Swift 2.3-> 3.0

是否从Swift2.3升级到3.0呢？

如果你有一个意义非常重大的Swift编码库（就像我们在VTS中做的一样），那么别犹豫了，赶快更新吧。另外为了项目的需要，Xcode的更新，跟随苹果的步伐，最新的技术，还是麻溜的跟上把。

但是如果你的Swift代码不是非常的繁重，那么你就直接忽略2.3吧。

下面有一些事情需要记住：

1.依赖库非常重要，确保你的依赖库支持2.3还是3.0-大多数的lib/framework是两者都支持的（Alamofire,Charts）.然而升级到3.0是你的唯一途径。新的发展将会在Swift3.0展开。所有的swift2.3版本的相关依赖库将会被冻结，也就是不在更新。

2.Xcode8.x 将会移除对Swift2.3的支持。Swift2.3注定是大型工程的一个过渡品。所以我们需要努力的升级到swift3.0

3.如果你有一个私有的CocoaPods用于分享代码在多个工程中，那么你不得不马上更新。

当然你可以只更新到2.3，但是你只能推迟，因为在发布Xcode9之前你必须移植到Swift3。

将问题从Swift2.2迁移到2.3

这个迁移可能将Range<T>到CountableRange<T>.但是CountableRange<T>只存在于Swift3

迁移者将会添加评论 /* 迁移者修复：使用变量类型为： DispatchSpecificKey*/去使用dispatch_queue_set_specific() 这是唯一未来相关在Swift3迁移中。

将问题从Swift2.x迁移到3.0

Swift3.0是一个打破一切的版本。swift3意味着打破一切并且移除了所有不利于一个语言长期发展的不利因素。这当然也意味着很多的东西被改变了。你也需要去改变这些使你的工程更加兼容。

Swift 2.3

Swift2.3是一个非常小的更新。具体多么小呢？实际上，这里仅仅只有一个改变。OC代码在编译时为空的检查在苹果的SDK中被更新。

// Swift 2.2
	let image = CIImage(MTLTexture: texture, options: options)
	let extent = image.extent // can crash

	// Swift 3.0
	let image = CIImage(MTLTexture: texture, options: options)!
	// or
	if let image = CIImage(MTLTexture: texture, options: options) {
	…
	}

其他的变化包括一些小的重命名和一些可选类型的初始化现在返回了他们应该返回的类型而不再是可选类型。这些应用就想从bundle初始化nib一样。

Swift3.0

重大的更改（你必须需要注意）

私有的定义已经被更改了，下面来介绍一下fileprivate

以前格式化的私有现在在Swift3.0中变成了fileprivate。一个fileprivate可以使用在一个Extension中

但是一个私有private的变量不可以使用在一个Extension的类中

// Before
	var myArray = [ 1 ,3, 2, 5, 4 ]
	myArray.sort() // returns a new sorted version of `myArray`
	myArray.sortInPlace() // sorts in place

	// After
	myArray.sort() // sorts in place
	myArray.sorted() // returns a new sorted version of `myArray`.

//before
	var myDate = Date()
	myDate.addTimeInterval(60) // OK

	//after
	let myOtherDate = Date()
	myOtherDate.addTimeInterval(60) // Error, as expected

	// Before
	let topLeft = bounds.origin
	let bottomRight = CGPoint(x: bounds.size.width, y: bounds.size.height)
	let path = CGPathCreateMutable()
	CGPathMoveToPoint(path, &transform, topLeft.x, topLeft.y)
	CGPathAddLineToPoint(path, &transform, CGRectGetMidX(bounds), bottomRight.y)
	CGPathAddLineToPoint(path, &transform, bottomRight.x, topLeft.y)
	CGPathAddLineToPoint(path, &transform, topLeft.x, topLeft.y)
	CGPathAddLineToPoint(path, &transform, topLeft.x, bottomRight.y)
	CGPathAddLineToPoint(path, &transform, bottomRight.x, bottomRight.y)
	CGPathAddLineToPoint(path, &transform, bottomRight.x, topLeft.y)
	CGContextAddPath(context, path)

	// After
	let topLeft = bounds.origin
	let bottomRight = CGPoint(x: bounds.size.width, y: bounds.size.height)
	let path = CGMutablePath()
	path.move(transform: &transform, x: topLeft.x, y: topLeft.y)
	path.addLine(transform: &transform, x: bounds.midX, y: bottomRight.y)
	path.addLine(transform: &transform, x: bottomRight.x, y: topLeft.y)
	path.addLine(transform: &transform, x: topLeft.x, y: topLeft.y)
	path.addLine(transform: &transform, x: topLeft.x, y: bottomRight.y)
	path.addLine(transform: &transform, x: bottomRight.x, y: bottomRight.y)
	path.addLine(transform: &transform, x: bottomRight.x, y: topLeft.y)
	context.addPath(path)

let path = UIBezierPath()
let point = CGPoint(x: 2, y: 3)
let string = NSString("Hello world!")
// Before
path.moveToPoint(point)
string.characterAtIndex(3)
// After
path.move(to: point)
string.character(at: 3)

// Before
let queue = dispatch_queue_create("com.test.myqueue", nil)
dispatch_async(queue) {
print("Hello World")
}
// After
let queue = DispatchQueue(label: "com.test.myqueue")
queue.asynchronously {
print("Hello World")
}

myIndex.successor() => myCollection.index(after: myIndex)
myIndex.predecessor() => myCollection.index(before: myIndex)
myIndex.advance(by: …) => myCollection.index(myIndex, offsetBy: …)

// before
words.sort(isOrderedBefore: >)
let sum = measurements.reduce(0, combine: +)

// after
words.sort(by: >)
let sum = measurements.reduce(0, +)

// Before
infix operator <> { precedence 100 associativity left }

// After
precedencegroup ComparisonPrecedence {
associativity: left
higherThan: LogicalConjunctionPrecedence
}
infix operator <> : ComparisonPrecedence

// Before
// HealthKit has the following strings:
HK_EXTERN NSString * const HKQuantityTypeIdentifierBodyMassIndex;
HK_EXTERN NSString * const HKQuantityTypeIdentifierBodyFatPercentage;
HK_EXTERN NSString * const HKQuantityTypeIdentifierHeight;
HK_EXTERN NSString * const HKQuantityTypeIdentifierBodyMass;
HK_EXTERN NSString * const HKQuantityTypeIdentifierLeanBodyMass;
// They were previously imported as swift Strings

// After
// Instead of importing this as a string, Swift will import this like so:
enum HKQuantityTypeIdentifier : String {
case BodyMassIndex
case BodyFatPercentage
case Height
case BodyMass
case LeanBodyMass
}

// Before
catch let error as AVError where error == .diskFull {
// AVError is an enum, so one only gets the equivalent of the code.
// There is no way to access the localized description (for example) or
// any other information typically stored in the ``userInfo`` dictionary.
}

// or
catch let error as NSError where error._domain == AVFoundationErrorDomain && error._code == AVFoundationErrorDomain.diskFull.rawValue {
// okay: userInfo is finally accessible, but still weakly typed
}

// And in usage:
catch let error as NSError where error._domain = AVFoundationErrorDomain {
if let time = error.userInfo[AVErrorTimeKey] as? CMTime {
// ...
}
}

// After
catch let error as AVError {
if let time = error.time {
// ...
}
}

// Before:
func curried(x: Int)(y: String) -> Float {
return Float(x) + Float(y)!
}

// After:
func curried(x: Int) -> (String) -> Float {
return {(y: String) -> Float in
return Float(x) + Float(y)!
}
}