How to change navigation drawer font?

You can do this in more easy way.

First go in style.xml and create there a new style.

 <style name="RobotoTextViewStyle" parent="android:Widget.TextView">
      <item name="android:fontFamily">@font/sans-serif-smallcaps</item>
 </style>

and the second is go to your navigation xml code and put the item text appearance there.

app:itemTextAppearance="@style/RobotoTextViewStyle"

now your final navigation code should be this.

<android.support.design.widget.NavigationView
        android:id="@+id/nav_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        app:itemTextAppearance="@style/RobotoTextViewStyle"
        android:fitsSystemWindows="true"
        app:headerLayout="@layout/nav_header_home"
        app:menu="@menu/activity_home_drawer" />

I found the answer: First create this class in your project:

import android.graphics.Paint;
import android.graphics.Typeface;
import android.text.TextPaint;
import android.text.style.TypefaceSpan;

public class CustomTypefaceSpan extends TypefaceSpan {

    private final Typeface newType;

    public CustomTypefaceSpan(String family, Typeface type) {
        super(family);
        newType = type;
    }

    @Override
    public void updateDrawState(TextPaint ds) {
        applyCustomTypeFace(ds, newType);
    }

    @Override
    public void updateMeasureState(TextPaint paint) {
        applyCustomTypeFace(paint, newType);
    }

    private static void applyCustomTypeFace(Paint paint, Typeface tf) {
        int oldStyle;
        Typeface old = paint.getTypeface();
        if (old == null) {
            oldStyle = 0;
        } else {
            oldStyle = old.getStyle();
        }

        int fake = oldStyle & ~tf.getStyle();
        if ((fake & Typeface.BOLD) != 0) {
            paint.setFakeBoldText(true);
        }

        if ((fake & Typeface.ITALIC) != 0) {
            paint.setTextSkewX(-0.25f);
        }

        paint.setTypeface(tf);
    }
}

Then add this method to your activity you want to change the font of navigation drawer menu:

private void applyFontToMenuItem(MenuItem mi) {
        Typeface font = Typeface.createFromAsset(getAssets(), "ds_digi_b.TTF");
        SpannableString mNewTitle = new SpannableString(mi.getTitle());
        mNewTitle.setSpan(new CustomTypefaceSpan("" , font), 0 , mNewTitle.length(),  Spannable.SPAN_INCLUSIVE_INCLUSIVE);
        mi.setTitle(mNewTitle);
}

and then add call the method you just added in your activity:

navView = (NavigationView) findViewById(R.id.navView);
Menu m = navView.getMenu();
for (int i=0;i<m.size();i++) {
    MenuItem mi = m.getItem(i);

    //for aapplying a font to subMenu ...
    SubMenu subMenu = mi.getSubMenu();
    if (subMenu!=null && subMenu.size() >0 ) {
        for (int j=0; j <subMenu.size();j++) {
            MenuItem subMenuItem = subMenu.getItem(j);
            applyFontToMenuItem(subMenuItem);
        }
    }

    //the method we have create in activity
    applyFontToMenuItem(mi);
}