Elegant way to use custom/standard attributes on Android

Original way:

You need to get/set attribute one by one.

// standard attributes
val textSet = intArrayOf(android.R.attr.text)
val ta = context.obtainStyledAttributes(attrs, textSet)
val text = ta.getString(0)
setText(text)
ta.recycle()
val backgroundSet = intArrayOf(android.R.attr.background)
val ba = context.obtainStyledAttributes(attrs, backgroundSet)
val background = ba.getDrawable(0)
setBackground(background)
ba.recycle()

// custom attributes
val ca = context.obtainStyledAttributes(attrs, R.styleable.MyCustomView)

val title = ca.getString(R.styleable.MyCustomView_title)
setTitle(title)

val titleColor = ca.getColor(R.styleable.MyCustomView_titleColor, Color.BLACK)
setTitleColor(titleColor)

ca.recycle()

Elegant way:

Use an array to include attributes you want to get and set

val set = intArrayOf(
        android.R.attr.background,
        android.R.attr.text,
        R.styleable.MyCustomView[R.styleable.MyCustomView_title],
        R.styleable.MyCustomView[R.styleable.MyCustomView_titleColor]
)
// for android limitation, before finding attrs, you need to sort all attrs you want to find in ascending order, otherwise some attrs cannot be found
// https://developer.android.com/reference/android/content/res/Resources.Theme.html#obtainStyledAttributes(android.util.AttributeSet, int[], int, int)
val sorted = set.apply { sort() }
val sa = context.obtainStyledAttributes(attrs, sorted)
val background = sa.getDrawable(sorted.indexOf(android.R.attr.background))
setBackground(background)

val text = sa.getText(sorted.indexOf(android.R.attr.text))
setText(text)

val title = sa.getText(sorted.indexOf(R.styleable.MyCustomView[R.styleable.MyCustomView_title]))
setTitle(title)

val titleColor = sa.getColor(sorted.indexOf(R.styleable.MyCustomView[R.styleable.MyCustomView_titleColor]))
setTitleColor(titleColor)

sa.recycle()

More elegant way:

Define a map, whose key is attribute you want to find, value is a function that you want to do after finding attribute.

val map = mapOf(
        android.R.attr.background to
                fun (a: TypedArray, i: Int) { background = a.getDrawable(i) },
        
        android.R.attr.text to
                fun (a: TypedArray, i: Int) = setText(a.getString(i)),
                
        R.styleable.MyCustomView[R.styleable.MyCustomView_title] to
                fun (a: TypedArray, i: Int) = setTitle(a.getString(i)),
        
        R.styleable.MyCustomView[R.styleable.MyCustomView_titleColor] to
                fun (a: TypedArray, i: Int) = setTitleColor(a.getColor(i, Color.BLACK))
)
setupAttrs(context, attrs, map)

/**
 * @param context application/activity context
 * @param attrs The attributes of the XML tag that is inflating the view.
 * @param map Map<Int, (TypedArray, Int) -> Unit>
 *  <Int> the attr resource id that you want to find in attrs,
 *  <(TypedArray, Int) -> Unit>, A callback function after finding specific attr resource id
 *  <TypedArray> contains real attribute values, <Int> is index in <TypedArray>
 */
fun setupAttrs(
        context: Context, attrs: AttributeSet, map: Map<Int, (TypedArray, Int) -> Unit>
) {
    // for android limitation, you need to sort all attrs you want to find in ascending order
    // then find attrs, otherwise some attrs cannot be found
    // https://developer.android.com/reference/android/content/res/Resources.Theme.html#obtainStyledAttributes(android.util.AttributeSet, int[], int, int)
    val sorted = map.toSortedMap()
    val set = sorted.keys.toIntArray().apply { sort() }

    // extract out attribute from your layout xml
    val a = context.obtainStyledAttributes(attrs, set)

    (0 until a.indexCount)
            .map(a::getIndex)
            .forEach {
                // map to what you want to do
                sorted[set[it]]?.invoke(a, it)
            }

    a.recycle()
}