WeakDetector.m

///////////////////////////////////////////////////////////////////////////////////////////
#pragma mark - Warn if we KVO a weak property

// Doesn't support key paths.
static BOOL PSPDFIsWeakProperty(id object, NSString *keyPath) {
    objc_property_t property = class_getProperty([object class], keyPath.UTF8String);
    if (property) {
        // https://developer.apple.com/library/mac/documentation/cocoa/conceptual/objcruntimeguide/articles/ocrtpropertyintrospection.html
        const char *attributes = property_getAttributes(property);
        return attributes && strstr(attributes, ",W");
    }
    return NO;
}

// Adding KVO to weak properties is a very bad idea.
// See http://www.componentix.com/blog/28/debug-usage-of-objectivec-weak-properties-with-kvo
__attribute__((constructor)) static void PSPDFWarnOnWeakPropertyKVO(void) {
    @autoreleasepool {
        SEL selector = NSSelectorFromString(@"pspdf_addObserver:forKeyPath:options:context:");
        PSPDFReplaceMethodWithBlock(NSObject.class, @selector(addObserver:forKeyPath:options:context:), selector, ^(NSObject *self, NSObject *observer, NSString *keyPath, NSKeyValueObservingOptions options, void *context) {
            if (PSPDFIsWeakProperty(self, keyPath)) {
                PSPDFLogError(@"WARNING: observing '%@' property, which is weak and so isn't fully KVO-compliant", keyPath);
            }
             ((void ( *)(id, SEL, NSObject*, NSString*, NSKeyValueObservingOptions, void *))objc_msgSend)(self, selector, observer, keyPath, options, context);
        });
    }
}

I only run this on debug builds, saves you from being stupid :)