Custom attributes with class reference

Question

I'm trying to create a custom attribute that behaves like tools:context, that is with

• Android Studio auto complete functionallity
• Project classname reference
• Support for auto refactory in case I change my class directory

This is my resources.xml

<declare-styleable name="RecyclerView">
    <attr name="adapter" format="string"></attr>
</declare-styleable>

This is the usage

    <example.com.br.appname.RecyclerView
         android:id="@+id/accounts"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:layout_marginTop="8dp"
         app:adapter="example.com.br.appname.AccountAdapter" >
    </example.com.br.appname.RecyclerView>

I've tried to use the format reference but it didn't compile as well.

Error:(17, 22) String types not allowed (at 'adapter' with value 'example.com.br.appname.AccountAdapter').

Answer 1

I don’t think this is possible currently. Other similar custom attrs I can think of, for instance app:layout_behavior from the design library, or simply app:layoutManager from RecyclerView all require the full classname, with none of your requirements.

It might be better to store these in a strings resource file, and remember to check it when refactoring class names.

You can consider filing a feature request, since Android Studio has this functionality in special cases (tools:context, class in <view> and <fragment> tags, classes in Manifest...), but I doubt they would add a new attribute format just for this.

Answer 2

so...

• apparently, YOU CAN!
• Google does this too.
• 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

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <declare-styleable name="MyCustomView">
        ....
        <attr name="give_me_a_class"/>
        ....
    </declare-styleable>

</resources>

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

<?xml version="1.0" encoding="utf-8"?>
<SomeLayout
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <!-- make sure to use $ dollar signs for nested classes -->
    <MyCustomView
        app:give_me_a_class="class.type.name.Outer$Nested/>

    <MyCustomView
        app:give_me_a_class="class.type.name.AnotherClass/>

</SomeLayout>

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()
            }
        }
    }