An NSPredicate that can recursively traverse an object graph?

问题

I'm trying to filter an array of objects that essentially form a tree-style graph. what i want to do is to filter out all objects from this array whose visible property is NO, or if its parent/grandparent/etc visible property is true (child objects can have the visible property be YES while its parent can be NO).

I'm unclear as to how i would go about this using NSPredicate syntax to keep searching the parent node until there are no parents or the visible property is found. Is there any way to go about this?

回答1:

Its been awhile since I asked this question, and I think I went in another direction with what i was doing, but there are some possibilities i realize now to solve what i wanted at the time:

Have the visible property method behave recursively instead of making the format predicate do that. This could be accomplished like so:

- (BOOL) isVisible {
  return visible && [parent isVisible];
}

//...
id filtered = [array filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:@"visible == YES"]];

Use block predicates instead of format predicates to do the recursive traversal:

[array filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL(id evaluatedObject, NSDictionary *bindings) {

    id obj = evaluatedObject;
    while (obj) {
      if (![obj isVisible]) return NO;
      obj = [obj parent];
    }
    return YES;
}]];

Or a combination of the two (which would be the most robust and readable I think).

回答2:

I'm not sure what you're looking to do is possible with a simple, single predicate. If it's a tree and you're using a predicate to get the nodes, you'd want and write a method that will traverse upwards and return a BOOL indicating whether it should be removed or not.

Then, just get your nodes and put them in an NSMutableArray and do a

for (int i = 0; i < [results count]; i++)
{
    if ([self shouldBeRemoved:[results objectAtIndex:i]])
    {
        [results removeObjectAtIndex:i];
        i--;
    }
}

Your shouldBeRemoved: method should be pretty a straightforward recursive method.

来源：https://stackoverflow.com/questions/2285467/an-nspredicate-that-can-recursively-traverse-an-object-graph

标签

objective-c

nspredicate