android set custom font to a paint

If you are using Android's new Fonts in XML for your fonts, then to get the typeface used for paint you can use:

val customTypeface = ResourcesCompat.getFont(context, R.font.myfont)

or if your min Android API >= 26

val customTypeface = resources.getFont(R.font.myfont)

Then to apply it to your paint object:

mTextPaint.typeface = customTypeface

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For more info check out https://developer.android.com/guide/topics/ui/look-and-feel/fonts-in-xml#fonts-in-code

If by "custom font" you mean a font that you are supplying as an asset, the following code should work:

Typeface plain = Typeface.createFromAsset(assetManager, pathToFont); 
Typeface bold = Typeface.create(plain, Typeface.DEFAULT_BOLD)
Paint paint = new Paint();
paint.setTypeface(bold);
canvas.drawText("Sample text in bold",0,0,paint);