I have a button that I want to be different widths depending on which kind of iPhone is being used. Obviously a larger width for the 7 Plus, and smaller on down the line. I click the button, click Vary for Traits, choose Height, change constraints for each iPhone, then click Done Varying and build, but it always keeps whatever the last constraint I edited was, no matter which device I run it on.
Vary For Traits has nothing to do with different iPhone sizes. All iPhones (in portrait) have identical traits; there is nothing to vary.
来源：https://stackoverflow.com/questions/41710979/ios-use-vary-for-traits-between-iphone-sizes