I want to start designing iOS app UI in Photoshop but I can't find up-to-date tutorials and informations about document resolution, ppi of document, getting retina ready and complete guide for it. Does anybody know any good tutorials?
This information is fragmented, like Android device resolution. Apple stopped caring about designers when they realised the iPod was way more successful than their pandering to the DTP community ever made them.
This, combined with a near pathological disdain for Adobe (largely warranted) means they don't provide useful templates or other resources to work in 1x, 2x and 3x at the same time.
And then there's the enormous fun of the scaling on the iPhone 6/6S/7 PLUS models.
As a starting point, that's not ideal, begin working in the lowest resolution of the smallest device you'll be targeting. Probably the iPhone 5/5S/SE's 4 inch display, which is 1136x640
This is a 2x screen. You don't really need 1x graphics anymore. But you do need 3x plus a scaling offset for the Plus series phones. All the iPads now run at 2x.