CustomEditTextView.kt

import android.content.Context
import android.graphics.Canvas
import android.util.AttributeSet
import androidx.appcompat.widget.AppCompatEditText

class CustomEditTextView: AppCompatEditText {

    private var padding = -1f
    private var prefix = ""

    constructor(context: Context) : super(context) {
        init(context, null)
    }

    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs) {
        init(context, attrs)
    }

    constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr) {
        init(context, attrs)
    }

    fun init(context: Context, attributeSet: AttributeSet?) {
        attributeSet?.let {
            val typedArray = context.obtainStyledAttributes(attributeSet, R.styleable.EditText)
            prefix = typedArray.getString(R.styleable.EditText_prefix)
            typedArray.recycle()
        }
    }

    fun prefix(prefix: String) = apply {
        this.prefix = prefix
        invalidate()
    }

    override fun onDraw(canvas: Canvas?) {
        super.onDraw(canvas)

        if (prefix.isNotEmpty()) {
            prefixSet()
            canvas?.drawText(prefix, padding, getLineBounds(0, null).toFloat(), paint)
        }
    }

    private fun prefixSet() {
        if (padding == -1f) {
            val widths = FloatArray(prefix.length)
            var textWidth = 0f

            paint.getTextWidths(prefix, widths)

            for (w in widths) {
                textWidth += w
            }

            padding = compoundPaddingLeft.toFloat()
            setPadding(
                    (textWidth + padding).toInt(),
                    paddingStart,
                    paddingTop,
                    paddingBottom
            )
        }
    }

    // get the text with prefix
    fun completedText() = prefix + text.toString()

}