How set Spannable object font with custom font

This is a late answer but will help others to solve the issue.

Use the following code:(I'm using Bangla and Tamil font)

  TextView txt = (TextView) findViewById(R.id.custom_fonts);  
        txt.setTextSize(30);
        Typeface font = Typeface.createFromAsset(getAssets(), "Akshar.ttf");
        Typeface font2 = Typeface.createFromAsset(getAssets(), "bangla.ttf");   
        SpannableStringBuilder SS = new SpannableStringBuilder("আমারநல்வரவு");
        SS.setSpan (new CustomTypefaceSpan("", font2), 0, 4,Spanned.SPAN_EXCLUSIVE_INCLUSIVE);
        SS.setSpan (new CustomTypefaceSpan("", font), 4, 11,Spanned.SPAN_EXCLUSIVE_INCLUSIVE);
        txt.setText(SS);

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The outcome is:

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CustomTypefaceSpan Class:

package my.app;
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);
}
}

---

Reference

If you are using Roboto, you can set a different TypefaceSpan in the constructor

TypefaceSpan typefaceSpan = new TypefaceSpan("sans-serif-medium");

textView.setSpan(typefaceSpan, indexStart, textLength, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);

We don't need to use CustomTypefaceSpan. Here is the solution using StyleSpan.

/**
* setCustomFontTypeSpan
* @param context
* @param source
* @param startIndex
* @param endIndex
* @param font
* @return
*/
public static SpannableString setCustomFontTypeSpan(Context context, String 
source, int startIndex, int endIndex, int font) {
      final SpannableString spannableString = new SpannableString(source);
      Typeface typeface = ResourcesCompat.getFont(context, font);
      spannableString.setSpan(new StyleSpan(typeface.getStyle()), 
      startIndex,endIndex,Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
return spannableString;
}

String source = "Hello world";
SpannableString string = setCustomFontTypeSpan(context, source, 6, 
source.length(), R.font.open_sans_bold);
textView.setText(string);

Create a CustomTypefaceSpan class:

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

public class CustomTypefaceSpan extends MetricAffectingSpan {

    private final Typeface typeface;

    public CustomTypefaceSpan(Typeface typeface) {
        this.typeface = typeface;
    }

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

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

    private static void applyCustomTypeFace(Paint paint, Typeface tf) {
        paint.setTypeface(tf);
    }
}

Use in the same way as the Android framework spans classes:

    TextView textView = (TextView) findViewById(R.id.custom_fonts);
    Typeface font = Typeface.createFromAsset(getAssets(), "Akshar.ttf");
    Typeface font2 = Typeface.createFromAsset(getAssets(), "bangla.ttf");
    SpannableStringBuilder spannableStringBuilder = new SpannableStringBuilder("আমারநல்வரவு");
    spannableStringBuilder.setSpan (new CustomTypefaceSpan(font2), 0, 4,Spanned.SPAN_EXCLUSIVE_INCLUSIVE);
    spannableStringBuilder.setSpan (new CustomTypefaceSpan(font), 4, 11,Spanned.SPAN_EXCLUSIVE_INCLUSIVE);
    textView.setText(spannableStringBuilder);

This answer is based on Imran Rana's answer but does not extend TypefaceSpan and then disable its functionality. CustomTypefaceSpan extends MetricAffectingSpan directly.

This answer shares a defect with Imran Rana's answer. The span is not parcelled. I.e if you do this (kotlin):

    val parcel = Parcel.obtain()
    TextUtils.writeToParcel(spannableStringBuilder, parcel, 0)
    parcel.setDataPosition(0)
    val sequence = TextUtils.CHAR_SEQUENCE_CREATOR.createFromParcel(parcel)
    parcel.recycle()

Any CustomTypefaceSpan objects set on spannableStringBuilder will not be marshalled and unmarshalled.