property type or class using reflection

Question

I was wondering if it's possible to determine the class or primitive type of an Objects properties. Getting all properties names and values is pretty easy. SO answer

So is there any way to get the properties class type while the property hast no value or nil value?

Example Code

@interface MyObject : NSObject
@property (nonatomic, copy) NSString *aString;
@property (nonatomic, copy) NSDate *aDate;
@property                   NSInteger aPrimitive;
@end

@implementation MyObject
@synthesize aString;
@synthesize aDate;
@synthesize aPrimitive;

- (void)getTheTypesOfMyProperties {
    unsigned int count;
    objc_property_t* props = class_copyPropertyList([self class], &count);
    for (int i = 0; i < count; i++) {
        objc_property_t property = props[i];

        // Here I can easy get the name or value
        const char * name = property_getName(property);

        // But is there any magic function that can tell me the type?
        // the property can be nil at this time
        Class cls = magicFunction(property);
    }
    free(props);
}

@end

Answer 1

After searching through Apples Documentation about objc Runtime and according to this SO answer I finally got it working. I just want to share my results.

unsigned int count;
objc_property_t* props = class_copyPropertyList([MyObject class], &count);
for (int i = 0; i < count; i++) {
    objc_property_t property = props[i];
    const char * name = property_getName(property);
    NSString *propertyName = [NSString stringWithCString:name encoding:NSUTF8StringEncoding];
    const char * type = property_getAttributes(property);
    NSString *attr = [NSString stringWithCString:type encoding:NSUTF8StringEncoding];

    NSString * typeString = [NSString stringWithUTF8String:type];
    NSArray * attributes = [typeString componentsSeparatedByString:@","];
    NSString * typeAttribute = [attributes objectAtIndex:0];
    NSString * propertyType = [typeAttribute substringFromIndex:1];
    const char * rawPropertyType = [propertyType UTF8String];

    if (strcmp(rawPropertyType, @encode(float)) == 0) {
        //it's a float
    } else if (strcmp(rawPropertyType, @encode(int)) == 0) {
        //it's an int
    } else if (strcmp(rawPropertyType, @encode(id)) == 0) {
        //it's some sort of object
    } else {
        // According to Apples Documentation you can determine the corresponding encoding values
    }

    if ([typeAttribute hasPrefix:@"T@"]) {
        NSString * typeClassName = [typeAttribute substringWithRange:NSMakeRange(3, [typeAttribute length]-4)];  //turns @"NSDate" into NSDate
        Class typeClass = NSClassFromString(typeClassName);
        if (typeClass != nil) {
            // Here is the corresponding class even for nil values
        }
    }

}
free(props);

Answer 2

Inspired by the ObjC answer by @arndt-bieberstein I have written a solution in Swift 3 (probably very similar - if not same - in earlier versions of Swift). You can find it on Github I am trying to make a pod of it but I am having issues getting pob lib lintto work with the Swift 3 code (CLI xcodebuild or Xcode 8 related problem probably.) Anyhow, the class method func getTypesOfProperties(inClass clazz: NSObject.Type) -> Dictionary<String, Any>? can extract the name and types of any Swift class that inherits from NSObject.

The work horse of the project are these methods, but checkout the full code on Github:

  func getTypesOfProperties(in clazz: NSObject.Type) -> Dictionary<String, Any>? {
        var count = UInt32()
        guard let properties = class_copyPropertyList(clazz, &count) else { return nil }
        var types: Dictionary<String, Any> = [:]
        for i in 0..<Int(count) {
            guard let property: objc_property_t = properties[i], let name = getNameOf(property: property) else { continue }
            let type = getTypeOf(property: property)
            types[name] = type
        }
        free(properties)
        return types
    }

   func getTypeOf(property: objc_property_t) -> Any {
        guard let attributesAsNSString: NSString = NSString(utf8String: property_getAttributes(property)) else { return Any.self }
        let attributes = attributesAsNSString as String
        let slices = attributes.components(separatedBy: "\"")
        guard slices.count > 1 else { return getPrimitiveDataType(withAttributes: attributes) }
        let objectClassName = slices[1]
        let objectClass = NSClassFromString(objectClassName) as! NSObject.Type
        return objectClass
    }

   func getPrimitiveDataType(withAttributes attributes: String) -> Any {
        guard let letter = attributes.substring(from: 1, to: 2), let type = primitiveDataTypes[letter] else { return Any.self }
        return type
    }

   func getNameOf(property: objc_property_t) -> String? {
        guard let name: NSString = NSString(utf8String: property_getName(property)) else { return nil }
        return name as String
    }

It can extract the NSObject.Type of all properties which class type inherits from NSObject such as NSDate (Swift3: Date), NSString(Swift3: String?) and NSNumber, however it is store in the type Any (as you can see as the type of the value of the Dictionary returned by the method). This is due to the limitations of value types such as Int, Int32, Bool. Since those types do not inherit from NSObject, calling .self on e.g. an Int - Int.self does not return NSObject.Type, but rather the type Any. Thus the method returns Dictionary<String, Any>? and not Dictionary<String, NSObject.Type>?.

You can use this method like this:

class Book: NSObject {
    let title: String
    let author: String?
    let numberOfPages: Int
    let released: Date
    let isPocket: Bool

    init(title: String, author: String?, numberOfPages: Int, released: Date, isPocket: Bool) {
        self.title = title
        self.author = author
        self.numberOfPages = numberOfPages
        self.released = released
        self.isPocket = isPocket
    }
}

guard let types = getTypesOfProperties(inClass: Book.self) else { return }
for (name, type) in types {
    print("'\(name)' has type '\(type)'")
}
// Prints:
// 'title' has type 'NSString'
// 'numberOfPages' has type 'Int'
// 'author' has type 'NSString'
// 'released' has type 'NSDate'
// 'isPocket' has type 'Bool'

You can also try to cast the Any to NSObject.Type, which will succeed for all properties inheriting from NSObject, then you can check the type using standard == operator:

func checkPropertiesOfBook() {
    guard let types = getTypesOfProperties(inClass: Book.self) else { return }
    for (name, type) in types {
        if let objectType = type as? NSObject.Type {
            if objectType == NSDate.self {
                print("Property named '\(name)' has type 'NSDate'")
            } else if objectType == NSString.self {
                print("Property named '\(name)' has type 'NSString'")
            }
        }
    }
}

If you declare this custom == operator:

func ==(rhs: Any, lhs: Any) -> Bool {
    let rhsType: String = "\(rhs)"
    let lhsType: String = "\(lhs)"
    let same = rhsType == lhsType
    return same
}

You can then even check the type of value types like this:

func checkPropertiesOfBook() {
    guard let types = getTypesOfProperties(inClass: Book.self) else { return }
    for (name, type) in types {
        if type == Int.self {
            print("Property named '\(name)' has type 'Int'")
        } else if type == Bool.self {
            print("Property named '\(name)' has type 'Bool'")
        }
    }
}

LIMITATIONS I have not yet been able to give this project support for when the value types are optionals. If you have declared a property in you NSObject subclass like this: var myOptionalInt: Int? my solution will not work, because the method class_copyPropertyList can't find those properties.

Does anyone have a solution for this?