Previously I’ve mentioned a number of components providing custom loading indicators most recently the Google material design inspired MMMaterialDesignSpinner.
Here’s an easy to use open source component loading indicator providing several different styles with a large degree of customizability called MBSimpleLoadingIndicator from Matt Brenman.
MBSimpleLoadindicator provides 3 different loading indicator styles, a straight line, half circle, and full circle and allows you to customize the inner and outer widths, and colors.
You can find MBSimpleLoadingIndicator on Github here.
A nice set of customizable loading indicators.