Swift is a general-purpose, multi-paradigm, compiled programming language developed by Apple Inc. for iOS, macOS, watchOS, tvOS, and Linux. Swift is designed to work with Apple’s Cocoa and Cocoa Touch frameworks and the large body of existing Objective-C code written for Apple products. It is built with the open source LLVM compiler framework and has been included in Xcode since version 6. On platforms other than Linux, it uses the Objective-C runtime library which allows C, Objective-C, C++ and Swift code to run within one program.
Apple intended Swift to support many core concepts associated with Objective-C, notably dynamic dispatch, widespread late binding, extensible programming, and similar features, but in a “safer” way, making it easier to catch software bugs; Swift has features addressing some common programming errors like null pointer dereferencing and provides syntactic sugar to help avoid the pyramid of doom. Swift supports the concept of protocol extensibility, an extensibility system that can be applied to types, structs, and classes, which Apple promotes as a real change in programming paradigms they term “protocol-oriented programming” (similar to traits).
Swift was introduced at Apple’s 2014 Worldwide Developers Conference (WWDC). It underwent an upgrade to version 1.2 during 2014 and a more major upgrade to Swift 2 at WWDC 2015. Initially, a proprietary language, version 2.2 was made open-source software under the Apache License 2.0 on December 3, 2015, for Apple’s platforms and Linux.
Different major versions have been released at an annual schedule with incompatible syntax and library invocations each, requiring significant source code rewrites. For larger codebases, this has caused many developers to dismiss Swift until a more stable version becomes available.
Swift is an alternative to the Objective-C language that employs modern programming-language theory concepts and strives to present a simpler syntax. During its introduction, it was described simply as “Objective-C without the C”.
By default, Swift does not expose pointers and other unsafe accessors, in contrast to Objective-C, which uses pointers pervasively to refer to object instances. Also, Objective-C’s use of a Smalltalk-like syntax for making method calls has been replaced with a dot-notation style and namespace system more familiar to programmers from other common object-oriented (OO) languages like Java or C#. Swift introduces true named parameters and retains key Objective-C concepts, including protocols, closures, and categories, often replacing former syntax with cleaner versions and allowing these concepts to be applied to other language structures, like enumerated types (enums)
Swift supports five access control levels for symbols: open, public, internal, file-private, and private. Unlike many object-oriented languages, these access controls ignore inheritance hierarchies: private indicates that a symbol is accessible only in the immediate scope, file-private indicates it is accessible only from within the file, internal indicates it is accessible within the containing module, public indicates it is accessible from any module, and open (only for classes and their methods) indicates that the class may be subclassed outside of the module.
An important new feature in Swift is option types, which allow references or values to operate in a manner similar to the common pattern in C, where a pointer may refer to a value or may be null. This implies that non-optional types cannot result in a null-pointer error; the compiler can ensure this is not possible.
Optional types are created with the Optional mechanism—to make an Integer that is nullable, one would use a declaration similar to var optional integer: Optional<Int>. As in C#, Swift also includes syntactic sugar for this, allowing one to indicate a variable is optional by placing a question mark after the type name, var optional integer: Int?. Variables or constants that are marked optional either have a value of the underlying type or are nil. Optional types wrap the base type, resulting in a different instance. String and String? are fundamentally different types, the latter has more in common with Int? than String.
In many object-oriented languages, objects are represented internally in two parts. The object is stored as a block of data placed on the heap, while the name (or “handle”) to that object is represented by a pointer. Objects are passed between methods by copying the value of the pointer, allowing the same underlying data on the heap to be accessed by anyone with a copy. In contrast, basic types like integers and floating point values are represented directly; the handle contains the data, not a pointer to it, and that data is passed directly to methods by copying. These styles of access are termed pass-by-reference in the case of objects, and pass-by-value for basic types.
Both concepts have their advantages and disadvantages. Objects are useful when the data is large, like the description of a window or the contents of a document. In these cases, access to that data is provided by copying a 32- or 64-bit value, versus copying an entire data structure. However, smaller values like integers are the same size as pointers (typically both are one word), so there is no advantage to passing a pointer, versus passing the value. Also, pass-by-reference inherently requires a dereferencing operation, which can produce noticeable overhead in some operations, typically those used with these basic value types, like mathematics.
Similarly to C# and in contrast to most other OO languages, Swift offers built-in support for objects using either pass-by-reference or pass-by-value semantics, the former using the class declaration and the latter using the struct. Structs in Swift have almost all the same features as classes: methods, implementing protocols and using the extension mechanisms. For this reason, Apple terms all data generically as instances, versus objects or values. Structs do not support inheritance, however.
The programmer is free to choose which semantics are more appropriate for each data structure in the application. Larger structures like windows would be defined as classes, allowing them to be passed around as pointers. Smaller structures, like a 2D point, can be defined as structs, which will be pass-by-value and allow direct access to their internal data with no dereference. The performance improvement inherent to the pass-by-value concept is such that Swift uses these types for almost all common data types, including Int and Double, and types normally represented by objects, like String and Array. Using value types can result in significant performance improvements in user applications as well.
To ensure that even the largest structs do not cause a performance penalty when they are handed off, Swift uses copy on write so that the objects are copied only if and when the program attempts to change a value in them. This means that the various accessors have what is in effect a pointer to the same data storage, but this takes place far below the level of the language, in the computer’s memory management unit (MMU). So while the data is physically stored as one instance in memory, at the level of the application, these values are separate and physical separation is enforced by copy on write only if needed
A key feature of Objective-C is its support for categories, methods that can be added to extend classes at runtime. Categories allow extending classes in-place to add new functions with no need to subclass or even have access to the original source code. An example might be to add spell checker support to the base NSString class, which means all instances of NSString in the application gain spell checking. The system is also widely used as an organizational technique, allowing related code to be gathered into library-like extensions. Swift continues to support this concept, although they are now termed extensions, and declared with the keyword extension. Unlike Objective-C, Swift can also add new properties accessors, types and enums to extant instances.
Another key feature of Objective-C is its use of protocols, known in most modern languages as interfaces. Protocols promise that a particular class implements a set of methods, meaning that other objects in the system can call those methods on any object supporting that protocol. This is often used in modern OO languages as a substitute for multiple inheritances, although the feature sets are not entirely similar. A common example of a protocol in Cocoa is the NSCopying protocol, which defines one method, copyWithZone, that implements deep copying on objects.
In Objective-C, and most other languages implementing the protocol concept, it is up to the programmer to ensure that the required methods are implemented in each class. Swift adds the ability to add these methods using extensions and to use generic programming (generics) to implement them. Combined, these allow protocols to be written once and support a wide variety of instances. Also, the extension mechanism can be used to add protocol conformance to an object that does not list that protocol in its definition
Swift uses the same runtime as the extant Objective-C system but requires iOS 7 or macOS 10.9 or higher. Swift and Objective-C code can be used in one program, and by extension, C and C++ also. In contrast to C, C++ code cannot be used directly from Swift. An Objective-C or C wrapper must be created between Swift and C++. In the case of Objective-C, Swift has considerable access to the object model and can be used to subclass, extend and use Objective-C code to provide protocol support. The converse is not true: a Swift class cannot be subclassed in Objective-C.
To aid the development of such programs, and the re-use of extant code, Xcode 6 offers a semi-automated system that builds and maintains a bridging header to expose Objective-C code to Swift. This takes the form of an additional header file that simply defines or imports all of the Objective-C symbols that are needed by the project’s Swift code. At that point, Swift can refer to the types, functions, and variables declared in those imports as though they were written in Swift. Objective-C code can also use Swift code directly, by importing an automatically maintained header file with Objective-C declarations of the project’s Swift symbols. For instance, an Objective-C file in a mixed project called “MyApp” could access Swift classes or functions with the code #import “MyApp-Swift.h”. Not all symbols are available through this mechanism, however—use of Swift-specific features like generic types, non-object optional types, sophisticated enums, or even Unicode identifiers may render a symbol inaccessible from Objective-C.
Swift also has limited support for attributes, metadata that is read by the development environment and is not necessarily part of the compiled code. Like Objective-C, attributes use the @ syntax, but the currently available set is small. One example is the @IBOutlet attribute, which marks a given value in the code as an outlet, available for use within Interface Builder (IB). An outlet is a device that binds the value of the on-screen display to an object in code.
Swift uses Automatic Reference Counting (ARC) to manage memory. Apple used to require manual memory management in Objective-C but introduced ARC in 2011 to allow for easier memory allocation and deallocation. One problem with ARC is the possibility of creating a strong reference cycle, where objects reference each other in a way that you can reach the object you started from by following references (e.g. A references B, B references A). This causes them to become leaked into memory as they are never released. Swift provides the keywords weak and unowned to prevent strong reference cycles. Typically a parent-child relationship would use a strong reference while a child-parent would use either weak reference, where parents and children can be unrelated, or unowned where a child always has a parent, but the parent may not have a child. Weak references must be optional variables since they can change and become nil.
A closure within a class can also create a strong reference cycle by capturing self-references. Self-references to be treated as weak or unowned can be indicated using a capture list.
A key element of the Swift system is its ability to be cleanly debugged and run within the development environment, using a read–eval–print loop (REPL), giving it interactive properties more in common with the scripting abilities of Python than traditional system programming languages. The REPL is further enhanced with the new concept playgrounds. These are interactive views running within the Xcode environment that respond to code or debugger changes on-the-fly. Playgrounds allow programmers to add in Swift code along with markdown documentation. If some code changes over time or with regard to some other ranged input value, the view can be used with the Timeline Assistant to demonstrate the output in an animated way. In addition, Xcode has debugging features for Swift development including breakpoints, step through and step overstatements, as well as UI element placement breakdowns for app developers.
Apple says that Swift “is the first industrial-quality systems programming language that is as expressive and enjoyable as a scripting language
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