Developer and Designer Hybrid@psousa
If you are interested in designing iPhone or Mac applications, chances are you already stumbled into the Human Interface Guidelines website from Apple, which offers tons of information on how to design and build applications for any of these platforms.
From design principles to practical rules on how to approach for example navigation or using alerts inside your applications, there's information here that you should take a good look.
What's also interesting is the Resources link because recently apple has updated their Design Resources for Sketch.
So now, if you want to design for example an iPhone application, you can use their official design resources by importing a Sketch Library.
This let's you access a collection of all the most common and important UI components you need to design an application and when Apple decides to do an update you will get notified of those changes.
Also these are official components so their design and measurements will be very accurate.