ObjectiveC Lightweight Generic

objc.m

@interface BaseViewModel : NSObject
- (void)baseMethod;
@end

@implementation BaseViewModel
- (void)baseMethod {}
@end

// ---------------------------------------------------------------------------------------------------------------------

@interface BaseViewController<__covariant ViewModelType: BaseViewModel *> : UIViewController
@property (nonatomic, strong) ViewModelType viewModel;
@end

@implementation BaseViewController

- (void)ha {
    [self.viewModel baseMethod];
}

@end

// =====================================================================================================================

@interface MyViewModel : BaseViewModel
- (void)myMethod;
@end

@implementation MyViewModel
- (void)myMethod {}
@end

// ---------------------------------------------------------------------------------------------------------------------

@interface MyViewController : BaseViewController<MyViewModel *>
@end

@implementation MyViewController

- (void)yo {
    [self.viewModel baseMethod];
    [self.viewModel myMethod];
}

@end