pie.progressLayer.strokeColor = self.primaryColor, there is a primaryColor line when "progress=0.0"

[daishuyi]

M13ProgressViewPie progressView. when I set pie.primaryColor, pie.progressLayer.strokeColor = self.primaryColor.CGColor. There is a primaryColor line when progress = 0.0. So I want to understand that why do you set strokeColor and not fillColor.