Runtime 函数 Swizzling 改变OC方法的调度顺序

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。