Android “?colorPrimary” vs “?attr/colorPrimary”?

Question

I can't find any information about the differences between:

android:textColor="?attr/colorPrimary"

vs

android:textColor="?colorPrimary"

I have read that "?attr" means attribute value specified in the current theme, but without "attr" it gives the same result (=color defined in my theme). It behaves similar with other attributes?

For example:

Does android:background="?attr/selectableItemBackground" equals android:background="?selectableItemBackground" ?

Here it's told that it differs.

Thanks very much.

Answer 1

Ok, I have finally found in the docs:

Because the system resource tool knows that an attribute resource is expected in this context, you do not need to explicitly state the type (which would be ?android:attr/textColorSecondary)—you can exclude the attr type.

So, I deducted it's like this: the AppCompat library attributes are treated as custom attributes and as a result we point them by a question mark without "android" keyword. And "attr" is type of resource which is optional for both platform and custom attributes (from docs). Android documentation