Swift 中的 @autoclosure

2018-01-14  本文已影响79人  萌面大道

由于种种原因，简书等第三方平台博客不再保证能够同步更新，欢迎移步 GitHub：https://github.com/kingcos/Perspective/。谢谢！

Date Notes Swift Xcode
2018-01-13 首次提交 4.0.3 9.2
@autoclosure

Perspective，即透视。笔者希望可以尽力将一些不是那么透彻的点透过 Demo 和 Source Code 而看到其本质。由于国内软件开发仍很大程度依赖国外的语言、知识，所以该系列文章中的术语将使用英文表述，除非一些特别统一的词汇或整段翻译时将使用中文，但也会在首次提及时标注英文。笔者英文水平有限，这样的目的也是尽可能减少歧义，但在其中不免有所错误，遗漏，还请大家多多批评、指正。

本文也会同步在笔者的 GitHub 的 Perspective 仓库：https://github.com/kingcos/Perspective，欢迎 Star 🌟。

— Kingcos

What

Closures are self-contained blocks of functionality that can be passed around and used in your code. Closures in Swift are similar to blocks in C and Objective-C and to lambdas in other programming languages.

— The Swift Programming Language (Swift 4.0.3)

Closure 在 Swift 等许多语言中普遍存在。熟悉 Objective-C 的同学一定对 Block 不陌生。两者其实是比较类似的，相较于 Block，Closure 的写法简化了许多，也十分灵活。

在 Swift 中，@ 开头通常代表着 Attribute。@autoclosure 属于 Type Attribute，意味着其可以对类型（Type）作出一些限定。

How

自动（Auto-）

func logIfTrue(_ predicate: () -> Bool) {
    if predicate() {
        print("True")
    }
}

// logIfTrue(predicate: () -> Bool)
logIfTrue { 1 < 2 }

func logIfTrueWithAutoclosure(_ predicate: @autoclosure () -> Bool) {
    if predicate() {
        print("True")
    }
}

// logIfTrueWithAutoclosure(predicate: Bool)
logIfTrueWithAutoclosure(1 < 2)

Closure 的 Delay Evaluation

var array = [1, 2, 3, 4, 5]

array.removeLast()
print(array.count)

var closure = { array.removeLast() }
print(array.count)

closure()
print(array.count)

// OUTPUT:
// 4
// 4
// 3

@escaping

func foo(_ bar: @autoclosure @escaping () -> Void) {
    DispatchQueue.main.async {
        bar()
    }
}

测试用例

inout

var demo: () -> () = {
    print("func - demo")
}

func foo(_ closure: @escaping () -> ()) {
    var closure = closure // Ignored the warning
    closure = {
        print("func - escaping closure")
    }
}

foo(demo)
demo()
// OUTPUT:
// func - demo

func bar(_ closure: inout () -> ()) {
    closure = {
        print("func - inout closure")
    }
}

bar(&demo)
demo()
// OUTPUT:
// func - inout closure

可变参数（Variadic Parameters）

// ERROR
func variadicAutoclosure(_ fn: @autoclosure () -> ()...) {
    for _ in fn {}
}

源代码用例

extension Bool {
  @_inlineable // FIXME(sil-serialize-all)
  @_transparent
  @inline(__always)
  public static func && (lhs: Bool, rhs: @autoclosure () throws -> Bool) rethrows
      -> Bool {
    return lhs ? try rhs() : false
  }

  @_inlineable // FIXME(sil-serialize-all)
  @_transparent
  @inline(__always)
  public static func || (lhs: Bool, rhs: @autoclosure () throws -> Bool) rethrows
      -> Bool {
    return lhs ? true : try rhs()
  }
}

@_inlineable // FIXME(sil-serialize-all)
@_transparent
public func ?? <T>(optional: T?, defaultValue: @autoclosure () throws -> T)
    rethrows -> T {
  switch optional {
  case .some(let value):
    return value
  case .none:
    return try defaultValue()
  }
}

@_inlineable // FIXME(sil-serialize-all)
@_transparent
public func ?? <T>(optional: T?, defaultValue: @autoclosure () throws -> T?)
    rethrows -> T? {
  switch optional {
  case .some(let value):
    return value
  case .none:
    return try defaultValue()
  }
}

// FIXME(ABI)#21 (Type Checker): rename to something descriptive.
@_inlineable // FIXME(sil-serialize-all)
public // COMPILER_INTRINSIC
func _undefined<T>(
  _ message: @autoclosure () -> String = String(),
  file: StaticString = #file, line: UInt = #line
) -> T {
  _assertionFailure("Fatal error", message(), file: file, line: line, flags: 0)
}

@_transparent
@available(OSX 10.12, iOS 10.0, tvOS 10.0, watchOS 3.0, *)
public func dispatchPrecondition(condition: @autoclosure () -> DispatchPredicate) {
    // precondition is able to determine release-vs-debug asserts where the overlay
    // cannot, so formulating this into a call that we can call with precondition()
    precondition(_dispatchPreconditionTest(condition()), "dispatchPrecondition failure")
}

@_inlineable // FIXME(sil-serialize-all)
@_transparent
public func assert(
  _ condition: @autoclosure () -> Bool,
  _ message: @autoclosure () -> String = String(),
  file: StaticString = #file, line: UInt = #line
) {
  // Only assert in debug mode.
  if _isDebugAssertConfiguration() {
    if !_branchHint(condition(), expected: true) {
      _assertionFailure("Assertion failed", message(), file: file, line: line,
        flags: _fatalErrorFlags())
    }
  }
}

@_inlineable // FIXME(sil-serialize-all)
@_transparent
public func precondition(
  _ condition: @autoclosure () -> Bool,
  _ message: @autoclosure () -> String = String(),
  file: StaticString = #file, line: UInt = #line
) {
  // Only check in debug and release mode.  In release mode just trap.
  if _isDebugAssertConfiguration() {
    if !_branchHint(condition(), expected: true) {
      _assertionFailure("Precondition failed", message(), file: file, line: line,
        flags: _fatalErrorFlags())
    }
  } else if _isReleaseAssertConfiguration() {
    let error = !condition()
    Builtin.condfail(error._value)
  }
}

@_inlineable // FIXME(sil-serialize-all)
@inline(__always)
public func assertionFailure(
  _ message: @autoclosure () -> String = String(),
  file: StaticString = #file, line: UInt = #line
) {
  if _isDebugAssertConfiguration() {
    _assertionFailure("Fatal error", message(), file: file, line: line,
      flags: _fatalErrorFlags())
  }
  else if _isFastAssertConfiguration() {
    _conditionallyUnreachable()
  }
}

@_inlineable // FIXME(sil-serialize-all)
@_transparent
public func preconditionFailure(
  _ message: @autoclosure () -> String = String(),
  file: StaticString = #file, line: UInt = #line
) -> Never {
  // Only check in debug and release mode.  In release mode just trap.
  if _isDebugAssertConfiguration() {
    _assertionFailure("Fatal error", message(), file: file, line: line,
      flags: _fatalErrorFlags())
  } else if _isReleaseAssertConfiguration() {
    Builtin.int_trap()
  }
  _conditionallyUnreachable()
}

@_inlineable // FIXME(sil-serialize-all)
@_transparent
public func fatalError(
  _ message: @autoclosure () -> String = String(),
  file: StaticString = #file, line: UInt = #line
) -> Never {
  _assertionFailure("Fatal error", message(), file: file, line: line,
    flags: _fatalErrorFlags())
}

@_inlineable // FIXME(sil-serialize-all)
@_transparent
public func _precondition(
  _ condition: @autoclosure () -> Bool, _ message: StaticString = StaticString(),
  file: StaticString = #file, line: UInt = #line
) {
  // Only check in debug and release mode. In release mode just trap.
  if _isDebugAssertConfiguration() {
    if !_branchHint(condition(), expected: true) {
      _fatalErrorMessage("Fatal error", message, file: file, line: line,
        flags: _fatalErrorFlags())
    }
  } else if _isReleaseAssertConfiguration() {
    let error = !condition()
    Builtin.condfail(error._value)
  }
}

@_inlineable // FIXME(sil-serialize-all)
@_transparent
public func _debugPrecondition(
  _ condition: @autoclosure () -> Bool, _ message: StaticString = StaticString(),
  file: StaticString = #file, line: UInt = #line
) {
  // Only check in debug mode.
  if _isDebugAssertConfiguration() {
    if !_branchHint(condition(), expected: true) {
      _fatalErrorMessage("Fatal error", message, file: file, line: line,
        flags: _fatalErrorFlags())
    }
  }
}

@_inlineable // FIXME(sil-serialize-all)
@_transparent
public func _sanityCheck(
  _ condition: @autoclosure () -> Bool, _ message: StaticString = StaticString(),
  file: StaticString = #file, line: UInt = #line
) {
#if INTERNAL_CHECKS_ENABLED
  if !_branchHint(condition(), expected: true) {
    _fatalErrorMessage("Fatal error", message, file: file, line: line,
      flags: _fatalErrorFlags())
  }
#endif
}

Why

It’s common to call functions that take autoclosures, but it’s not common to implement that kind of function.

NOTE

Overusing autoclosures can make your code hard to understand. The context and function name should make it clear that evaluation is being deferred.

— The Swift Programming Language (Swift 4.0.3)

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