首先加入一个小知识：
SEL、Method、IMP的含义及区别
在运行时，类（Class）维护了一个消息分发列表来解决消息的正确发送。每一个消息列表的入口是一个方法（Method），这个方法映射了一对键值对，其中键是这个方法的名字（SEL），值是指向这个方法实现的函数指针 implementation（IMP）。
伪代码表示：
Class {
MethodList (
Method{
SEL:IMP；
}
Method{
SEL:IMP；
}
);
};Method Swizzling就是改变类的消息分发列表来让消息解析时从一个选择器（SEL）对应到另外一个的实现（IMP），同时将原始的方法实现混淆到一个新的选择器（SEL）。
对Swizzling方法封装
//
// NSObject+Swizzling.h
// Swizzling
//
// Created by peter.zhang on 2016/12/14.
// Copyright © 2016年 Peter. All rights reserved.
//
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
@interface NSObject (Swizzling)
/**
* Adds a new method to a class with a given name and implementation.
*
* @param originalSelector 原来的方法
* @param swizzledSelector 替换成的方法
*
*/
+ (void)methodSwizzlingWithOriginalSelector:(SEL)originalSelector
bySwizzledSelector:(SEL)swizzledSelector;
@end
//
// NSObject+Swizzling.m
// Swizzling
//
// Created by peter.zhang on 2016/12/14.
// Copyright © 2016年 Peter. All rights reserved.
//
#import "NSObject+Swizzling.h"
@implementation NSObject (Swizzling)
+ (void)methodSwizzlingWithOriginalSelector:(SEL)originalSelector bySwizzledSelector:(SEL)swizzledSelector{
Class class = [self class];
//原有方法
Method originalMethod = class_getInstanceMethod(class, originalSelector);
//替换原有方法的新方法
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
//先尝试給源SEL添加IMP，这里是为了避免源SEL没有实现IMP的情况
BOOL didAddMethod = class_addMethod(class,originalSelector,
method_getImplementation(swizzledMethod),
method_getTypeEncoding(swizzledMethod));
if (didAddMethod) {//添加成功：说明源SEL没有实现IMP，将源SEL的IMP替换到交换SEL的IMP
class_replaceMethod(class,swizzledSelector,
method_getImplementation(originalMethod),
method_getTypeEncoding(originalMethod));
} else {//添加失败：说明源SEL已经有IMP，直接将两个SEL的IMP交换即可
method_exchangeImplementations(originalMethod, swizzledMethod);
}
}
@end
-------------------------------以上是对Swizzling方法封装类别--------------------------------
runtime有很多用途：改变ViewController的生命周期、app热更新、改变系统方法调度（解决获取索引、添加、删除元素越界崩溃问题）等。今天主要说数组或者字典的越界crash问题。
啥都不是了，你把Swizzling方法封装类别添加到工程中：
以可变数组为例子：
//
// NSMutableArray+Security.h
// Swizzling
//
// Created by peter.zhang on 2016/12/14.
// Copyright © 2016年 Peter. All rights reserved.
//
#import <Foundation/Foundation.h>
@interface NSMutableArray (Security)
@end
//
// NSMutableArray+Security.m
// Swizzling
//
// Created by peter.zhang on 2016/12/14.
// Copyright © 2016年 Peter. All rights reserved.
//
#import "NSMutableArray+Security.h"
#import "NSObject+Swizzling.h"
@implementation NSMutableArray (Security)
+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
[objc_getClass("__NSArrayM") methodSwizzlingWithOriginalSelector:@selector(removeObject:) bySwizzledSelector:@selector(safeRemoveObject:) ];
[objc_getClass("__NSArrayM") methodSwizzlingWithOriginalSelector:@selector(addObject:) bySwizzledSelector:@selector(safeAddObject:)];
[objc_getClass("__NSArrayM") methodSwizzlingWithOriginalSelector:@selector(removeObjectAtIndex:) bySwizzledSelector:@selector(safeRemoveObjectAtIndex:)];
[objc_getClass("__NSArrayM") methodSwizzlingWithOriginalSelector:@selector(insertObject:atIndex:) bySwizzledSelector:@selector(safeInsertObject:atIndex:)];
[objc_getClass("__NSArrayM") methodSwizzlingWithOriginalSelector:@selector(objectAtIndex:) bySwizzledSelector:@selector(safeObjectAtIndex:)];
});
}
- (void)safeAddObject:(id)obj {
if (obj == nil) {
NSLog(@"%s can add nil object into NSMutableArray", __FUNCTION__);
} else {
[self safeAddObject:obj];
}
}
- (void)safeRemoveObject:(id)obj {
if (obj == nil) {
NSLog(@"%s call -removeObject:, but argument obj is nil", __FUNCTION__);
return;
}
[self safeRemoveObject:obj];
}
- (void)safeInsertObject:(id)anObject atIndex:(NSUInteger)index {
if (anObject == nil) {
NSLog(@"%s can't insert nil into NSMutableArray", __FUNCTION__);
} else if (index > self.count) {
NSLog(@"%s index is invalid", __FUNCTION__);
} else {
[self safeInsertObject:anObject atIndex:index];
}
}
- (id)safeObjectAtIndex:(NSUInteger)index {
if (self.count == 0) {
NSLog(@"%s can't get any object from an empty array", __FUNCTION__);
return nil;
}
if (index > self.count) {
NSLog(@"%s index out of bounds in array", __FUNCTION__);
return nil;
}
return [self safeObjectAtIndex:index];
}
- (void)safeRemoveObjectAtIndex:(NSUInteger)index {
if (self.count <= 0) {
NSLog(@"%s can't get any object from an empty array", __FUNCTION__);
return;
}
if (index >= self.count) {
NSLog(@"%s index out of bound", __FUNCTION__);
return;
}
[self safeRemoveObjectAtIndex:index];
}
@end
然后你在工程中用可变数组的增删改查都不会crash了。