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NSInvocation的巧妙用法

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作者 裘俊云 9 分钟阅读

iOS消息转发机制中介绍，Objective-C 是动态语言，所有的消息都是在 Runtime 进行派发的。

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OBJC_EXPORT void objc_msgSend(void /* id self, SEL op, ... */ ) OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0);

objc_msgSend是 C 函数，苹果不提倡我们直接使用该函数来向对象消息。

performSelector

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- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;

  • 在 ARC 场景下 performSelector 可能会造成内存泄漏

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    //#pragma clang diagnostic push
//#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
    if (data.cellCustomSEL) {
      	// warning: PerformSelector may cause a leak because its selector is unknown
        [self performSelector:data.cellCustomSEL withObject:cell withObject:data];
    }
//#pragma clang diagnostic pop

  • performSelector 至多接收 2 个参数，如果参数多余 2 个，我们就无法使用 performSelector 来向对象发送消息了。

  • performSelector 限制参数类型为 id，以标量数据(int double NSInteger 等)为参数的方法使用 performSelector 调用会出现各种各样诡异的问题

NSInvocation

场景一 注册某一类方法或是模块为多方提供统一调用

场景介绍

见Demo

多个客户端需要调用同一个服务的多个能力，每个客户端都要跟服务器制定一套自己的沟通协议，但是调用是服务的能力其实都是同一个，如下图

screenshot-20210825-150727

导致代码复用性极差，改版后如下：

screenshot-20210825-151338

其中的关键技术用的就是NSInvocation来实现的。

首先我们要整合多个客户端，定义用同一套协议，不能各自为营。

其次就是对于协议能够调用到正确的Native模块中。

实战

定义层

假设我们定义协议如下：

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{
  "handlerName": "bridge_share",
  "data": {
    "type": "wechat"
  }
}

桥接类我们这样设计：

  1. 公共父类BridgeModule，所有的需要与客户机沟通的能力或是模块都需要继承至BridgeModule来实现。

  2. 定义标准规范方法

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    - (void)bridge_share:(id)data nativeBidgeResponseCallback:(Callback)responseCallback
使用层

先总结

  1. 中继器注册所有桥接类和类中的方法
  2. 客户机开始调用，我们能够拿到handlerName=bridge_share
  3. 通过handlerName我们就能根据第一步在中继器中找到我们需要执行NSInvocation的类和方法了

  1. 中继器注册所有桥接类和类中的方法

    使用runtime方法获取所有继承至BridgeModule的类和方法

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    gMessageModuleClassMap = [NSMutableDictionary new];
   
int count = objc_getClassList(NULL,0);
NSMutableArray * moduleClasses = [NSMutableArray new];
Class *classes = (Class *)malloc(sizeof(Class) * count);
objc_getClassList(classes, count);
   
for (int i = 0; i < count; i++) {
    if ([BridageModule class] == class_getSuperclass(classes[i])) {
        [moduleClasses addObject:classes[i]];
    }
}
free(classes);
   
for (Class moduleClass in moduleClasses) {
    unsigned int count;
       
    Method *methodList = class_copyMethodList(moduleClass, &count);
    for (unsigned int i = 0; i < count; i++) {
        Method method = methodList[i];
        NSString *methodName = NSStringFromSelector(method_getName(method));
        if ([methodName hasSuffix:@":nativeBidgeResponseCallback:"]) {
            NSArray *components = [methodName componentsSeparatedByString:@":"];
            NSString *messageName = components.firstObject;
            if (messageName.length > 0) {
                [gMessageModuleClassMap setObject:moduleClass forKey:messageName];
            }
        }
    }
    free(methodList);
}

    这样之后gMessageModuleClassMap中就保存着我们定义的协议handlerName和要执行他的类

  2. 客户机开始调用，我们能够拿到handlerName=bridge_share

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    Class moduleClass = gMessageModuleClassMap[handlerName];
BridgeModule *nativeModule = [[moduleClass alloc] init];
NSString *methodString = [NSString stringWithFormat:@"%@%@", handlerName, @":nativeBidgeResponseCallback:"];
SEL selector = NSSelectorFromString(methodString);

  3. 通过handlerName我们就能根据第一步在中继器中找到我们需要执行NSInvocation的类和方法了

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    NSMethodSignature *signature = [nativeModule methodSignatureForSelector:selector];
if (!signature) {
    return NO;
}
   
id requestData = message.data;
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
invocation.target = nativeModule;
[invocation setSelector:selector];
[invocation setArgument:&requestData atIndex:2];
   
if (message.callback) {
    Block_copy((__bridge void *)message.callback);
    [invocation setArgument:&message.callback atIndex:3];
}
[invocation invoke];

场景二 block

有关block使用NSInvocation，问题是如何获得block的MethodSignature，block并没有selector，我们可以通过获取block的编码的Signature来使用-[NSMethodSignature signatureWithObjCTypes:]

其中Clang官方文档和stackoverflow都有获取block的函数编码的讲解。

iOS的lua热修复详解的热修复中，block的方法替换采用的是CTObjectiveCRuntimeAdditions的解法

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struct CTBlockLiteral {
    void *isa; // initialized to &_NSConcreteStackBlock or &_NSConcreteGlobalBlock
    int flags;
    int reserved;
    void (*invoke)(void *, ...);
    struct block_descriptor {
        unsigned long int reserved;	// NULL
        unsigned long int size;         // sizeof(struct Block_literal_1)
        // optional helper functions
        void (*copy_helper)(void *dst, void *src);     // IFF (1<<25)
        void (*dispose_helper)(void *src);             // IFF (1<<25)
        // required ABI.2010.3.16
        const char *signature;                         // IFF (1<<30)
    } *descriptor;
    // imported variables
};

enum {
    CTBlockDescriptionFlagsHasCopyDispose = (1 << 25),
    CTBlockDescriptionFlagsHasCtor = (1 << 26), // helpers have C++ code
    CTBlockDescriptionFlagsIsGlobal = (1 << 28),
    CTBlockDescriptionFlagsHasStret = (1 << 29), // IFF BLOCK_HAS_SIGNATURE
    CTBlockDescriptionFlagsHasSignature = (1 << 30)
};
typedef int CTBlockDescriptionFlags;



@interface SpaBlockDescription : NSObject

@property (nonatomic, readonly) CTBlockDescriptionFlags flags;
@property (nonatomic, readonly) NSMethodSignature *blockSignature;
@property (nonatomic, readonly) unsigned long int size;
@property (nonatomic, readonly) id block;

- (id)initWithBlock:(id)block;
@end
  
  
@implementation SpaBlockDescription

- (id)initWithBlock:(id)block
{
    if (self = [super init]) {
        _block = block;
        
        struct CTBlockLiteral *blockRef = (__bridge struct CTBlockLiteral *)block;
        _flags = blockRef->flags;
        _size = blockRef->descriptor->size;
        
        if (_flags & CTBlockDescriptionFlagsHasSignature) {
            void *signatureLocation = blockRef->descriptor;
            signatureLocation += sizeof(unsigned long int);
            signatureLocation += sizeof(unsigned long int);
            
            if (_flags & CTBlockDescriptionFlagsHasCopyDispose) {
                signatureLocation += sizeof(void(*)(void *dst, void *src));
                signatureLocation += sizeof(void (*)(void *src));
            }
            
            const char *signature = (*(const char **)signatureLocation);
            _blockSignature = [NSMethodSignature signatureWithObjCTypes:signature];
        }
    }
    return self;
}

- (NSString *)description
{
    return [NSString stringWithFormat:@"%@: %@", [super description], _blockSignature.description];
}

@end

Demo工程中用的如下

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struct Block_literal_1 {
    void *isa; // initialized to &_NSConcreteStackBlock or &_NSConcreteGlobalBlock
    int flags;
    int reserved;
    void (*invoke)(void *, ...);
    struct Block_descriptor_1 {
        unsigned long int reserved;     // NULL
        unsigned long int size;         // sizeof(struct Block_literal_1)
        // optional helper functions
        // void (*copy_helper)(void *dst, void *src);     // IFF (1<<25)
        // void (*dispose_helper)(void *src);             // IFF (1<<25)
        // required ABI.2010.3.16
        // const char *signature;                         // IFF (1<<30)
        void* rest[1];
    } *descriptor;
    // imported variables
};

enum {
    BLOCK_HAS_COPY_DISPOSE =  (1 << 25),
    BLOCK_HAS_CTOR =          (1 << 26), // helpers have C++ code
    BLOCK_IS_GLOBAL =         (1 << 28),
    BLOCK_HAS_STRET =         (1 << 29), // IFF BLOCK_HAS_SIGNATURE
    BLOCK_HAS_SIGNATURE =     (1 << 30),
};

static const char *__BlockSignature__(id blockObj)
{
    struct Block_literal_1 *block = (__bridge void *)blockObj;
    struct Block_descriptor_1 *descriptor = block->descriptor;
    assert(block->flags & BLOCK_HAS_SIGNATURE);
    int offset = 0;
    if(block->flags & BLOCK_HAS_COPY_DISPOSE)
        offset += 2;
    return (char*)(descriptor->rest[offset]);
}

// NSInvocation* invocation = [NSInvocation invocationWithMethodSignature:[NSMethodSignature signatureWithObjCTypes:__BlockSignature__(block)]];
// invocation.target = block;

场景三 热修复中的消息转发

以这个iOS的lua热修复详解为例子，先简单介绍先热修复中方法修复原理：

热修复核心逻辑，方法替换，将需要热修的方法实现指向_objc_msgForward，原实现指向我们新定义的sel

图片

然后hookforwardInvocation，使他指向我们实现的方法。

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static void replaceMethod(Class klass, SEL sel)
{
    if (klass == nil || sel == nil) {
        return ;
    }
    SEL originSelector = spa_originForSelector(sel);
    Method targetMethod = class_getInstanceMethod(klass, sel);
    if (targetMethod) {
        const char *typeEncoding = method_getTypeEncoding(targetMethod);
        class_addMethod(klass, originSelector, method_getImplementation(targetMethod), typeEncoding);
        spa_swizzleForwardInvocation(klass);
        // We use forwardInvocation to hook in.
        class_replaceMethod(klass, sel, spa_getMsgForwardIMP(klass, sel), typeEncoding);
        
        [[SpaClass replacedClassMethods] addObject:@{@"class":NSStringFromClass(klass), @"sel":NSStringFromSelector(sel)}];
    }
}

static void spa_swizzleForwardInvocation(Class klass)
{
    NSCParameterAssert(klass);
    // get origin forwardInvocation impl, include superClass impl，not NSObject impl, and class method to kClass
    SEL originForwardSelector = NSSelectorFromString(SPA_ORIGIN_FORWARD_INVOCATION_SELECTOR_NAME);
    if (![klass instancesRespondToSelector:originForwardSelector]) {
        Method originalMethod = class_getInstanceMethod(klass, @selector(forwardInvocation:));
        IMP originalImplementation = method_getImplementation(originalMethod);
        class_addMethod(klass, NSSelectorFromString(SPA_ORIGIN_FORWARD_INVOCATION_SELECTOR_NAME), originalImplementation, "v@:@");
        
    }
    // If there is no method, replace will act like class_addMethod.
    class_replaceMethod(klass, @selector(forwardInvocation:), (IMP)__SPA_ARE_BEING_CALLED__, "v@:@");
}

static void __SPA_ARE_BEING_CALLED__(__unsafe_unretained NSObject *self, SEL selector, NSInvocation *invocation)

参考文档

NSInvocation 调用Block

Block-ABI-Apple

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