- Set it up, so your iPhone is your second screen (desktop and iPhone have to be on the same wifi)
- Open the app layout you're working on in Photoshop
- Duplicate the Photoshop screen (Window > Arrange > New Window for X)
- Then pull that duplicated window onto the iPhone screen
- 100% viewsize, then hit F to fullscreen that window
- You can now design away, but instantly see the live layout on the actual device, in retina, fullscreen. You can even simulate scrolling with your mouse, pass it around the office to show people, hide/show different layouts to compare...
Same works for Android etc of course.
Citymapper
Product Hunt