Swift 4.2 现已正式发布！Swift 4.2 基于 Swift 4 的优势，提供更快的编译时间，改进调试体验，更新标准库以及融合二进制兼容性。

泛型改进

随着对 conditional conformance 的支持，Swift 4.2 在泛型方面取得了重大进展，像是减少样板代码的数量，使更多代码可重用等等。要了解有关 conditional conformance 的更多信息，请参阅：

• SE-0143 Conditional conformances

标准库更新

Swift 4.2 中的标准库包含许多新特性，比如 Hashable 协议的改进以及新的统一随机化函数（randomization functions）和协议集。

• SE-0197 Adding in-place removeAll(where:) to the Standard Library

• SE-0199 Adding toggle to Bool

• SE-0202 Random Unification

• SE-0204 Add last(where:) and lastIndex(where:) Methods

• SE-0206 Hashable Enhancements

• SE-0207 Add an allSatisfy algorithm to Sequence

二进制兼容性

Swift 4.2 使语言更接近稳定的 ABI ，为未来的 Swift 版本实现二进制兼容性。访问 ABI Dashboard 可了解有关此项工作当前进展的更多详情。

其他语言和编译器更新

Swift 4.2 包含许多对开发者体验有显着影响的改进，包括：

• Support for batch mode compilation resulting in faster build times

• Change in calling convention for retain/release cycle to reduce code size and improve runtime performance

• Lazier SIL deserialization with -Onone

• Recursive metadata support

• More compact reflection metadata

• Default arguments are inlined at call sites

• Some long-standing bug fixes, including init declname, switch with multiple case labels per block, inheritance of generic initializers, and more

Swift 4.2 还实现了 Swift Evolution 上的一些提案：

• SE-0054 Abolish ImplicitlyUnwrappedOptional type. Read even more details about the reimplementation here.

• SE-0079 Allow using optional binding to upgrade self from a weak to strong reference

• SE-0193 Cross-module inlining and specialization

• SE-0194 Derived Collection of Enum Cases

• SE-0195 Introduce User-defined “Dynamic Member Lookup” Types

• SE-0196 Compiler Diagnostic Directives

• SE-0205 withUnsafePointer(to:_:) and withUnsafeBytes(of:_:) for immutable values

• SE-0210 Add an offset(of:) method to MemoryLayout

• SE-0212 Compiler Version Directive

包管理器更新

Swift 4.2 为 Swift 包管理器引入了新功能：

• 支持批处理模式. Swift targets will be now compiled using the Swift compiler’s batch mode.

• Improved scheme generation logic. The scheme generation logic is improved and generates schemes as follows:

• One scheme containing all regular and test targets of the root package.

• One scheme per executable target containing the test targets whose dependencies intersect with the dependencies of the executable target.

• Automatic Xcode project generation. The generate-xcodeproj has a new –watch option to watch the file system and automatically regenerate the Xcode project if needed. This uses the watchman tool to watch the required files.

Swift 4.2 还实现了 Swift Evolution 上关于包管理器的一些提案：

• SE-0201 Package Manager Local Dependencies. SwiftPM now supports declaring dependency on a package using its path on disk instead of the git URL. This requires updating the package’s tools version to 4.2.

• SE-0208 Package Manager System Library Targets. The PackageDescription API in tools version 4.2 supports a new type of target “system library target”, which moves the current system-module packages feature from package to target level.

• SE-0209 Package Manager Swift Language Version API Update. swiftLanguageVersions property in PackageDescription manifest API for tools version 4.2 is changed from an array of Integers to an array of SwiftVersion enum.

迁移说明和更新细节可查阅：

• https://swift.org/blog/swift-4-2-released/
  

下载地址：

• https://swift.org/download/