[iOS] 让某个 view 总是在最顶端/前面（front）

Objective-C

#import <QuartzCore/QuartzCore.h>
myAlwaysOnTopView.layer.zPosition = MAXFLOAT;

Swift 3

myAlwaysOnTopView.layer.zPosition = .greatestFiniteMagnitude

参考资料：https://stackoverflow.com/questions/6511443/how-to-make-a-uiview-always-appear-at-the-front