Styjun

I am trying to achieve an effect in my app of where the user scrolls down, the opacity of a view will change from 0 to 1.

To achieve this, I've created a ColorDrawable with my desired color, blue, and then set its alpha to 0.

val actionBarBackground = ColorDrawable(ContextCompat.getColor(it, R.color.myBlue))
(activity as? AppCompatActivity)?.supportActionBar?.setBackgroundDrawable(actionBarBackground)

However, after increasing the alpha, it does not change. I've tried printing the value of actionBarBackground but its still 0...

// This is called inside a scrollview callback that calculates an alpha value between 0 and 255
actionBarBackground.alpha = 255
Log.d(TAG, "Alpha: $actionBarBackground.alpha") // Prints: Alpha: 0

Any ideas why the alpha of the ColorDwarable does not change? Thank you.

Thanks to @Jon Goodwin's comment, I finally fixed the problem.

For some reason, changing the alpha value on a ColorDrawable in Kotlin, does not seem to have any effect (it used to work in Java).

However, replacing this ColorDrawable with the Drawable you get from calling .mutate() on the ColorDrawable, makes the alpha changes work.

My final, working code from my question:

val actionBarBackground = ColorDrawable(ContextCompat.getColor(it, R.color.myBlue)).mutate()
// Keep in mind that actionBarBackground now is a Drawable, not a ColorDrawable
(activity as? AppCompatActivity)?.supportActionBar?.setBackgroundDrawable(actionBarBackground)

actionBarBackground.alpha = 255
Log.d(TAG, "Alpha: $actionBarBackground.alpha") // Prints: Alpha: 255
// This also works when called form inside a ScrolView Listener, to fade the actionbar background.

I guess I should answer, as Lucas P. says:

However, replacing this ColorDrawable with the Drawable you get from
calling .mutate() on the ColorDrawable, makes the alpha changes work.

But it is not for some reason, there is a reason:

mutate() added in API level 3

open fun mutate(): Drawable

A mutable BitmapDrawable still shares its Bitmap with any other
Drawable that comes from the same resource.

mutate()
Added in API level 3

public Drawable mutate ()

Make this drawable mutable. This operation cannot be reversed. A
mutable drawable is guaranteed to not share its state with any other
drawable. This is especially useful when you need to modify properties
of drawables loaded from resources. By default, all drawables
instances loaded from the same resource share a common state; if you
modify the state of one instance, all the other instances will receive
the same modification. Calling this method on a mutable Drawable will
have no effect.

reference mutate

Kotlin distinguishes between mutable and immutable collections, automatically --cool (if you know what that means).

An immutable class is a class whose state cannot be changed once it
has been created.
Mutability