In Objective-C, how can I achieve something like this:
Class propertyClass = [self classForPropertyWithName:#"a_property"];
So you could break it into two steps.
Get a property by name (via -valueForKey:)
Get the class of the property (via -class)
Something like this maybe (note that this will fail for non object types):
Class propertyClass = [[self valueForKey:#"a_property"] class];
You should get the object with property named "key" using this method:
- (id)valueForKey:(NSString *)key
ie:
object = [valueForKey:#"propertyName"];
Then to get the class type you simply call the class method which is available for all object types except primitives.
Class classType = [object class];
I think the answer you are looking for is here. Code:
Class c = NSClassFromString(#"RootViewController");
Another option:
id object = // Fetch here;
Class c = [object class];
NSString *className = NSStringFromClass(c);
Documentation: link
Related
I am trying to learn reflection in Objective-C. I've found some great information on how to dump the property list of a class, especially here, but I want to know if it is possible to set a property's value using reflection.
I have a dictionary of keys (property names) and values (all NSStrings). I want to use Reflection to get the property and then set its value to the value in my dictionary. Is this possible? Or am I dreaming?
This has nothing to do with the dictionary. I am merely using the dictionary to send in the values.
Like this question, but for Objective C.
- (void)populateProperty:(NSString *)value
{
Class clazz = [self class];
u_int count;
objc_property_t* properties = class_copyPropertyList(clazz, &count);
for (int i = 0; i < count ; i++)
{
const char* propertyName = property_getName(properties[i]);
NSString *prop = [NSString stringWithCString:propertyName encoding:NSUTF8StringEncoding]];
// Here I have found my prop
// How do I populate it with value passed in?
}
free(properties);
}
Objective C properties automatically conform to the NSKeyValueCoding protocol. You can use setValue:forKey: to set any property value by a string property name.
NSDictionary * objectProperties = #{#"propertyName" : #"A value for property name",
#"anotherPropertyName" : #"MOAR VALUE"};
//Assuming class has properties propertyName and anotherPropertyName
NSObject * object = [[NSObject alloc] init];
for (NSString * propertyName in objectProperties.allKeys)
{
NSString * propertyValue = [objectProperties valueForKey:propertyName];
[object setValue:propertyValue
forKey:propertyName];
}
The NSKeyValueCoding protocol, which NSObject implements (see NSKeyValueCoding.h), contains the method -setValuesForKeysWithDictionary:. This method takes exactly the kind of dictionary you describe and sets the appropriate properties (or ivars) of the reciever.
This is absolutely reflection; the code in setValuesForKeysWithDictionary: accesses the properties by the names you give it, and will even find the appropriate ivar if no setter method exists.
NSClassFromString(aClassName) returns a class object of the class named aClassName. Great.
Now how can I call a class method on that class object? For
Class moduleClass = NSClassFromString(aClassName);
Xcode won't allow me to either call
AppModule* appModuleClass = moduleClass;
[appModuleClass classMethod] // actually that's an object instance...
or
[((AppModule)moduleClass) classMethod]; // C-style cast not allowed
What am I missing here? Thanks.
To call a class method do so:
[NSClassFromString(aClassName) performSelector:#selector(classMethod)];
Maybe this could work:
objc_msgSend(NSClassFromString(aClassName), #selector(classMethod));
You can use
__typeof([AppModule class]) cls = NSClassFromString(#"aClassName");
if ( [cls respondsToSelector:#selector(classMethod)] ) {
[cls classMethod];
}
I have a problem with classes. This is my code:
self.shapeClass = [HWRectangle class];
if ([_shapeClass isKindOfClass:[HWRectangle class]]) {
NSLog(#"Class created as: %s", [_shapeClass description]);
}
I thought that the program will do the logging in this case, but it doesn't. Do you have any idea why not?
because: if ([_shapeClass isKindOfClass:[HWRectangle class]])
_shapeClass should be an instance of the class you are testing, unless you are really testing for class comparisons. So, this method is instance to class comparison, not class to class comparison.
For bonus points, your format string should be: NSLog(#"Class created as: %#", [_shapeClass description])
(unless you have overridden the NSObject method (which you should not do))
isKindOfClass checks the class of a variable. You say that shapeCalls = [HWRectangle class]. The result of [HWRectangle class] is of the class "Class". So if you compare this with the class of HWRectangle you will find that the two are not the same.
I have an object in objective-c at runtime, from which I only know the KVC key and I need to detect the return value type (e.g. I need to know if its an NSArray or NSMutableArray) of this property, how can I do that?
You're talking about runtime property introspection, which happens to be something that Objective-C is very good at.
In the case you describe, I'm assuming you have a class like this:
#interface MyClass
{
NSArray * stuff;
}
#property (retain) NSArray * stuff;
#end
Which gets encoded in XML something like this:
<class>
<name>MyClass</name>
<key>stuff</key>
</class>
From this information, you want to recreate the class and also give it an appropriate value for stuff.
Here's how it might look:
#import <objc/runtime.h>
// ...
Class objectClass; // read from XML (equal to MyClass)
NSString * accessorKey; // read from XML (equals #"stuff")
objc_property_t theProperty =
class_getProperty(objectClass, accessorKey.UTF8String);
const char * propertyAttrs = property_getAttributes(theProperty);
// at this point, propertyAttrs is equal to: T#"NSArray",&,Vstuff
// thanks to Jason Coco for providing the correct string
// ... code to assign the property based on this information
Apple's documentation (linked above) has all of the dirty details about what you can expect to see in propertyAttrs.
Cheap answer: use the NSObject+Properties source here.
It implements the same methodology described above.
The preferred way is to use the methods defined in the NSObject Protocol.
Specifically, to determine if something is either an instance of a class or of a subclass of that class, you use -isKindOfClass:. To determine if something is an instance of a particular class, and only that class (ie: not a subclass), use -isMemberOfClass:
So, for your case, you'd want to do something like this:
// Using -isKindOfClass since NSMutableArray subclasses should probably
// be handled by the NSMutableArray code, not the NSArray code
if ([anObject isKindOfClass:NSMutableArray.class]) {
// Stuff for NSMutableArray here
} else if ([anObject isKindOfClass:NSArray.class]) {
// Stuff for NSArray here
// If you know for certain that anObject can only be
// an NSArray or NSMutableArray, you could of course
// just make this an else statement.
}
This is really a comment addressing an issue raised by Greg Maletic in response to answer provided by e.James 21APR09.
Agreed that Objective-C could use a better implementation for getting these attributes.
Below is a method I quickly threw together to retrieve attributes of a single object property:
- (NSArray*) attributesOfProp:(NSString*)propName ofObj:(id)obj{
objc_property_t prop = class_getProperty(obj.class, propName.UTF8String);
if (!prop) {
// doesn't exist for object
return nil;
}
const char * propAttr = property_getAttributes(prop);
NSString *propString = [NSString stringWithUTF8String:propAttr];
NSArray *attrArray = [propString componentsSeparatedByString:#","];
return attrArray;
}
Partial list of attribute keys:
R Read-only
C Copy of last value assigned
& Reference to last value assigned
N Nonatomic property
W Weak reference
Full list at Apple
You can use isKindOfClass message
if([something isKindOfClass:NSArray.class])
[somethingElse action];
If you know that the property is defined :
id vfk = [object valueForKey:propertyName];
Class vfkClass = vfk.class;
And compare with isKindOfClass, isSubClass, etc.
I am working on an object factory to keep track of a small collection of objects. The objects can be of different types, but they will all respond to createInstance and reset. The objects can not be derived from a common base class because some of them will have to derive from built-in cocoa classes like NSView and NSWindowController.
I would like to be able to create instances of any suitable object by simply passing the desired classname to my factory as follows:
myClass * variable = [factory makeObjectOfClass:myClass];
The makeObjectOfClass: method would look something like this:
- (id)makeObjectOfClass:(CLASSNAME)className
{
assert([className instancesRespondToSelector:#selector(reset)]);
id newInstance = [className createInstance];
[managedObjects addObject:newInstance];
return newInstance;
}
Is there a way to pass a class name to a method, as I have done with the (CLASSNAME)className argument to makeObjectOfClass: above?
For the sake of completeness, here is why I want to manage all of the objects. I want to be able to reset the complete set of objects in one shot, by calling [factory reset];.
- (void)reset
{
[managedObjects makeObjectsPerformSelector:#selector(reset)];
}
You can convert a string to a class using the function: NSClassFromString
Class classFromString = NSClassFromString(#"MyClass");
In your case though, you'd be better off using the Class objects directly.
MyClass * variable = [factory makeObjectOfClass:[MyClass class]];
- (id)makeObjectOfClass:(Class)aClass
{
assert([aClass instancesRespondToSelector:#selector(reset)]);
id newInstance = [aClass createInstance];
[managedObjects addObject:newInstance];
return newInstance;
}
I have right a better tutorial on that , please checkout
https://appengineer.in/2014/03/13/send-class-name-as-a-argument-in-ios/
It's pretty easy to dynamically specify a class, in fact you can just reference it by it's name:
id string = [[NSClassFromString(#"NSString") alloc] initWithString:#"Hello!"];
NSLog( #"%#", string );
One other tip, I would avoid using the nomenclature 'managed object' since most other Cocoa programmers will read that as NSManagedObject, from Core Data. You may also find it easier to use a global NSNotification (that all your reset-able objects subscribe to) instead of managing a collection of different types of objects, but you're more informed to make that decision than I am.
The bit of the answer missing from the other answers is that you could define a #protocol containing your +createInstance and +reset methods.
It sounds like you want something like:
- (id)makeObjectOfClassNamed:(NSString *)className
{
Class klass = NSClassFromString(className);
assert([klass instancesRespondToSelector:#selector(reset)]);
id newInstance = [klass createInstance];
[managedObjects addObject:newInstance];
return newInstance;
}
This would assume a class method named +createInstance. Or you could just use [[klass alloc] init].
To call it:
MyClass *variable = [factory makeObjectOfClassNamed:#"MyClass"];
Depending on what you're trying to do, it might be better to pass around class objects than strings, e.g.:
MyClass *variable = [factory makeObjectOfClass:[MyClass class]];