Create a callback using a lambda

Question

I'm having a difficult time trying how to figure out how to create a callback in Kotlin using lambdas. I have a custom TextInputEditText and I want to implement a function that the activity can call when text changes.

Here is my custom EditText:

class EditTextEx : TextInputEditText, TextWatcher {
    constructor(context: Context?) : super(context)
    constructor(context: Context?, attrs: AttributeSet?) : super(context, attrs)
    constructor(context: Context?, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr)


    override fun onTextChanged(s: CharSequence, start: Int, before: Int, count: Int) {
      // Call the callback onTextAvailable with the EditText's text (s.toString)
    }

    override fun afterTextChanged(p0: Editable?) {
    }

    override fun beforeTextChanged(p0: CharSequence?, p1: Int, p2: Int, p3: Int) {
    }

}

In my activity I want to have a callback that gets called when the onTextChanged event gets called. The callback in the custom control sends only the text back to the client. So in my activity, I want something like this:

editText.onTextAvailable(text -> do something )

EpicPandaForce · Accepted Answer · 2019-06-14 18:35:35Z

1

It's actually quite easy to do, look:

inline fun EditText.onTextChanged(crossinline onTextChange: (String) -> Unit): TextWatcher {
    val textWatcher = object: TextWatcher {
        override fun afterTextChanged(editable: Editable) {
            onTextChange(editable.toString())
        }

        override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) {}
        override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {}
    }
    this.addTextChangeListener(textWatcher)
    return textWatcher
}

Now you can call

editText.onTextChanged { text -> /* do something */ }

answered Jun 14, 2019 at 18:35

EpicPandaForce

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3 Comments

Big Star Over a year ago

So callbacks are primarily done using extension functions? I came across this but couldn't figure it out: stackoverflow.com/a/48761541/11647945 Is that a different approach that accomplishes the same thing?

Shermano Over a year ago

Not exactly, extension is just a way to provide new functionalities to a given class, callbacks are just interfaces that you can put it anywhere

EpicPandaForce Over a year ago

You can wrap callback methods with your preferred signature using extension functions.

Johann · Accepted Answer · 2019-06-15 08:44:55Z

In addition to the solution by @EpicPandaForce, there are a couple other solutions. If you want to stick with using a class as you've shown in your example, then you can do this:

class EditTextEx : TextInputEditText, TextWatcher {
    constructor(context: Context?) : super(context)
    constructor(context: Context?, attrs: AttributeSet?) : super(context, attrs)
    constructor(context: Context?, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr)

    private var mOnTextWatcherCallback: (m: String) -> Unit = {}

    override fun onTextChanged(s: CharSequence, start: Int, before: Int, count: Int) {
        if (mOnTextWatcherCallback != null)
            mOnTextWatcherCallback(s.toString())
    }

    override fun afterTextChanged(p0: Editable?) {
    }

    override fun beforeTextChanged(p0: CharSequence?, p1: Int, p2: Int, p3: Int) {
    }

    fun onTextChange(callback: (text: String) -> Unit) {
        mOnTextWatcherCallback = callback
    }
}

Then in your activity create a function:

fun onTextChange(text: String) {
  // Do something with the text.
}

And then setup your callback as follows:

my_edittext.onTextChange(::onTextChange)

This solution allows you to re-use the same onTextChange function for other controls that want to use it as well.

If you prefer to use an interface to define the callback, do this:

class EditTextEx : TextInputEditText, TextWatcher {
    constructor(context: Context?) : super(context)
    constructor(context: Context?, attrs: AttributeSet?) : super(context, attrs)
    constructor(context: Context?, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr)

    private var mOnTextWatcherCallback: ITextWatcher? = null

    override fun onTextChanged(s: CharSequence, start: Int, before: Int, count: Int) {
        if (mOnTextWatcherCallback != null)
            mOnTextWatcherCallback!!.onTextChanged(s.toString())
    }

    override fun afterTextChanged(p0: Editable?) {
    }

    override fun beforeTextChanged(p0: CharSequence?, p1: Int, p2: Int, p3: Int) {
    }

    fun onTextChange(callback: ITextWatcher) {
        mOnTextWatcherCallback = callback
    }
}

Then in your activity, create the callback as follows:

val textChangeHandler = object: ITextWatcher {
    override fun onTextChanged(text: String) {
        var t = text
    }
}

And then setup your callback for your edittext controls as follows:

my_edittext.onTextChange(textChangeHandler)

Shermano · Accepted Answer · 2019-06-14 17:48:33Z

0

Try something like this:

fun getEditTextChange(editText: EditText, onTextChange: (String) -> Unit){
        val tw = object: TextWatcher {
            private var region = Locale.getDefault().language

            override fun afterTextChanged(s: Editable?) {
                onTextChange.invoke(s.toString())
            }

            override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) {}
            override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {}

        }
        editText.addTextChangedListener(tw)
    }

Hope it helps

answered Jun 14, 2019 at 17:48

Shermano

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