添加“计算属性”内容

source/chapter2/10_Properties.md

@@ -105,3 +105,36 @@ Swift编程语言中把这些理论统一用属性来实现。Swift中的属性
 
 ## 计算属性
 
+除存储属性外，类、结构体和枚举可以定义*计算属性*，计算属性不直接存储值，而是提供一个getter来获取值，一个可选的setter来间接设置值。
+
+```
+struct Point {
+    var x = 0.0, y = 0.0
+}
+struct Size {
+    var width = 0.0, height = 0.0
+}
+struct Rect {
+    var origin = Point()
+    var size = Size()
+    var center: Point {
+    get {
+        let centerX = origin.x + (size.width / 2)
+        let centerY = origin.y + (size.height / 2)
+        return Point(x: centerX, y: centerY)
+    }
+    set(newCenter) {
+        origin.x = newCenter.x - (size.width / 2)
+        origin.y = newCenter.y - (size.height / 2)
+    }
+    }
+}
+var square = Rect(origin: Point(x: 0.0, y: 0.0),
+    size: Size(width: 10.0, height: 10.0))
+let initialSquareCenter = square.center
+square.center = Point(x: 15.0, y: 15.0)
+println("square.origin is now at (\(square.origin.x), \(square.origin.y))")
+// prints "square.origin is now at (10.0, 10.0)”
+
+```
+