalChange.refreshListListeners.add(::refreshList)
//or
SignalChange.refreshListListeners.add { refreshList() }

fun refreshList() {

}

edited Jan 22 '19 at 20:37

answered Jan 22 '19 at 18:09

Sergio

27,326
8
128
149

With the posted Java code I was able to signal a change from anywhere in the program. Can the Delegates.observable do that? – Samuel Jan 22 '19 at 19:07
I edited my answer. When you set value to `property1` Delegates.observable fires off and invokes `refreshListRequest()` on each listener. – Sergio Jan 22 '19 at 19:20
This seems the same as fun sendRefreshSignal() for(l in refreshListListeners) l.refreshPersonsList() but the issue I am having is how to invoke: fun addRefreshListListener(l: InterfaceRefreshPersonsList) refreshListListeners.add(l) from the FragmentBrowse class – Samuel Jan 22 '19 at 20:07
it seems you need implement `SignalChange` as `Singleton`. And then you will be able to add listeners like this. I've edited my answer, now `SignalChange ` is a `Singleton` – Sergio Jan 22 '19 at 20:32
You gave us 2 answers which is great. Is one better than the other? – Jevon Jan 31 '21 at 20:22

score 13 · Answer 2 · answered May 14 '20 at 20:11

13

The simple way to Listen to variable in KOTLIN

private var myVariableName by Delegates.observable(0) { property, oldValue, newValue ->
             Log.d(TAG,"New Value $newValue")
             Log.d(TAG,"Old Value $oldValue")
        }

for more info read https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.properties/-delegates/

answered May 14 '20 at 20:11

Ahmad Darwesh

553
4
12

1

Thank you. How do you assign a type for myVariableName ? I tried ```myVariableName: String by Delegates....``` and it does not work – Jack' Jun 06 '21 at 19:31
2

Oh, I get it. Assigning 0 as the initial value automatically makes myVariableName as an Int. So we should do ```myVariableName by Delegates.observable("Initial Text")``` for a String – Jack' Jun 06 '21 at 19:34

How to set up a listener for a variable in Kotlin

2 Answers2