Defining custom attrs

The traditional approach is full of boilerplate code and clumsy resource handling. That's why I made the Spyglass framework. To demonstrate how it works, here's an example showing how to make a custom view that displays a String title.

Step 1: Create a custom view class.

public class CustomView extends FrameLayout {
    private TextView titleView;

    public CustomView(Context context) {
        super(context);
        init(null, 0, 0);
    }

    public CustomView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(attrs, 0, 0);
    }

    public CustomView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init(attrs, defStyleAttr, 0);
    }

    @RequiresApi(21)
    public CustomView(
            Context context, 
            AttributeSet attrs,
            int defStyleAttr,
            int defStyleRes) {

        super(context, attrs, defStyleAttr, defStyleRes);
        init(attrs, defStyleAttr, defStyleRes);
    }

    public void setTitle(String title) {
        titleView.setText(title);
    }

    private void init(AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        inflate(getContext(), R.layout.custom_view, this);

        titleView = findViewById(R.id.title_view);
    }
}

Step 2: Define a string attribute in the values/attrs.xml resource file:

<resources>
    <declare-styleable name="CustomView">
        <attr name="title" format="string"/>
    </declare-styleable>
</resources>

Step 3: Apply the @StringHandler annotation to the setTitle method to tell the Spyglass framework to route the attribute value to this method when the view is inflated.

@HandlesString(attributeId = R.styleable.CustomView_title)
public void setTitle(String title) {
    titleView.setText(title);
}

Now that your class has a Spyglass annotation, the Spyglass framework will detect it at compile-time and automatically generate the CustomView_SpyglassCompanion class.

Step 4: Use the generated class in the custom view's init method:

private void init(AttributeSet attrs, int defStyleAttr, int defStyleRes) {
    inflate(getContext(), R.layout.custom_view, this);

    titleView = findViewById(R.id.title_view);

    CustomView_SpyglassCompanion
            .builder()
            .withTarget(this)
            .withContext(getContext())
            .withAttributeSet(attrs)
            .withDefaultStyleAttribute(defStyleAttr)
            .withDefaultStyleResource(defStyleRes)
            .build()
            .callTargetMethodsNow();
}

That's it. Now when you instantiate the class from XML, the Spyglass companion interprets the attributes and makes the required method call. For example, if we inflate the following layout then setTitle will be called with "Hello, World!" as the argument.

<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:width="match_parent"
    android:height="match_parent">

    <com.example.CustomView
        android:width="match_parent"
        android:height="match_parent"
        app:title="Hello, World!"/>
</FrameLayout>

The framework isn't limited to string resources has lots of different annotations for handling other resource types. It also has annotations for defining default values and for passing in placeholder values if your methods have multiple parameters.

Have a look at the Github repo for more information and examples.

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

<?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()
            }
        }
    }

• HERE is the official documentation for creating custom attributes and Views