Objective-C & KeyValueCoding: How to avoid an exception with valueForKeyPath:?

Question

I've got an object of type id and would like to know if it contains a value for a given keyPath:

[myObject valueForKeyPath:myKeyPath];

Now, I wrap it into a @try{ } @catch{} block to avoid exceptions when the given keypath isn't found. Is there a nicer way to do this? Check if the given keypath exists without handling exceptions?

Thanks a lot,

Stefan

Answer 1

You could try this:

if ([myObject respondsToSelector:NSSelectorFromString(myKeyPath)])
{
}

However, that may not correspond to the getter you have, especially if it is a boolean value. If this doesn't work for you, let me know and I'll write you up something using reflection.

Answer 2

For NSManagedObjects, an easy solution is to look at the object's entity description and see if there's an attribute with that key name. If there is, you can also take it to the next step and see what type of an attribute the value is.

Here's a simple method that given any NSManagedObject and any NSString as a key, will always return an NSString:

- (NSString *)valueOfItem:(NSManagedObject *)item asStringForKey:(NSString *)key {

    NSEntityDescription *entity = [item entity];
    NSDictionary *attributesByName = [entity attributesByName];
    NSAttributeDescription *attribute = attributesByName[key];

    if (!attribute) {
        return @"---No Such Attribute Key---";
    }
    else if ([attribute attributeType] == NSUndefinedAttributeType) {
        return @"---Undefined Attribute Type---";
    }
    else if ([attribute attributeType] == NSStringAttributeType) {
        // return NSStrings as they are
        return [item valueForKey:key];
    }
    else if ([attribute attributeType] < NSDateAttributeType) {
        // this will be all of the NSNumber types
        // return them as strings
        return [[item valueForKey:key] stringValue];
    }
        // add more "else if" cases as desired for other types

    else {
        return @"---Unacceptable Attribute Type---";
    }
}

If the key is invalid or the value can't be made into a string, the method returns an NSString error message (change those blocks to do whatever you want for those cases).

All of the NSNumber attribute types are returned as their stringValue representations. To handle other attribute types (e.g.: dates), simply add additional "else if" blocks. (see NSAttributeDescription Class Reference for more information).