Basic extension of Android's TextView to allow declaring the use of custom fonts via xml. Assumes fonts are inside 'assets/fonts/' but this can be changed in `applyFont(..)`.

attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="CustomFontTextView">
        <attr name="fontFile" format="string" />
    </declare-styleable>
</resources>

CustomFontTextView.java

package foo.bar.baz;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Typeface;
import android.support.annotation.NonNull;
import android.util.AttributeSet;

import foo.bar.R;

/**
 * TextView that just uses a specified font file from 'assets/fonts/'.
 * Specify the font in xml with 'fontFile="my-font.ttf"'
 */
public class CustomFontTextView extends android.support.v7.widget.AppCompatTextView {
    public CustomFontTextView(Context context, AttributeSet attrs) {
        super(context, attrs);
        applyFont(attrs);
    }

    public CustomFontTextView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        applyFont(attrs);
    }

    private void applyFont(@NonNull AttributeSet attrs) {
        final TypedArray attributes = getContext().obtainStyledAttributes(attrs, R.styleable.CustomFontTextView);
        try {
            final String fontFile = attributes.getString(R.styleable.CustomFontTextView_fontFile);
            final Typeface font = Typeface.createFromAsset(getContext().getAssets(), "fonts/" + fontFile);
            setTypeface(font, getTypeface().getStyle());
        } finally {
            attributes.recycle();
        }
    }
}

sample xml

<foo.bar.baz.CustomFontTextView
            android:id="@+id/foo_text"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="BAR!"
            app:fontFile="Comic-Sans.ttf" />