How does one eliminate Objective-C #try #catch blocks like this? - objective-c

Question

I'm a developer from Python world used to using exceptions. I found in many places that using exceptions is not so wise here, and did my best to convert to NSErrors when needed. but then I encounter this:
NSMutableArray *results;
for (NSDictionary *dict in dicts)
{
// Memory management code omitted
SomeModel *model = [[SomeModel alloc] init];
model.attr1 = [[dict objectForKey:#"key1"] integerValue];
model.attr2 = [[dict objectForKey:#"key2"] integerValue];
model.attr3 = [[dict objectForKey:#"key3"] integerValue];
model.attr4 = [[dict objectForKey:#"key4"] integerValue];
[results addObject:model];
}
with some of the objects in dict containing NSNull, which would result an "unrecognized selector" exception. In that case, I want to drop that datum completely. My first instinct is to wrap the whole content of the for block into a #try-#catch block:
NSMutableArray *results;
for (NSDictionary *dict in dicts)
{
#try
{
SomeModel *model = [[SomeModel alloc] init];
model.attr1 = [[dict objectForKey:#"key1"] integerValue];
model.attr2 = [[dict objectForKey:#"key2"] integerValue];
model.attr3 = [[dict objectForKey:#"key3"] integerValue];
model.attr4 = [[dict objectForKey:#"key4"] integerValue];
[results addObject:model];
}
#catch(NSException *exception)
{
// Do something
}
}
But is this a good approach? I can't come up with a solution without repeating checks on each variable, which is really ugly IMO. Hopefully there are alternatives to this that haven't occur to me. Thanks in advance.

Answer 1

The proper Objective-C way to do this would be:
for (NSDictionary *dict in dicts)
{
if (! [dict isKindOfClass:[NSDictionary class]])
continue;
// ...
}
Testing if a receiver can respond to a message before sending it is a typical pattern in Objective-C.
Also, take note that exceptions in Objective-C are always a programmer error and are not used for normal execution flow.

Answer 2

Many people use a category on NSDictionary for these cases:
- (id)safeObjectForKey:(id)aKey
{
id obj = [self objectForKey:aKey];
if ([obj isKindOfClass:[NSNull class]])
{
return nil;
}
return obj;
}
You still need to make sure, that your dict is an actual dictionary instance.

Answer 3

In the end I decided to solve the problem using KVC. Something like this:
- (id)initWithPropertyDictionary:(NSDictionary *)dict
lookUpTable:(NSDictionary *)keyToProperty
{
self = [self init];
for (NSString *key in dict)
{
NSString *propertyName;
if ([keyToProperty objectForKey:key])
propertyName = [keyToProperty objectForKey:key];
else
propertyName = key;
if ([[dict objectForKey:key] isKindOfClass:[NSNull class]])
{
[self release];
return nil;
}
else
{
[self setValue:[dict objectForKey:key] forKey:propertyName];
}
}
}
The setback of this resolution is that I'll have to use NSNumber for my properties, but for JSON data there is really no distinction between floating numbers and integers, so this is fine.
And if you really want primitive types, you can couple this method with custom setters that converts those NSNumbers into appropriate types.
With this, all you need to do is check for nil before adding the object into the array. Much cleaner everywhere except the model class.
Thanks to jaydee3 for inspiring me to focus on changing the model class.