要不要来点 Swift

作者：Jacob Bandes-Storch， 原文链接 ，原文日期：2015-11-28

译者： mmoaay ；校对：千叶知风；定稿： shanks

label.textAlignment = NSTextAlignmentCenter;

label.textAlignment = NSTextAlignment.Center

// 更简明的:
label.textAlignment = .Center

animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)

extension CAMediaTimingFunction
{
    // 这个属性会在第一次被访问时初始化。
    // (需要添加 @nonobjc 来防止编译器
    //  给 static（或者 final）属性生成动态存取器。)
    @nonobjc static let EaseInEaseOut = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)

    // 另外一个选择就是使用计算属性, 它同样很有效,
    // 但 *每次* 被访问时都会重新求值：
    static var EaseInEaseOut: CAMediaTimingFunction {
        // .init 是 self.init 的简写
        return .init(name: kCAMediaTimingFunctionEaseInEaseOut)
    }
}

animation.timingFunction = .EaseInEaseOut

CGContextSetFillColorWithColor(UIGraphicsGetCurrentContext(),
    CGColorCreate(CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB), [0.792, 0.792, 0.816, 1]))

extension CGContext
{
    static func currentContext() -> CGContext? {
        return UIGraphicsGetCurrentContext()
    }
}

extension CGColorSpace
{
    static let GenericRGB = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB)
}

CGContextSetFillColorWithColor(.currentContext(),
    CGColorCreate(.GenericRGB, [0.792, 0.792, 0.816, 1]))

spaceConstraint = NSLayoutConstraint(
    item: label,
    attribute: .Leading,
    relatedBy: .Equal,
    toItem: button,
    attribute: .Trailing,
    multiplier: 1, constant: 20)
widthConstraint = NSLayoutConstraint(
    item: label,
    attribute: .Width,
    relatedBy: .LessThanOrEqual,
    toItem: nil,
    attribute: .NotAnAttribute,
    multiplier: 0, constant: 200)

spaceConstraint.active = true
widthConstraint.active = true

spaceConstraint = label.leadingAnchor.constraintEqualToAnchor(button.trailingAnchor, constant: 20)
widthConstraint = label.widthAnchor.constraintLessThanOrEqualToConstant(200)
spaceConstraint.active = true
widthConstraint.active = true

spaceConstraint = label.constrain(.Leading, .Equal, to: button, .Trailing, plus: 20)
widthConstraint = label.constrain(.Width, .LessThanOrEqual, to: 200)

// "设置 label 的 leading edge 和 button 的 trailing edge 相距 20"
// "设置 label 的 width 小于等于 200。"

extension UIView
{
    func constrain(
        attribute: NSLayoutAttribute,
        _ relation: NSLayoutRelation,
        to otherView: UIView,
        _ otherAttribute: NSLayoutAttribute,
        times multiplier: CGFloat = 1,
        plus constant: CGFloat = 0,
        atPriority priority: UILayoutPriority = UILayoutPriorityRequired,
        identifier: String? = nil)
        -> NSLayoutConstraint
    {
        let constraint = NSLayoutConstraint(
            item: self,
            attribute: attribute,
            relatedBy: relation,
            toItem: otherView,
            attribute: otherAttribute,
            multiplier: multiplier,
            constant: constant)
        constraint.priority = priority
        constraint.identifier = identifier
        constraint.active = true
        return constraint
    }
    
    func constrain(
        attribute: NSLayoutAttribute,
        _ relation: NSLayoutRelation,
        to constant: CGFloat,
        atPriority priority: UILayoutPriority = UILayoutPriorityRequired,
        identifier: String? = nil)
        -> NSLayoutConstraint
    {
        let constraint = NSLayoutConstraint(
            item: self,
            attribute: attribute,
            relatedBy: relation,
            toItem: nil,
            attribute: .NotAnAttribute,
            multiplier: 0,
            constant: constant)
        constraint.priority = priority
        constraint.identifier = identifier
        constraint.active = true
        return constraint
    }
}

// 触摸开始 / 鼠标按下:

let touchPos = touch.locationInView(container)
objectOffset = CGPoint(x: object.center.x - touchPos.x, y: object.center.y - touchPos.y)

// 触摸移动 / 鼠标拖动:

let touchPos = touch.locationInView(container)
object.center = CGPoint(x: touchPos.x + objectOffset.x, y: touchPos.y + objectOffset.y)
``` 

在这段代码里面我们只做了简单的加法和减法，但因为 `CGPoint` [包含](https://en.wikipedia.org/wiki/User:Giraffedata/comprised_of) `x` 和 `y`，所以每个表达式我们都要写两次。所以我们需要一些简化操作的函数。

`objectOffset` 代表触摸位置和对象位置的距离。描述这种距离最好的方式并不是 `CGPoint`，而是不那么为人所知的 [CGVector](https://developer.apple.com/library/ios/documentation/GraphicsImaging/Reference/CGGeometry/#//apple_ref/c/tdef/CGVector)， 它不使用 `x` 和 `y`，而是用 `dx` 和 `dy` 来表示距离或者 “deltas“。

![这里写图片描述](/img/articles/help-yourself-to-some-swift/201512151428497511452821418.99237)

所以两个点相减得到一个向量就比较符合逻辑了，这样一来我们就得到了 `-` 操作符的一个重载：

然后，相反滴，把一个向量和一个点相加得到另外一个点：

现在下面的代码看起来就感觉很好了！

```swift
// 触摸开始:
objectOffset = object.center - touch.locationInView(container)

// 触摸移动:
object.center = touch.locationInView(container) + objectOffset

练习：想一想其它可以用在点和向量上的操作符，并对它们进行重载。

建议： -(CGPoint, CGVector) 、 *(CGVector, CGFloat) 和 -(CGVector) 。

a += b   // 等价于 "a = a + b"
a %= b   // 等价于 "a = a % b"

infix operator ??= { associativity right precedence 90 assignment } // 匹配其它的赋值操作符

/// 如果 `lhs` 为 `nil`, 把 `rhs` 的值赋给它
func ??=<T>(inout lhs: T?, @autoclosure rhs: () -> T)
{
    lhs = lhs ?? rhs()
}

a ??= b   // 等价于 "a = a ?? b"

extension dispatch_queue_t
{
    final func async(block: dispatch_block_t) {
        dispatch_async(self, block)
    }
    
    // 这里的 `block` 需要是 @noescape 的, 但不能是链接中这样的： <http://openradar.me/19770770>
    final func sync(block: dispatch_block_t) {
        dispatch_sync(self, block)
    }
}

mySerialQueue.sync {
    print("I’m on the queue!")
    threadsafeNum++
}

dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0).async {
    expensivelyReticulateSplines()
    print("Done!")
    
    dispatch_get_main_queue().async {
        print("Back on the main queue.")
    }
}

extension dispatch_queue_t
{
    final func sync<Result>(block: () -> Result) -> Result {
        var result: Result?
        dispatch_sync(self) {
            result = block()
        }
        return result!
    }
}

// 在串行队列上抓取一些数据
let currentItems = mySerialQueue.sync {
    print("I’m on the queue!")
    return mutableItems.copy()
}

extension dispatch_queue_t
{
    final func async(group: dispatch_group_t, _ block: dispatch_block_t) {
        dispatch_group_async(group, self, block)
    }
}

extension dispatch_group_t
{
    final func waitForever() {
        dispatch_group_wait(self, DISPATCH_TIME_FOREVER)
    }
}

let group = dispatch_group_create()

concurrentQueue.async(group) {
    print("I’m part of the group")
}

concurrentQueue.async(group) {
    print("I’m independent, but part of the same group")
}

group.waitForever()
print("Everything in the group has now executed")

@interface MyClass : NSObject

/// @返回 @c 东西的下标, 如果没有提供就返回 NSNotFound。
- (NSUInteger)indexOfThing:(id)thing NS_REFINED_FOR_SWIFT;

@end

// 当导出到 Swift 时, 它就变成了:

public class MyClass: NSObject
{
    public func __indexOfThing(thing: AnyObject) -> UInt
}

extension MyClass
{
    /// - 返回: 给定 `thing` 的下标, 如果没有就返回 `nil`。
    func indexOfThing(thing: AnyObject) -> Int?
    {
        let idx = Int(__indexOfThing(thing)) // 调用原始方法
        if idx == NSNotFound { return nil }
        return idx
    }
}