How to access constructor argument that's not member variable in init function?

Question

I have a custom layout as below

class CustomComponent : FrameLayout {

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

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

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

    init {
        LayoutInflater.from(context).inflate(R.layout.view_custom_component, this, true)
    }

    fun initAttrs(attrs: AttributeSet?) {
        val typedArray = context.obtainStyledAttributes(attrs, R.styleable.custom_component_attributes, 0, 0)
        my_title.text = resources.getText(typedArray
                .getResourceId(R.styleable.custom_component_attributes_custom_component_title, R.string.component_one))
        typedArray.recycle()
    }
}

Now for each constructor, I have to explicitly call initAttrs(attrs) as I can't find way to access attrs in my init function.

Is there a way I could access attrs in init function, so I could call initAttrs(attrs) from init instead of having to explicitly call it in each of the constructor?

Answer 1

Use a constructor with default arguments:

class CustomComponent @JvmOverloads constructor(
  context: Context, 
  attrs: AttributeSet? = null, 
  defStyle: Int = 0
) : FrameLayout(context, attrs, defStyle) {

    fun init {
      // Initialize your view
    }
}

The @JvmOverloads annotation tells Kotlin to generate three overloaded constructors so they can be called in Java as well.

In your init function, attrs becomes available as a nullable type:

fun init {
  LayoutInflater.from(context).inflate(R.layout.view_custom_component, this, true)

  attrs?.let {
        val typedArray = context.obtainStyledAttributes(it, R.styleable.custom_component_attributes, 0, 0)
        my_title.text = resources.getText(typedArray
                .getResourceId(R.styleable.custom_component_attributes_custom_component_title, R.string.component_one))
        typedArray.recycle()
  }
}

Note the usage of it in the let block.