在日常代码中，我们常常要重复用到view的一些属性，比如view.center.x、view.frame.origin.y等等，每次用到都要写一遍view.center.x之类会让代码看起来很麻烦，所以必要用extension来扩展一些属性。
贴代码：

import UIKit
/// MARK - UIView
extension UIView {
    
    // MARK: - 常用位置属性
    public var left:CGFloat {
        get {
            return self.frame.origin.x
        }
        set(newLeft) {
            var frame = self.frame
            frame.origin.x = newLeft
            self.frame = frame
        }
    }
    
    public var top:CGFloat {
        get {
            return self.frame.origin.y
        }
        
        set(newTop) {
            var frame = self.frame
            frame.origin.y = newTop
            self.frame = frame
        }
    }
    
    public var width:CGFloat {
        get {
            return self.frame.size.width
        }
        
        set(newWidth) {
            var frame = self.frame
            frame.size.width = newWidth
            self.frame = frame
        }
    }
    
    public var height:CGFloat {
        get {
            return self.frame.size.height
        }
        
        set(newHeight) {
            var frame = self.frame
            frame.size.height = newHeight
            self.frame = frame
        }
    }
    
    public var right:CGFloat {
        get {
            return self.left + self.width
        }
    }
    
    public var bottom:CGFloat {
        get {
            return self.top + self.height
        }
    }
    
    public var centerX:CGFloat {
        get {
            return self.center.x
        }
        
        set(newCenterX) {
            var center = self.center
            center.x = newCenterX
            self.center = center
        }
    }
    
    public var centerY:CGFloat {
        get {
            return self.center.y
        }
        
        set(newCenterY) {
            var center = self.center
            center.y = newCenterY
            self.center = center
        }
    }
}

当你需要使用一个视图的y轴顶点坐标的时候，只需要view.top即可。
其他类型的extension也是如此，通过这种方法，会让我们的代码更加简洁。