很早就看过一些关于 Method Swizzling 的博客，看完后一直没有一个恰当的使用场景能操练一下。最近写一个 Demo 的时候发现要在很多控制器里写导航栏的返回按钮，本来是复制一下或者继承一下就行的。但是复制这种做法挺蠢的，继承会让代码耦合性增加。这个时候我就突然的想到了 Method Swizzling，然后做了一个尝试。
创建一个 UIViewController 的分类，引入#import <objc/runtime.h>头文件。基本代码如下：
@implementation UIViewController (BackButton)
+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
SEL originalSelector = @selector(viewWillAppear:);
SEL swizzledSelector = @selector(swizzled_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);
}
});
}
#pragma mark - Method Swizzling
- (void)swizzled_viewWillAppear:(BOOL)animated {
[self swizzled_viewWillAppear:animated];
//在这里判断哪个控制器不需要返回按钮
if (![self isMemberOfClass:NSClassFromString(@"ViewController")]) {
UIImage *image = [UIImage imageNamed:@"goBack_blue.png"];
UIButton *leftButton = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, image.size.width, image.size.height)];
[leftButton setImage:image forState:UIControlStateNormal];
[leftButton addTarget:self action:@selector(goBack) forControlEvents:UIControlEventTouchUpInside];
UIBarButtonItem *leftBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:leftButton];
self.navigationItem.leftBarButtonItem = leftBarButtonItem;
}
NSLog(@"swizzled_viewWillAppear");
}
- (void)goBack {
[self.navigationController popViewControllerAnimated:YES];
}
@end
这样就实现了，下篇见～
