Tinkering with a predicate builder in Objective-C.

IHPredicateBuilder.m

/* I'm trying to come up with with a general predicate builder in Objective-C, that can build NSPredicates for
arbitrary classes. I want to write something like this:
	
	predicate = [[[[[builder name] contains:@"ilja"] age] greaterThan:@20] predicate];

But the compiler insists on sprinkling ids everywhere.
	
	predicate = [[(id)[[(id) [(id) builder name] contains:@"ilja"] age] greaterThan:@20] predicate];
	
Any ideas how to omit the 'id' casts? Also, need a good idea for or and not methods.

Feel free to paste this code into CodeRunner.

*/

#import <Foundation/Foundation.h>


@interface Person: NSObject

@property NSString *name;
@property NSNumber *age; /* I' rather have an int property, but the compiler doesn't like sending messages to int */

@end

@implementation Person

@end

@interface IHPredicateBuilder: NSObject

+ (id) predicateBuilderForClass:(Class) inClass;

- (id) contains:(NSString*) value;
- (id) equals:(id) value;
- (id) lessThan:(NSNumber*) value;
- (id) lessThanOrEqual:(NSNumber*) value;
- (id) greaterThan:(NSNumber*) value;
- (id) greaterThanOrEqual:(NSNumber*) value;

/* No idea how to this so far
- (id) not:(IHPredicateBuilder*) predicate;
- (id) or:(IHPredicateBuilder*) inPredicates;

*/

@property Class targetClass;

@property NSString *lastKey;
@property NSPredicateOperatorType lastOperatorType;

@property NSPredicate *predicate;

@end

@implementation IHPredicateBuilder


+ (id) predicateBuilderForClass:(Class) inClass
{
	IHPredicateBuilder *result = [[self alloc] init];
	
	result.targetClass = inClass;
	
	return result;
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
	
	NSString *selectorName = NSStringFromSelector( aSelector);
	
	if ([self.targetClass instancesRespondToSelector:aSelector]) {
		NSString *selectorName = NSStringFromSelector( aSelector);
		
		self.lastKey = selectorName;
		
		return [self methodSignatureForSelector:@selector(mirror)];
	}
	return [super methodSignatureForSelector:aSelector];

}


- (void)forwardInvocation:(NSInvocation *)anInvocation
{
	if ([self.targetClass instancesRespondToSelector:anInvocation.selector]) {
		anInvocation.selector = @selector(mirror);
		[anInvocation invokeWithTarget:self];
		return;		
	}
	[super forwardInvocation:anInvocation];
}

- (id) mirror
{
	return self;
}

- (id) contains:(NSString*) inValue
{
	[self addPredicateWithOperatorType: NSContainsPredicateOperatorType value:inValue];
	return self;
}

- (id) equals:(id) inValue
{
	[self addPredicateWithOperatorType: NSEqualToPredicateOperatorType value:inValue];
	return self;
}

- (id) lessThan:(NSNumber*) inValue
{
	[self addPredicateWithOperatorType: NSLessThanPredicateOperatorType value:inValue];
	return self;
}

- (id) lessThanOrEqual:(NSNumber*) inValue
{
	[self addPredicateWithOperatorType: NSLessThanOrEqualToPredicateOperatorType value:inValue];
	return self;
}

- (id) greaterThan:(NSNumber*) inValue
{
	[self addPredicateWithOperatorType: NSGreaterThanPredicateOperatorType value:inValue];
	return self;
}

- (id) greaterThanOrEqual:(NSNumber*) inValue
{
	[self addPredicateWithOperatorType: NSGreaterThanOrEqualToPredicateOperatorType value:inValue];
	return self;
}

- (void) addPredicateWithOperatorType:(NSPredicateOperatorType) inType value:(id) inValue
{
	NSPredicate *newPredicate = [NSComparisonPredicate predicateWithLeftExpression:[NSExpression expressionForKeyPath:self.lastKey]
	rightExpression:[NSExpression expressionForConstantValue:inValue]
	modifier:NSDirectPredicateModifier
	type:inType 
	options:NSDiacriticInsensitivePredicateOption];
	
	if (self.predicate == nil) {
		self.predicate = newPredicate;
	} else {
		self.predicate = [NSCompoundPredicate andPredicateWithSubpredicates:@[self.predicate, newPredicate]];;
	}
}

/* No idea yet 
- (id) or:(NSArray<IHPredicateBuilder*>*) inPredicates
{
}

- (id) not:(IHPredicateBuilder*) predicate
{
}
*/

@end

int main(int argc, char *argv[]) {
	@autoreleasepool {
		
		Person *dummy = [[Person alloc] init];
		
		dummy.name = @"ilja";
		dummy.age = @100;
		
		IHPredicateBuilder *builder = [IHPredicateBuilder predicateBuilderForClass:Person.class];
		NSPredicate *predicate = [[(id)[[(id) [(id) builder name] contains:@"ilja"] age] greaterThan:@20] predicate];
		
		NSLog (@"should evaluate to 1: %d", [predicate evaluateWithObject:dummy]);
		
		dummy.age = @5;
		
		NSLog (@"should evaluate to 0: %d", [predicate evaluateWithObject:dummy]);
	}
}

Would UIAppearance be a good inspiration? Where you call, for example: [[UINavigationBar appearance] setTintColor:...]? The biggest difference is that you want to "re-implement" the properties of Person returning the IHPredicateBuilder.

[UINavigationBar appearance] returns an object conforming to protocol UIAppearance . I'm wondering why they can send messages such as setTintColor there without the compiler raising an error message. 🤷‍♂️

I bet the magic lies in UI_APPEARANCE_SELECTOR

So that's defined as
#define UI_APPEARANCE_SELECTOR attribute((annotate("ui_appearance_selector")))

Looks like I'd need to hack the compiler to get rid of the warnings?