我很高兴发现 Swift 3 对 #keyPath() 的实现，这将消除拼写错误并在编译时强制执行 key 路径确实存在。比手动输入字符串要好得多。

https://github.com/apple/swift-evolution/blob/master/proposals/0062-objc-keypaths.md

class MyObject {
    @objc var myString: String = "default"
}

// Works great
let keyPathString = #keyPath(MyObject.myString)

Swift docs list the type作为“属性名称”传递给 #keyPath()。

The property name must be a reference to a property that is available in the Objective-C runtime. At compile time, the key-path expression is replaced by a string literal.

是否可以单独保存这个“属性名称”，然后传递给 #keyPath() 来创建一个 String？

let propertyName = MyObject.myString // error. How do I save?
let string = #keyPath(propertyName)

是否支持要求属性名称属于特定类型？

// something like this
let typedPropertyName: MyObject.PropertyName = myString // error
let string = #keyPath(typedPropertyName)

最终目标是与需要 NSExpression 作为关键路径的 API 进行交互。我想编写将有效的属性名称作为参数而不是随机键路径字符串的便捷方法。理想情况下，由特定类型实现的属性名称。

func doSomethingForSpecificTypeProperty(_ propertyName: MyObject.PropertyName) {

    let keyPathString = #keyPath(propertyName)

    let expression = NSExpression(forKeyPath: keyPathString)

    // ...
}

最佳答案

这看起来不太可能。

下面是编译器解析关键路径表达式的代码:

///   expr-keypath:
///     '#keyPath' '(' unqualified-name ('.' unqualified-name) * ')'
///
ParserResult<Expr> Parser::parseExprKeyPath() {
  // Consume '#keyPath'.
  SourceLoc keywordLoc = consumeToken(tok::pound_keyPath);

  // Parse the leading '('.
  if (!Tok.is(tok::l_paren)) {
    diagnose(Tok, diag::expr_keypath_expected_lparen);
    return makeParserError();
  }
  SourceLoc lParenLoc = consumeToken(tok::l_paren);

  // Handle code completion.
  SmallVector<Identifier, 4> names;
  SmallVector<SourceLoc, 4> nameLocs;
  auto handleCodeCompletion = [&](bool hasDot) -> ParserResult<Expr> {
    ObjCKeyPathExpr *expr = nullptr;
    if (!names.empty()) {
      expr = ObjCKeyPathExpr::create(Context, keywordLoc, lParenLoc, names,
                                     nameLocs, Tok.getLoc());
    }

    if (CodeCompletion)
      CodeCompletion->completeExprKeyPath(expr, hasDot);

    // Eat the code completion token because we handled it.
    consumeToken(tok::code_complete);
    return makeParserCodeCompletionResult(expr);
  };

  // Parse the sequence of unqualified-names.
  ParserStatus status;
  while (true) {
    // Handle code completion.
    if (Tok.is(tok::code_complete))
      return handleCodeCompletion(!names.empty());

    // Parse the next name.
    DeclNameLoc nameLoc;
    bool afterDot = !names.empty();
    auto name = parseUnqualifiedDeclName(
                  afterDot, nameLoc, 
                  diag::expr_keypath_expected_property_or_type);
    if (!name) {
      status.setIsParseError();
      break;
    }

    // Cannot use compound names here.
    if (name.isCompoundName()) {
      diagnose(nameLoc.getBaseNameLoc(), diag::expr_keypath_compound_name,
               name)
        .fixItReplace(nameLoc.getSourceRange(), name.getBaseName().str());
    }

    // Record the name we parsed.
    names.push_back(name.getBaseName());
    nameLocs.push_back(nameLoc.getBaseNameLoc());

    // Handle code completion.
    if (Tok.is(tok::code_complete))
      return handleCodeCompletion(false);

    // Parse the next period to continue the path.
    if (consumeIf(tok::period))
      continue;

    break;
  }

  // Parse the closing ')'.
  SourceLoc rParenLoc;
  if (status.isError()) {
    skipUntilDeclStmtRBrace(tok::r_paren);
    if (Tok.is(tok::r_paren))
      rParenLoc = consumeToken();
    else
      rParenLoc = PreviousLoc;
  } else {
    parseMatchingToken(tok::r_paren, rParenLoc,
                       diag::expr_keypath_expected_rparen, lParenLoc);
  }

  // If we cannot build a useful expression, just return an error
  // expression.
  if (names.empty() || status.isError()) {
    return makeParserResult<Expr>(
             new (Context) ErrorExpr(SourceRange(keywordLoc, rParenLoc)));
  }

  // We're done: create the key-path expression.
  return makeParserResult<Expr>(
           ObjCKeyPathExpr::create(Context, keywordLoc, lParenLoc, names,
                                   nameLocs, rParenLoc));
}

此代码首先在括号内创建一个以句点分隔的名称列表，然后尝试将它们解析为表达式。它接受一个表达式而不是任何 Swift 类型的数据；它接受代码，而不是数据。

关于swift - 传给#keyPath()的非字符串 "property name"能否独立保存？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/42122786/