原创：转载请注明出处
首先，因为遇到问题如下：
class MainTabBar: UITabBar {
override init(frame: CGRect) {
super.init(frame: frame)
self.translucent = false
self.backgroundImage = UIImage(named: "tabbar")
}
// //在子类重写父类的必要构造器时，必须在子类的构造器前也添加required修饰符，表明该构造器要求也应用于继承链后面的子类。
required init?(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
}
}
继承关系链：
UITabBar -> UIView ->
public class UIView : UIResponder, NSCoding, UIAppearance, UIAppearanceContainer, UIDynamicItem, UITraitEnvironment, UICoordinateSpace, UIFocusEnvironment
好，在UITabBar、UIView 中init方法并没有显示写出 Required
原因在此：
If you override any of a class's designated initializers, you don't inherit any other designated initializers. But UIView adopts the NSCoding protocol, which requires an init(coder:)initializer. So you must implement that one too:
required init(coder decoder: NSCoder) { super.init(coder: decoder) }
By the way: Xcode 会提示自动补全代码
参考链接：
1.http://stackoverflow.com/questions/26081287/required-initializer-initcoder-must-be-provided-by-subclass-of-uitablevi
2.https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Initialization.html#//apple_ref/doc/uid/TP40014097-CH18-XID_307