传给#keyPath()的非字符串"property name"能独立保存吗？

Question

我很高兴找到 Swift 3 对 #keyPath() 的实现，这将消除拼写错误并在编译时强制执行关键路径确实存在。比手动输入字符串好多了。

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 "property name" 传递到 #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 "property name"，然后传递给 #keyPath() 创建一个字符串？

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，属性名称由特定类型实现。

func doSomethingForSpecificTypeProperty(_ propertyName: MyObject.PropertyName) {

    let keyPathString = #keyPath(propertyName)

    let expression = NSExpression(forKeyPath: keyPathString)

    // ...
}

Answer 1

看起来不可能。

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 period-separated 个名称的列表，然后尝试将它们解析为表达式。它接受表达式而不是任何 Swift 类型的数据；它接受 code，而不是 data。

Answer 2

刚刚提出类似问题并找到 this article。您可以使用 KeyPath 泛型用于这些目的

swift4 中的短代码如下所示：

let getName = \Person.name
print(p[keyPath: getName])

// or just this:
print(p[keyPath: \Person.name])

传给#keyPath()的非字符串"property name"能独立保存吗？

Can the non-string "property name" passed to #keyPath() be saved independently?

key-value-observing

kvc

swift

keypaths