How set Spannable object font with custom font

I have Spannable object which I want to set its font by a custom font I have loaded before.

Typeface tf = Typeface.createFromAsset(getAssets(), "fonts/font_Name.ttf");
Spannable span1 = /*Spannable Item*/;

/// I want to set span1 to have tf font face???
/// Here where I want help.

EDIT :
My problem is that I want to set two different custom fonts for the text view so I am working with the Spannable

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);
}
}

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Reference