Objc Runtime在项目中该怎么用

从以下四个方面讲述Objc Runtime在项目中的使用场景，使用的例子来自于github上的开源项目FDFullscreenPopGesture、GVUserDefaults 以及系统中KVO的底层实现例子

Method Swizzling
动态方法添加
isa Swizzling
消息转发

Method Swizzling

Method Swizzling简单的讲就是方法替换，是一种hook技术，一个典型的Method Swizzling例子如下，注释部分说明了为什么这么做。

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];
        // When swizzling a class method, use the following:
        // Class class = object_getClass((id)self);
        SEL originalSelector = @selector(viewWillAppear:);
        SEL swizzledSelector = @selector(xxx_viewWillAppear:);
        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
        // 在进行Swizzling的时候，我们需要用class_addMethod先进行判断一下原有类中是否有要替换的方法的实现。
        BOOL didAddMethod = class_addMethod(class,
                                            originalSelector,
                                            method_getImplementation(swizzledMethod),
                                            method_getTypeEncoding(swizzledMethod));
        if (didAddMethod) {
            // 如果class_addMethod返回YES，说明当前类中没有要替换方法的实现，我们需要在父类中去寻找。这个时候就需要用到method_getImplementation去获取class_getInstanceMethod里面的方法实现。然后再进行class_replaceMethod来实现Swizzling。
            class_replaceMethod(class,
                                swizzledSelector,
                                method_getImplementation(originalMethod),
                                method_getTypeEncoding(originalMethod));
        } else {
            // 如果class_addMethod返回NO，说明当前类中有要替换方法的实现，所以可以直接进行替换，调用method_exchangeImplementations即可实现Swizzling。
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

#pragma mark - Method Swizzling
- (void)xxx_viewWillAppear:(BOOL)animated {
    // 由于我们进行了Swizzling，所以其实在原来的- (void)viewWillAppear:(BOOL)animated方法中，调用的是- (void)xxx_viewWillAppear:(BOOL)animated方法的实现。所以不会造成死循环。相反的，如果这里把[self xxx_viewWillAppear:animated];改成[self viewWillAppear:animated];就会造成死循环。因为外面调用[self viewWillAppear:animated];的时候，会交换方法走到[self xxx_viewWillAppear:animated];这个方法实现中来，然后这里又去调用[self viewWillAppear:animated]，就会造成死循环了。
    [self xxx_viewWillAppear:animated];
    NSLog(@"viewWillAppear: %@", self);
}

FDFullscreenPopGesture 这个库使用的就是Method Swizzling技术实现的全屏手势返回的效果

UINavigationController (FDFullscreenPopGesture)分类的load方法中替换了系统的pushViewController:animated:方法

+ (void)load
{
    // Inject "-pushViewController:animated:"
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];
        
        SEL originalSelector = @selector(pushViewController:animated:);
        SEL swizzledSelector = @selector(fd_pushViewController:animated:);
        
        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
        BOOL success = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));
        if (success) {
            class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        } else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

在替换的方法中禁用了系统的边缘返回手势，添加了自定义的手势来处理

- (void)fd_pushViewController:(UIViewController *)viewController animated:(BOOL)animated
{
    if (![self.interactivePopGestureRecognizer.view.gestureRecognizers containsObject:self.fd_fullscreenPopGestureRecognizer]) {
        
        // Add our own gesture recognizer to where the onboard screen edge pan gesture recognizer is attached to.
        [self.interactivePopGestureRecognizer.view addGestureRecognizer:self.fd_fullscreenPopGestureRecognizer];
        
        // Forward the gesture events to the private handler of the onboard gesture recognizer.
        NSArray *internalTargets = [self.interactivePopGestureRecognizer valueForKey:@"targets"];
        id internalTarget = [internalTargets.firstObject valueForKey:@"target"];
        SEL internalAction = NSSelectorFromString(@"handleNavigationTransition:");
        self.fd_fullscreenPopGestureRecognizer.delegate = self.fd_popGestureRecognizerDelegate;
        [self.fd_fullscreenPopGestureRecognizer addTarget:internalTarget action:internalAction];
        
        // Disable the onboard gesture recognizer.
        self.interactivePopGestureRecognizer.enabled = NO;
    }
    
    // Handle perferred navigation bar appearance.
    [self fd_setupViewControllerBasedNavigationBarAppearanceIfNeeded:viewController];
    
    // Forward to primary implementation.
    if (![self.viewControllers containsObject:viewController]) {
        [self fd_pushViewController:viewController animated:animated];
    }
}

然后在自定义的手势的回调方法gestureRecognizerShouldBegin中处理手势的返回

- (BOOL)gestureRecognizerShouldBegin:(UIPanGestureRecognizer *)gestureRecognizer
{
    // Ignore when no view controller is pushed into the navigation stack.
    if (self.navigationController.viewControllers.count <= 1) {
        return NO;
    }
    
    // Ignore when the active view controller doesn't allow interactive pop.
    UIViewController *topViewController = self.navigationController.viewControllers.lastObject;
    if (topViewController.fd_interactivePopDisabled) {
        return NO;
    }
    
    // Ignore when the beginning location is beyond max allowed initial distance to left edge.
    CGPoint beginningLocation = [gestureRecognizer locationInView:gestureRecognizer.view];
    CGFloat maxAllowedInitialDistance = topViewController.fd_interactivePopMaxAllowedInitialDistanceToLeftEdge;
    if (maxAllowedInitialDistance > 0 && beginningLocation.x > maxAllowedInitialDistance) {
        return NO;
    }
    
    // Ignore pan gesture when the navigation controller is currently in transition.
    if ([[self.navigationController valueForKey:@"_isTransitioning"] boolValue]) {
        return NO;
    }
    
    // Prevent calling the handler when the gesture begins in an opposite direction.
    CGPoint translation = [gestureRecognizer translationInView:gestureRecognizer.view];
    BOOL isLeftToRight = [UIApplication sharedApplication].userInterfaceLayoutDirection == UIUserInterfaceLayoutDirectionLeftToRight;
    CGFloat multiplier = isLeftToRight ? 1 : - 1;
    if ((translation.x * multiplier) <= 0) {
        return NO;
    }
    
    return YES;
}

动态方法添加

GVUserDefaults 是一个属性和NSUserDefaults之间实现自动写入和读取的开源库，该库（当前最新版本 1.0.2）使用到的技术就是动态方法添加，项目中使用到的主要技术点列举如下：

class_copyPropertyList获取类的property列表
property_getName获取property名称
property_getAttributes获取property的属性，可以参考Property Attribute Description Examples
sel_registerName注册SEL
class_addMethod添加方法

主要看generateAccessorMethods方法的实现，该方法自动生成属性对应的Getter和Setter方法，关键的地方有添加了注释

- (void)generateAccessorMethods {
    unsigned int count = 0;
    // 获取类的属性列表
    objc_property_t *properties = class_copyPropertyList([self class], &count);

    self.mapping = [NSMutableDictionary dictionary];

    for (int i = 0; i < count; ++i) {
        objc_property_t property = properties[i];
        // 获取property名称
        const char *name = property_getName(property);
        // 获取property的属性
        const char *attributes = property_getAttributes(property);

        char *getter = strstr(attributes, ",G");
        if (getter) {
            // getter修饰的属性：@property (nonatomic, getter=zytCustomerGetter) float customerGetter; -> "Tf,N,GzytCustomerGetter"
            getter = strdup(getter + 2);
            getter = strsep(&getter, ",");
        } else {
            getter = strdup(name);
        }
        SEL getterSel = sel_registerName(getter);
        free(getter);

        char *setter = strstr(attributes, ",S");
        if (setter) {
            // setter 修饰的属性：@property (nonatomic, setter=zytCustomerSetter:) float customerSetter; -> Tf,N,SzytCustomerSetter:
            setter = strdup(setter + 2);
            setter = strsep(&setter, ",");
        } else {
            asprintf(&setter, "set%c%s:", toupper(name[0]), name + 1);
        }
        // 注册SEL
        SEL setterSel = sel_registerName(setter);
        free(setter);

	// 同一个属性的`Getter`或者`Setter`方法在`self.mapping`对应的值是一样的，
        NSString *key = [self defaultsKeyForPropertyNamed:name];
        [self.mapping setValue:key forKey:NSStringFromSelector(getterSel)];
        [self.mapping setValue:key forKey:NSStringFromSelector(setterSel)];

        IMP getterImp = NULL;
        IMP setterImp = NULL;
        char type = attributes[1];
        switch (type) {
            case Short:
            case Long:
            case LongLong:
            case UnsignedChar:
            case UnsignedShort:
            case UnsignedInt:
            case UnsignedLong:
            case UnsignedLongLong:
                getterImp = (IMP)longLongGetter;
                setterImp = (IMP)longLongSetter;
                break;

            case Bool:
            case Char:
                getterImp = (IMP)boolGetter;
                setterImp = (IMP)boolSetter;
                break;

            case Int:
                getterImp = (IMP)integerGetter;
                setterImp = (IMP)integerSetter;
                break;

            case Float:
                getterImp = (IMP)floatGetter;
                setterImp = (IMP)floatSetter;
                break;

            case Double:
                getterImp = (IMP)doubleGetter;
                setterImp = (IMP)doubleSetter;
                break;

            case Object:
                getterImp = (IMP)objectGetter;
                setterImp = (IMP)objectSetter;
                break;

            default:
                free(properties);
                [NSException raise:NSInternalInconsistencyException format:@"Unsupported type of property \"%s\" in class %@", name, self];
                break;
        }

        char types[5];

        snprintf(types, 4, "%c@:", type);
        // 添加方法
        class_addMethod([self class], getterSel, getterImp, types);
        
        snprintf(types, 5, "v@:%c", type);
        // 添加方法
        class_addMethod([self class], setterSel, setterImp, types);
    }

    free(properties);
}

其中property_getAttributes(property)获取到的property属性字符串的第二位的类型信息可以参考Apple官方文档

Type Encodings

对应的程序中定义了TypeEncodings枚举

enum TypeEncodings {
    Char                = 'c',
    Bool                = 'B',
    Short               = 's',
    Int                 = 'i',
    Long                = 'l',
    LongLong            = 'q',
    UnsignedChar        = 'C',
    UnsignedShort       = 'S',
    UnsignedInt         = 'I',
    UnsignedLong        = 'L',
    UnsignedLongLong    = 'Q',
    Float               = 'f',
    Double              = 'd',
    Object              = '@'
};

比如对象类型的属性，经过如下的处理

getterImp = (IMP)objectGetter;
setterImp = (IMP)objectSetter;

//...

class_addMethod([self class], getterSel, getterImp, types);
class_addMethod([self class], setterSel, setterImp, types);

使用属性Getter或者Setter最终会调用以下的方法

static id objectGetter(GVUserDefaults *self, SEL _cmd) {
    NSString *key = [self defaultsKeyForSelector:_cmd];
    return [self.userDefaults objectForKey:key];
}

static void objectSetter(GVUserDefaults *self, SEL _cmd, id object) {
    NSString *key = [self defaultsKeyForSelector:_cmd];
    if (object) {
        [self.userDefaults setObject:object forKey:key];
    } else {
        [self.userDefaults removeObjectForKey:key];
    }
}

这里用到的defaultsKeyForSelector方法定义如下，把属性Getter或者Setter方法映射为对应的属性的保存的Key的字符串，同一个属性的Getter或者Setter方法在self.mapping对应的值是一样的，详细的保存映射信息到self.mapping中可以查看generateAccessorMethods方法。

- (NSString *)defaultsKeyForSelector:(SEL)selector {
    return [self.mapping objectForKey:NSStringFromSelector(selector)];
}

isa Swizzling

系统的KVO的实现是基于isa swizzling实现的，创建一个NSObject的分类模拟KVO的实现，主要用到技术点

objc_allocateClassPair添加一个类
objc_registerClassPair注册添加的类
object_setClass修改当前类的class，也就是isa指针
class_addMethod类动态添加方法
objc_msgSend使用底层C的方法执行方法调用
objc_setAssociatedObject、objc_getAssociatedObject设置和获取关联对象

主要的思路如下：

- (void)ytt_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context 方法是类似系统添加KVO监听的方法，该方法中
- 保存 observer 和 keyPath 参数；动态的创建和注册KVO类；
- 使用object_setClass修改当前对象的isa指针；
- class_addMethod动态添加一个监听的keyPath属性对应的Setter方法
keyPath属性对应的Setter方法的实现：
- 使用object_setClass修改对象的isa指针为原始的类，后面使用Setter方法设置新值调用的objc_msgSend方法才能正确执行；
- 使用objc_getAssociatedObject获取绑定的关联对象中keyPath，还原出Getter方法和Setter方法，使用Getter方法获取旧的值，使用Setter方法设置新值；
- 使用objc_getAssociatedObject获取绑定的关联对象中observer，向 observer 对象发送属性变换的消息；
- 最后重置对象的isa指针为动态创建的KVO类，下一次的流程才能正常执行

@implementation NSObject (YTT_KVO)

- (void)ytt_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context {
    
    // 保存keypath
    objc_setAssociatedObject(self, "keyPath", keyPath, OBJC_ASSOCIATION_COPY_NONATOMIC);
    objc_setAssociatedObject(self, "observer", observer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);

    // 获取当前类
    Class selfClass = self.class;
    
    // 动态创建KVO类
    const char * className = NSStringFromClass(selfClass).UTF8String;
    char kvoClassName[1000];
    sprintf(kvoClassName, "%s%s", "YTT_KVO_", className);
    Class kvoClass = objc_allocateClassPair(selfClass, kvoClassName, 0);
    if (!kvoClass) {
        // Nil if the class could not be created (for example, the desired name is already in use).
        kvoClass = NSClassFromString([NSString stringWithUTF8String:kvoClassName]);
    }
    objc_registerClassPair(kvoClass);
    
    // 修改当前类指向为动态创建的KVO子类，kvoClass继承自selfClass
    object_setClass(self, kvoClass);
    
    // 动态添加一个方法:setXxx()
    SEL sel = NSSelectorFromString([NSString stringWithFormat:@"set%@:", keyPath.capitalizedString]);
    class_addMethod(kvoClass, sel, (IMP)setValue, NULL);
}

void setValue(id self, SEL _cmd, id value) {
    
    // 保存当前的Class，重置Class使用
    Class selfClass = [self class];
    // 设置Class为原始Class
    object_setClass(self, [self superclass]);
    // 获取keyPath
    NSString* keyPath = objc_getAssociatedObject(self, "keyPath");

    // KVO 回调参数
    NSMutableDictionary* change = [NSMutableDictionary dictionary];
    change[NSKeyValueChangeNewKey] = value;
    
    // 获取旧的值
    SEL getSel = NSSelectorFromString([NSString stringWithFormat:@"%@", keyPath]);
    if ([self respondsToSelector:getSel]) {
        id ret = ((id(*)(id, SEL, id))objc_msgSend)(self, getSel, value);
        if (ret) {
            change[NSKeyValueChangeOldKey] = ret;
        }
    }
    
    // 给原始类设置数据
    SEL setSel = NSSelectorFromString([NSString stringWithFormat:@"set%@:", keyPath.capitalizedString]);
    if ([self respondsToSelector:setSel]) {
        ((void(*)(id, SEL, id))objc_msgSend)(self, setSel, value);
    }
    
    // 发送通知
    id observer = objc_getAssociatedObject(self, "observer");
    SEL observerSel = @selector(ytt_observeValueForKeyPath:ofObject:change:context:);
    if ([observer respondsToSelector:observerSel]) {
        ((void(*) (id, SEL, NSString*, id, id ,id))(void *)objc_msgSend)(observer, observerSel, keyPath, self, change, nil);
    }
    
    // 重置class指针，这样再次调用对象方法会走到这里面
    object_setClass(self, selfClass);
}

@end

消息转发

消息转发的步骤有以下四个：

动态方法解析：resolveClassMethod:(SEL)sel和resolveInstanceMethod:(SEL)sel处理类方法和实例方法，可以在该方法中使用class_addMethod动态添加方法处理，返回YES表示该步骤可处理，否则表示不可处理，继续执行下一步
备援接收者：forwardingTargetForSelector:(SEL)aSelector方法返回一个可以处理该消息的对象完成消息的转发处理
完整的消息转发：forwardInvocation:(NSInvocation *)anInvocation，如果以上两个步骤都不能处理，最终会执行到这一步，anInvocation对象包含了消息详细信息，包括id target、SEL selector、参数和返回值信息等等。该方法需要和methodSignatureForSelector:(SEL)aSelector方法一起使用，可以使用NSInvocation的实例方法invokeWithTarget:(id)target完成消息的转发
如果以上的步骤都不能处理，最终会由NSObject的方法doesNotRecognizeSelector:(SEL)aSelector，抛出一个unrecognized selector sent to instance xxx的异常

GVUserDefaults 是一个属性和NSUserDefaults之间实现自动写入和读取的开源库，该库（0.4.1之前的旧版本）使用到的技术就是消息转发

GVUserDefaults库中使用的是消息转发中的动态方法解析resolveInstanceMethod方法，Setter方法的消息转发给accessorSetterC函数处理，Getter方法的消息转发给accessorGetterC函数处理，accessorSetter和accessorGetter最终还是通过NSUserDefaults实现属性对应的Key的设置和读取，关键的代码如下：

+ (BOOL)resolveInstanceMethod:(SEL)aSEL {
    NSString *method = NSStringFromSelector(aSEL);

    if ([method isEqualToString:@"transformKey:"] || [method isEqualToString:@"setupDefaults"]) {
        // Prevent endless loop for optional (and missing) category methods
        return [super resolveInstanceMethod:aSEL];
    }

    if ([method hasPrefix:@"set"]) {
        class_addMethod([self class], aSEL, (IMP) accessorSetter, "v@:@");
        return YES;
    } else {
        class_addMethod([self class], aSEL, (IMP) accessorGetter, "@@:");
        return YES;
    }
}

- (NSString *)_transformKey:(NSString *)key {
    if ([self respondsToSelector:@selector(transformKey:)]) {
        return [self performSelector:@selector(transformKey:) withObject:key];
    }

    return key;
}

id accessorGetter(GVUserDefaults *self, SEL _cmd) {
    NSString *key = NSStringFromSelector(_cmd);
    key = [self _transformKey:key];
    return [[NSUserDefaults standardUserDefaults] objectForKey:key];
}

void accessorSetter(GVUserDefaults *self, SEL _cmd, id newValue) {
    NSString *method = NSStringFromSelector(_cmd);
    NSString *key = [[method stringByReplacingCharactersInRange:NSMakeRange(0, 3) withString:@""] stringByReplacingOccurrencesOfString:@":" withString:@""];
    key = [key stringByReplacingCharactersInRange:NSMakeRange(0,1) withString:[[key substringToIndex:1] lowercaseString]];
    key = [self _transformKey:key];

    // Set value of the key anID to newValue
    [[NSUserDefaults standardUserDefaults] setObject:newValue forKey:key];
}