Method swizzling for NSTimer is not working

Question

I am trying to method swizzling for NSTimer in a category. But the swizzling method swizzling_invalidate is never call when send message invalidate to NSTimer object.

#import "NSTimer+Test.h"
#import <objc/runtime.h>

@implementation NSTimer (Test)

+ (void)load {
    Method originalMethod, swizzlingMethod;
    originalMethod = class_getInstanceMethod(self, @selector(invalidate));
    swizzlingMethod = class_getInstanceMethod(self, @selector(swizzling_invalidate));
    method_exchangeImplementations(originalMethod, swizzlingMethod);
}

- (void)swizzling_invalidate {
    [self swizzling_invalidate];
}

@end

It is so strange that method swizzling for other NSObject is working. Like that:

#import "NSUserDefaults+Timing.h"
#import <objc/runtime.h>

@implementation NSUserDefaults (Timing)

+ (void)load {
    Method original, swizzling;
    original = class_getInstanceMethod([self class], @selector(synchronize));
    swizzling = class_getInstanceMethod([self class], @selector(swizzling_synchronize));
    method_exchangeImplementations(original, swizzling);
}

- (BOOL)swizzling_synchronize {
    NSDate *start = [NSDate date];
    BOOL returnValue = [self swizzling_synchronize];
    NSLog(@"%f", [[NSDate date] timeIntervalSinceDate:start]);
    return returnValue;
}

@end

What exactly is not working? Is `load` not called or `swizzling_invalidate`? How did notice the error? What did you try? — Nikolai Ruhe, Jul 14 '14 at 08:05
In NSTimer category, I have try method swizzling on other NSObject, it work. It is strange method swizzling not working on NSTimer selector. @Caleb — yehuanwen, Jul 14 '14 at 08:06
`swizzling_invalidate ` is not calling when `[timer invalidate]`.@NikolaiRuhe — yehuanwen, Jul 14 '14 at 08:07
What are you trying to achieve by swizzling `invalidate`? Perhaps there is a better way to achieve the same result. — mttrb, Jul 14 '14 at 08:57
Because I want to remove associated object for NSTimer when `invalidate`. Any ideas to run my own code after `invalidate`? @mttrb — yehuanwen, Jul 14 '14 at 09:04

mttrb · Answer 1 · 2014-07-14T08:56:28.727

The NSTimer documentation says:

You should not attempt to subclass NSTimer.

This almost certainly means that the internals of NSTimer are not simple and that method swizzling will be tricky.

I suspect what this actually means is that NSTimer is implemented as a class cluster or something similar. The fact that NSTimer is toll-free bridged to Core Foundation strongly suggests it might be.

This article, Playing with NSTimer, suggests that NSTimer is actually an abstract class.

score 0 · Answer 2 · answered Jul 14 '14 at 08:07

0

You have to make sure that your load method is called only once. Otherwise the loading of NSTimer subclasses would switch swizzling back to the original implementation.

answered Jul 14 '14 at 08:07

Nikolai Ruhe

81,520
17
180
200

It is called only once. I can add `dispatch_once` to make sure it is called once. But it won't worked either. – yehuanwen Jul 14 '14 at 08:10

Method swizzling for NSTimer is not working

2 Answers2