程序崩溃对于app来说是最致命的bug，而数组越界便是其中最重要的原因之一。我们可以应用Method Swizzling知识来避免这一问题。
1.我们来创建一个类别，继承于NSArray：

121.png

2.然后在.m文件中导入 objc/runtime.h头文件

#import "NSArray+EM.h"  
#import <objc/runtime.h>  

@implementation NSArray (EM)  
+(void)load {  
Method fromMethod = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(objectAtIndex:));  
Method toMethod = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(em_objectAtIndex:));  
method_exchangeImplementations(fromMethod, toMethod);  
}  

- (void)em_objectAtIndex:(NSUInteger)index {  
    if (self.count - 1 < index) {  
      @try {  
        return[self em_objectAtIndex:index];  
      }  
      @catch (NSException *exception) {  
        NSLog(@"-------- %s Crash Because Method %s -------\n",class_getName(self.class),__func__);  
        NSLog(@"%@", [exception callStackSymbols]);  
      }  
      @finally {  
          
      }  
  }else {  
    return [self em_objectAtIndex:index];  
  }  
 }  
@end  

细心的人会发现，下面这个方法会导致递归，其实不会的。
这实际上是运用了方法替换，当我们在其他类中 调用NSArray的objectAtIndex方法时，会被自动替换为执行em_objectAtIndex:方法，而当我们调用em_objectAtIndex:方法时，会被替换为执行objectAtIndex:方法。我们也可以用这种方法同样解决NSMutableArray。

#import "NSMutableArray+EM.h"  
#import <objc/runtime.h>  
@implementation NSMutableArray (EM)  
+(void)load {  
Method fromMethod = 
class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(objectAtIndex:));  
Method toMethod = 
class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(em_objectAtIndex:));  
method_exchangeImplementations(fromMethod, toMethod);  
 }  

- (void)em_objectAtIndex:(NSUInteger)index {  
  if (self.count - 1 < index) {  
      @try {  
        return[self em_objectAtIndex:index];  
    }  
      @catch (NSException *exception) {  
        NSLog(@"-------- %s Crash Because Method %s -------\n",class_getName(self.class),__func__);  
        NSLog(@"%@", [exception callStackSymbols]);  
      }  
      @finally {  
          
      }  
  }else {  
      return [self em_objectAtIndex:index];  
  }  
}  

@end  

还可以判断NSMutableDictionary 的value值是否为nil

#import "NSMutableDictionary+EM.h"  
#import <objc/runtime.h>  
@implementation NSMutableDictionary (EM)  
+ (void)load {  
  
Method fromMethod = class_getInstanceMethod(objc_getClass("__NSDictionaryM"), @selector(setObject:forKey:));  
Method toMethod = class_getInstanceMethod(objc_getClass("__NSDictionaryM"), @selector(em_setObject:forKey:));  
method_exchangeImplementations(fromMethod, toMethod);  
}  

- (void)em_setObject:(id)emObject forKey:(NSString *)key {  
    if (emObject == nil) {  
      @try {  
          [self em_setObject:emObject forKey:key];  
      }  
      @catch (NSException *exception) {  
        NSLog(@"---------- %s Crash Because Method %s  ----------\n", class_getName(self.class), __func__);  
        NSLog(@"%@", [exception callStackSymbols]);  
        emObject = [NSString stringWithFormat:@""];  
        [self em_setObject:emObject forKey:key];  
      }  
      @finally {}  
  }else {  
      [self em_setObject:emObject forKey:key];  
  }  
}  
@end  

下面我们列举一些常用的类簇的“真身”：

222.png

注：转载自 MiAo_EM的博客