I use custom view to set a UIButton on the navigation bar.
But the text on the button like below:

is not like the default button text that clear.

I tried to add another label on the button, but it seems failed.
I also tied code like this, but seems also not working.
doneButton.titleLabel.layer.shadowColor = [UIColor blackColor].CGColor;
doneButton.titleLabel.layer.shadowOffset = CGSizeMake(0, -1.0);
These code just works well.
I got it from https://github.com/boctor/idev-recipes which is very great for custom UI fulfillment.