Custom leftBarButtonItem and interactivePopGestureRecognizer Freeze

iOS7有个系统app都有个新功能，支持手势返回。也就是navigationViewController.interactivePopGestureRecognizer

如果你乖乖的用系统自带的UIBarButtonItem来定义navigationViewController的leftBarButtonItem的话，那么一切都ok。但是如果你是通过initWithCustomView:来定义leftBarButtonItemd的话，你需要在你父ViewController的ViewDidAppear方法里手动开启它, 当然顺便请设置好Gesture的delegate.

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    if ([self.navigationController respondsToSelector:@selector(interactivePopGestureRecognizer)]) {
        self.navigationController.interactivePopGestureRecognizer.delegate = self;
    }
}

可是，可是，这样做还不够。你可以快速pop一个viewcontroller并且快速手势返回，这时候，你的整个屏幕就freeze了。

怎么办呢？

看了网上的整体思路就是，你当pop的时候，把navigationController.interactivePopGestureRecognizer.enabled设为NO，当你navigationController:didShowViewController:animated:（nav的delegate）时候再打开。

具体到代码就是：

在navigationViewController里
- (void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated
  {
  if ([self respondsToSelector:@selector(interactivePopGestureRecognizer)])
```
self.interactivePopGestureRecognizer.enabled = NO;    
```
  [super pushViewController:viewController animated:animated];
  }
在你ViewController里
- (void)navigationController:(UINavigationController *)navigationController
```
didShowViewController:(UIViewController *)viewController
             animated:(BOOL)animate
```
  {
  if ([self.navigationController respondsToSelector:@selector(interactivePopGestureRecognizer)])
```
self.navigationController.interactivePopGestureRecognizer.enabled = YES;
```
  }

问题来了，如何令所有的navigationViewController都能有我们经过处理后的pushViewController:animated:方法。可以看到，这里我是subclass了。

这里我主要想介绍另外一种objc runtime的方法， method_swizzle。

#import <objc/runtime.h>

@implementation UINavigationController (PopGesture)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];

        // When swizzling a class method, use the following:
        // Class class = object_getClass((id)self);

        SEL originalSelector = @selector(pushViewController:animated:);
        SEL swizzledSelector = @selector(xxx_pushViewController:animated:);

        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);
        }
    });
}

#pragma mark - Method Swizzling

- (void)xxx_pushViewController:(UIViewController *)viewController animated:(BOOL)animated
{
    NSLog(@"xxx_pushViewController: %@", self);
    if ([self respondsToSelector:@selector(interactivePopGestureRecognizer)])
        self.interactivePopGestureRecognizer.enabled = NO;

    [self xxx_pushViewController:viewController animated:animated];
}

@end

这样就不要subclass了。