iOS_Runtime是什么？原理？作用？怎么实现weak？使用

Runtime：

1、什么是runtime？

就是在程序运行的过程中，有一套C语言级别的API，它把代码从OC转换成C

 

2、原理：

OC是基于C，并添加了面向对象的特性，将很多静态语言在编译和链接时做的事放到了runtime运行时来处理

C：函数的调用在编译时就知道会调用哪个函数

OC：在编译的时候并不知道，只在正在运行时才会根据函数名称找到对应的函数

 

3、作用

获取属性、方法、成员变量、协议（包括私有的）

给分类动态添加属性、方法

字典转模型

拦截并替换方法

实现NSCoding的归档和反归档

 

4、Runtime是怎么实现weak？(我有在一次面试中遇到)

Runtime对注册的类会进行布局，对于weak对象会放入一个hash表中，用weak指向的`对象的内存地址`作为key。当对象的引用计数为0时会调用dealloc方法，此时会在weak表中搜索，将所有weak对象置为nil。

Key：对象内存地址 — value：n个weak对象

 

5、使用

• 替换ViewController生命周期方法
• 解决获取索引、添加、删除元素越界crash问题
• 防止按钮重复暴力点击
• 全局更换控件初始效果
• App热修复
• App异常加载占位图通用类封装
• 全局修改导航栏返回按钮 (去掉title)

以下是使用内容：

• `NSObject`的`Category`里实现方法替换，方便需要的类直接调用：

// NSObject+Swizzling.h
+ (void)methodSwizzlingWithOriginalSelector:(SEL)originalSelector bySwizzledSelector:(SEL)swizzledSelector;


// NSObject+Swizzling.m
+ (void)methodSwizzlingWithOriginalSelector:(SEL)originalSelector bySwizzledSelector:(SEL)swizzledSelector {
  Class class = [self class];
  // 原有方法
  Method originalMethod = class_getInstanceMethod(class, originalSelector);
  // 新方法
  Method swizzledMetod = class_getInstanceMethod(class, swizzledSelector);
  // 尝试添加`origin`方法
  BOOL didAddMethod = class_addMethod(class,
                                      originalSelector,
                                      method_getImplementation(swizzledMetod),
                                      method_getTypeEncoding(swizzledMetod));
  if (didAddMethod) { // 之前没有实现`origin`方法
    class_replaceMethod(class,
                        swizzledSelector,
                        method_getImplementation(originalMethod),
                        method_getTypeEncoding(originalMethod));
  } else {
    method_exchangeImplementations(originalMethod, swizzledMetod);
  }
}

• `UINavigationController`的`Category`里替换相应的方法，并添加需要的处理

// UIViewController+Swizzling.m
+ (void)load {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    [self methodSwizzlingWithOriginalSelector:@selector(viewWillDisappear:) bySwizzledSelector:@selector(mo_viewWillDisappear:)];
  });
}

- (void)mo_viewWillDisappear:(BOOL)animated {
  [self mo_viewWillDisappear:animated];
  // 添加埋点等
}

• `NSMutableArray`的`Category`里拦截增删改方法，进行判空处理 （其他的NSArray、NSDictionary、NSMutableArray、NSMutableDictionary等也可以做相应的处理，防止越界crash）

•  类      真身

   NSArray __NSArrayI

   NSMutableArray __NSArrayM

   NSDictionary __NSDictionaryI

   NSMutableDictionary __NSDictionaryM

// NSMutableArray+Swizzling.m
+ (void)load {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    // objc_getClass("__NSArrayM") 没有用 [self class] 处理，是因为运行时用了`类簇` 运行时正在的类是前者
    [objc_getClass("__NSArrayM") methodSwizzlingWithOriginalSelector:@selector(addObject:) bySwizzledSelector:@selector(safeAddObject:)];
    [objc_getClass("__NSArrayM") methodSwizzlingWithOriginalSelector:@selector(insertObject:atIndex:) bySwizzledSelector:@selector(safeInsertObject:atIndex:)];
    [objc_getClass("__NSArrayM") methodSwizzlingWithOriginalSelector:@selector(objectAtIndex:) bySwizzledSelector:@selector(safeObjectAtIndex:)];
    [objc_getClass("__NSArrayM") methodSwizzlingWithOriginalSelector:@selector(removeObjectAtIndex:) bySwizzledSelector:@selector(safeRemoveObjectAtIndex:)];
    [objc_getClass("__NSArrayM") methodSwizzlingWithOriginalSelector:@selector(removeObject:) bySwizzledSelector:@selector(safeRemoveObject:) ];
  });
}
- (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];
}

• UIButton的Category中替换 sendAction:(SEL)action to:(id)target forEvent:(UIEvent *)event  改用 afterDelay 调用，并添加标志位，防止暴力点击，具体代码如下：

// UIButton+Swizzling.m
+ (void)load {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    [self methodSwizzlingWithOriginalSelector:@selector(sendAction:to:forEvent:) bySwizzledSelector:@selector(mo_SendAction:to:forEvent:)];
  });
}

// 当按钮点击事件 sendAction 时将会执行 mo_SendAction
- (void)mo_SendAction:(SEL)action to:(id)target forEvent:(UIEvent *)event {
  if (self.isIgnore) { // 不需要被hook
    [self mo_SendAction:action to:target forEvent:event];
    return;
  }
  if ([NSStringFromClass(self.class) isEqualToString:@"UIButton"]) {
    if (self.isIgnoreEvent) {
      return;
    } else {
      self.timeInterval = self.timeInterval == 0 ? defaultInterval : self.timeInterval; // 是否自定义，否则用默认值
      [self performSelector:@selector(resetState) withObject:nil afterDelay:defaultInterval];
    }
  }
  // 此处 methodA 和 methodB方法IMP互换了，实际上执行 sendAction；所以不会死循环
  self.isIgnoreEvent = YES;
  [self mo_SendAction:action to:target forEvent:event]; // 执行系统的原有方法
}
// 还有些属性，没添加，详情见Demo

• `UILabel`的`Category`中拦截初始化方法，并设置font

// UILabel+Swizzling.m
+ (void)load {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    [self methodSwizzlingWithOriginalSelector:@selector(init) bySwizzledSelector:@selector(mo_Init)];
    [self methodSwizzlingWithOriginalSelector:@selector(initWithFrame:) bySwizzledSelector:@selector(mo_InitWithFrame:)];
    [self methodSwizzlingWithOriginalSelector:@selector(awakeFromNib) bySwizzledSelector:@selector(mo_AwakeFromNib)];
  });
}
- (instancetype)mo_Init {
  id __self = [self mo_Init];
  UIFont *font = [UIFont fontWithName:@"Zapfino" size:self.font.pointSize];
  if (font) {
    self.font = font;
  }
  return __self;
}
- (instancetype)mo_InitWithFrame:(CGRect)rect {
  id __self = [self mo_InitWithFrame:rect];
  UIFont *font = [UIFont fontWithName:@"Zapfino" size:self.font.pointSize];
  if (font) {
    self.font = font;
  }
  return __self;
}
- (void)mo_AwakeFromNib {
  [self mo_AwakeFromNib];
  UIFont *font = [UIFont fontWithName:@"Zapfino" size:self.font.pointSize];
  if (font) {
    self.font = font;
  }
}

使用Demo地址

参考1 参考2