How to add custom attributes to a custom view in Android?

Defining attributes in attrs.xml: Create a file res/values/attrs.xml (or add to an existing one). In it, define <declare-styleable> with the name of the custom View and list <attr> for each custom attribute, specifying their format (format).

<?xml version="1.0" encoding="utf-8"?>  
<resources>  
    <declare-styleable name="MyCustomView">  
        <attr name="customText" format="string"/>  
        <attr name="customColor" format="color"/>  
        <attr name="customEnabled" format="boolean"/>  
    </declare-styleable>  
</resources>  

Using attributes in XML layout: In the XML layout where the custom View is used, add the specified attributes using the app namespace.

<com.example.MyCustomView  
    android:layout_width="wrap_content"  
    android:layout_height="wrap_content"  
    xmlns:app="http://schemas.android.com/apk/res-auto"  
    app:customText="Hello Custom View"  
    app:customColor="@color/colorPrimary"  
    app:customEnabled="true"/>  

Reading attributes in View code: In the constructor of the custom View (usually the one that takes Context and AttributeSet), use TypedArray and obtainStyledAttributes to read the attribute values specified in XML.

class MyCustomView @JvmOverloads constructor(  
    context: Context,  
    attrs: AttributeSet? = null,  
    defStyleAttr: Int = 0  
) : View(context, attrs, defStyleAttr) {  

    // Variables to store attribute values  
    private var customText: String? = null  
    private var customColor: Int = 0  
    private var customEnabled: Boolean = false  

    init {  
        // Obtain TypedArray with attributes  
        val typedArray = context.obtainStyledAttributes(attrs, R.styleable.MyCustomView, defStyleAttr, 0)  

        try {  
            // Read attribute values  
            customText = typedArray.getString(R.styleable.MyCustomView_customText)  
            customColor = typedArray.getColor(R.styleable.MyCustomView_customColor, 0) // Default value added  
            customEnabled = typedArray.getBoolean(R.styleable.MyCustomView_customEnabled, false) // Default value added  

            // Now you can use customText, customColor, and customEnabled to configure the View  
            // For example:  
            // if (customEnabled) { /* ... */ }  

        } finally {  
            // Important! Recycle TypedArray to free resources  
            typedArray.recycle()  
        }  
    }  

    // ... rest of the View logic ...  
}  

The obtainStyledAttributes method returns a TypedArray, from which attribute values can be extracted by their indices (generated by the R class). After use, call recycle() to free resources.