Defining custom attrs

Question

I need to implement my own attributes like in com.android.R.attr

Found nothing in official documentation so I need information about how to define these

Answer 1

if you omit the format attribute from the attr element, you can use it to reference a class from XML layouts.

• example from attrs.xml.
• Android Studio understands that the class is being referenced from XML
  • i.e.
    • Refactor > Rename works
    • Find Usages works
    • and so on...

don't specify a format attribute in .../src/main/res/values/attrs.xml

    
        ....
        
        ....
    

use it in some layout file .../src/main/res/layout/activity__main_menu.xml

parse the class in your view initialization code .../src/main/java/.../MyCustomView.kt

class MyCustomView(
        context:Context,
        attrs:AttributeSet)
    :View(context,attrs)
{
    // parse XML attributes
    ....
    private val giveMeAClass:SomeCustomInterface
    init
    {
        context.theme.obtainStyledAttributes(attrs,R.styleable.ColorPreference,0,0).apply()
        {
            try
            {
                // very important to use the class loader from the passed-in context
                giveMeAClass = context::class.java.classLoader!!
                        .loadClass(getString(R.styleable.MyCustomView_give_me_a_class))
                        .newInstance() // instantiate using 0-args constructor
                        .let {it as SomeCustomInterface}
            }
            finally
            {
                recycle()
            }
        }
    }