案例一：避免向一个数组中加入nil时导致崩溃

• 交换方法实现swizzle(+load 和 dispatch_once_t 能有效保证安全)；
• 获取类的实例方法class_getInstanceMethod，对应获取类的类方法class_getClassMethod

+(void)load{
    [self swizzleMethod:@selector(addObject:) anotherMethod:@selector(safeAddObject:)];
}

+ (void)swizzleMethod:(SEL)oneSel anotherMethod:(SEL)anotherSel{
     static dispatch_once_t onceToken;
     dispatch_once(&onceToken, ^{
     Method oneMethod = class_getInstanceMethod(NSClassFromString(@"__NSArrayM"), oneself);//类的真名
     Method anotherMethod = class_getInstanceMethod(self, anotherSel);
     if (!oneMethod || !anotherMethod) return;
     method_exchangeImplementations(oneMethod, anotherMethod);
});
}

- (void)safeAddObject:(id)obj {
     if (obj != nil) {
          [self safeAddObject:obj];//不会导致递归
     } else {
          NSLog(@"数组不能加入nil object");
     }
}
@end

案例二：防止按钮重复点击

• 关联属性Associated object

#import "UIControl+Safety.h"
#import <objc/runtime.h>
static const char kSafety_ignoreEventTime;//按钮点击的时间
static const char kSafety_acceptEventInterval;//按钮点击的时间间隔
@interface UIControl ()
@property (nonatomic, assign) NSTimeInterval safety_ignoreEventTime;
@end
@implementation UIControl (Safety)
+(void)load {
    [self swizzleMethod:@selector(sendAction:to:forEvent:) anotherMethod:@selector(safety_sendAction:to:forEvent:)];
}
+ (void)swizzleMethod:(SEL)oneSel anotherMethod:(SEL)anotherSel {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Method oneMethod = class_getInstanceMethod(self, oneSel);
        Method anotherMethod = class_getInstanceMethod(self, anotherSel);
        if (!oneMethod || !anotherMethod) return;
        method_exchangeImplementations(oneMethod, anotherMethod);
    });
}
- (void)safety_sendAction:(SEL)action to:(id)target forEvent:(UIEvent *)event {
    if (NSDate.date.timeIntervalSince1970 - self.safety_ignoreEventTime < self.safety_acceptEventInterval) {
        return;
    }
    
    if (self.safety_acceptEventInterval > 0) {
        self.safety_ignoreEventTime = NSDate.date.timeIntervalSince1970;
    }
    [self safety_sendAction:action to:target forEvent:event];
}

#pragma mark property 关联属性
- (NSTimeInterval)safety_ignoreEventTime {
    return [objc_getAssociatedObject(self, &kSafety_ignoreEventTime) doubleValue];
}

- (void)setSafety_ignoreEventTime:(NSTimeInterval)safety_ignoreEventTime {
    objc_setAssociatedObject(self, &kSafety_ignoreEventTime, @(safety_ignoreEventTime), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (NSTimeInterval)safety_acceptEventInterval {
    return [objc_getAssociatedObject(self, &kSafety_acceptEventInterval) doubleValue];
}

- (void)setSafety_acceptEventInterval:(NSTimeInterval)safety_acceptEventInterval {
    objc_setAssociatedObject(self, &kSafety_acceptEventInterval, @(safety_acceptEventInterval), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
@end

案例三：字典转模型原理