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

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.