I am using a category on UIViewController to swizzle the viewWillAppear: method across multiple ViewControllers.
@implementation UIViewController (Tracking)
+(void)load
{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
SEL originalSelector = @selector(viewWillAppear:);
SEL swizzledSelector = @selector(xx_viewWillAppear:);
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
BOOL didAddMethod = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));
if (didAddMethod)
{
class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
}else{
method_exchangeImplementations(originalMethod, swizzledMethod);
}
});
}
-(void)xx_viewWillAppear:(BOOL)animated
{
[self xx_viewWillAppear:animated];
NSLog(@"VC loaded");
}
@end
When I load the initial VC from Storyboard, the swizzled method is never called. It is called only if the viewWillAppear method in VC is commented out. However when I remove the category and move the code into the individual VC, both the methods are called.
#import "ViewController.h"
#import "UIViewController+Tracking.h"
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
}
-(void)viewWillAppear:(BOOL)animated
{
NSLog(@"Inside Controller1");
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
@end
Is it ok to implement the viewWillAppear in VC if it is going to be swizzled? If not, how do I handle this, if I am required to write some code in VC's viewWillAppear?
You need to call super
in your viewWillAppear:
from ViewController
, otherwise the method from the base class (UIViewController
) won't execute, and that's the one that you swizzled.
The following version of viewWillAppear:
should give you the expected results:
-(void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
NSLog(@"Inside Controller1");
}