核心数据的托管对象属性作为参数

Question

Any 第一个语句没有 return 任何值，而第二个语句工作正常。是否可以将属性名称指定为参数之一以避免硬编码？

let predicate = NSPredicate(format: "%@ == %@", "id","553178666d61d70c24fe4221")

let predicate = NSPredicate(format: "id == %@", "553178666d61d70c24fe4221")

感谢@FreeNickname

，这是一个完整的解决方案

  class func searchForObject(propertyName:NSString,property:NSString,className:String,single:Bool )->AnyObject?{
        let fetchRequest = NSFetchRequest(entityName: className)
        let predicate = NSPredicate(format: "%K == %@", argumentArray: [propertyName, property])
        var appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
        fetchRequest.predicate = predicate
          // Execute the fetch request, and cast the results to an array of LogItem objects
        var error:NSError?
        if let fetchResults = appDelegate.managedObjectContext!.executeFetchRequest(fetchRequest, error: &error) as [AnyObject]? {
            if let e = error{
                println(e.debugDescription)

            }

            if single {
                var object: AnyObject? = fetchResults.first

                return object;
            }
            else{
                return fetchResults
            }
        }
    return nil;
    }

Answer 1

尝试使用 %K 而不是 %@ 作为属性名称。像这样：

let predicate = NSPredicate(format: "%K == %@", "id","553178666d61d70c24fe4221")

来源：Predicates Syntax (developer.apple.com) 来自文档：

The format string supports printf-style format specifiers such as %x (see Formatting String Objects). Two important format specifiers are %@ and %K.

%@ is a var arg substitution for an object value—often a string, number, or date.

%K is a var arg substitution for a key path.

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