Objective-C - iVar 作用域方法变量？

Question

之前在 Objective-C 里胡思乱想，遇到了一个很常见的情况:

我有一个类，它不是单例，它需要一个在方法调用之间共享的变量，比如 static，但每个实例都需要它自己的变量。然而，这个变量只需要在一个特定的方法中使用，我们称之为-foo。

我想做的是拥有一个宏，我们称它为 ivar，它可以让我执行以下操作:

@implementation MyClass 

-(foo)
{
    ivar int someVal = 10; // default value, ivar scoped variable.
}

-(bar)
{
    someVal = 5; // error, outside of `foo`'s scope.
}

@end

变量的定义方式对我来说并不重要(无论是像 OBJC_IVAR(Type, Name, Default) 还是 ivar someType someName = value 这样的宏)，只要因为它满足以下要求:

• 具有线程安全
• 可以在另一个方法中使用同名变量(但值不同)
• 无类型(不管变量是什么类型)
• 默认值支持
• 变量可以在一行中声明(我不应该为了在我的代码中放置一个变量而写 15 行代码)

我目前正在自己​​开发 Objective-C++ 实现，我只是想知道是否还有其他人对如何执行此操作有任何想法(或现有工具)。

显然，这不一定要用真正的 iVar 来完成。更有可能的是，这应该在运行时使用关联对象完成，这也为我们管理释放。

Answer 1

在花费了大量时间之后，我相信我已经有了一个完全可用的 Objective-C++ 解决方案。一些功能:

• 变量是唯一的。只要范围不同，它们的值就是独立的
• 每个实例都有自己的值
• 线程安全(由关联对象完成)

• 简单的变量声明:

  • 宏重载:只指定你需要的信息

  • 定义 OBJC_IVAR 的可能方法:

    OBJC_IVAR(); // creates a warning, does nothing
    OBJC_IVAR(Name); // creates an ivar named 'Name' of type 'id'
    OBJC_IVAR(Type, Name); // creates an ivar named 'Name' of type 'Type'
    OBJC_IVAR(Type, Name, Default); // creates an ivar named 'Name', of type 'Type', and a default value of 'Default' (which is only executed once);
    

• C++ 模板的完整类型支持(__weak、__strong、__autoreleasing、volatile 等. 都支持)

• 子类不与其父类(super class)共享变量(因此不会发生冲突，变量实际上仅限于它们的范围)。
• 可以毫无问题地用于单例
• 速度很快，查找一个变量大约需要 15-30 个 CPU 周期，一旦查找，设置它所花的时间与任何其他变量一样长。
• 大部分艰苦的工作都是由预处理器完成的，这允许更快的代码
• 只需拖放到现有的 Xcode 项目中，不依赖于自定义处理器

实现的一些小缺点:

• 对象必须有所有权说明符(C++ 引用的限制:Reference to non-const type 'id' with no explicit ownership)。通过向变量类型添加 __strong、__weak 或 __autoreleasing 即可轻松修复

• 实现很难阅读。因为它非常依赖 C++ 模板和 Objective-C 的协调工作，所以很难仅仅改变“一件事”并希望它起作用。我已对实现添加了大量评论，希望这能减轻一些负担。

• Method swizzling 主要会混淆这一点。这不是最大的问题，但如果您开始尝试方法混合，如果您得到意想不到的结果，请不要感到惊讶。

• 不能在 C++ 对象中使用。不幸的是，C++ 不支持运行时属性，就像 objective-c 那样，所以我们不能指望我们的变量最终会被清理。因此，在 C++ 对象中不能使用 OBJC_IVAR。不过，我有兴趣看到它的实现。

• #line 会把它搞得一团糟，所以不要使用它。

版本历史

• 1.0:初始版本
• 1.1:更新了 OBJC_IVAR_NAME 以仅依赖预处理器。因此，我们不能使用 __func__。

所以，事不宜迟，这是代码:

OBJC_IVAR.hpp

//
//  OBJC_IVAR.h
//  TestProj
//
//  Created by Richard Ross on 8/17/12.
//  Copyright (c) 2012 Ultimate Computer Services, Inc. All rights reserved.
//
#ifndef OBJC_IVAR_HPP
#define OBJC_IVAR_HPP

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

#import "NSValue+CppObject.h"

// Argument counting algorithm. Not too complex
#define __NARG(_1, _2, _3, _4, _5, VAL, ...) VAL
#define NARG(...) __NARG(__VA_ARGS__, 5, 4, 3, 2, 1, 0)

// Different implementations based on number of parameters passed in
#define __OBJC_IVAR(N, ...) _OBJC_IVAR_ ## N (__VA_ARGS__)
#define _OBJC_IVAR(N, ...) __OBJC_IVAR(N, __VA_ARGS__)

// Usage: OBJC_IVAR(Type (optional), Name (required), Default (optional))
#define OBJC_IVAR(...) _OBJC_IVAR(NARG(__VA_ARGS__), __VA_ARGS__)

// create a unique name. we use '__COUNTER__' here to support scoping on the same line, for compressed source code
#define __OBJC_IVAR_STRINGIFY_NAME(file, line, name, counter) @file ":" #line " " #name ":" #counter
#define _OBJC_IVAR_NAME(file, line, name, counter) __OBJC_IVAR_STRINGIFY_NAME(file, line, name, counter)
#define OBJC_IVAR_NAME(name) _OBJC_IVAR_NAME(__FILE__, __LINE__, name, __COUNTER__)

// old style creation. advantage: uses __func__ to determine calling function
// #define OBJC_IVAR_NAME(Name) [NSString stringWithFormat:@"%s:%i %s:%s:%i", __FILE__, __LINE__, __func__, #Name, __COUNTER__]

// implemenations for each of the overloads
#define _OBJC_IVAR_0(...) _Pragma("message \"Cannot call OBJC_IVAR with 0 params!\"")
#define _OBJC_IVAR_1(Name) _OBJC_IVAR_2(__strong id, Name)

// first major implemenation. because we do no assignment here, we don't have to check for is_set
#define _OBJC_IVAR_2(Type, Name) Type& Name = (_OBJC_IVAR::IMPL<Type>(self, OBJC_IVAR_NAME(Name)))

// this is where things get fun. we have 'OBJC_IVAR_CUR_NAME', instead of calling OBJC_IVAR_NAME
// multiple times, because we must ensure that COUNTER does not change during the course of the macro
// this is the 'inner bowels' of C, and it's quite hacky. Returns a reference to an associated object
// which is wrapped in a NSValue. Note that we only evaluate 'default' once throught the course of the
// application's cycle, so you can feel free to put intensive loading code there.
static NSString *_OBJC_IVAR_CUR_NAME;
#define _OBJC_IVAR_3(Type, Name, Default) Type& Name = (_OBJC_IVAR::IS_SET(self, (_OBJC_IVAR_CUR_NAME = OBJC_IVAR_NAME(Name))) ? _OBJC_IVAR::IMPL<Type>(self, _OBJC_IVAR_CUR_NAME) : _OBJC_IVAR::IMPL<Type>(self, _OBJC_IVAR_CUR_NAME, Default))

// namespace to wrap al lof our functions
namespace _OBJC_IVAR
{
    // internal dictionary of all associated object names, so that we don't run
    // into memory management issues.  we use a set here, because we should never
    // have duplicate associated object names.
    static NSMutableSet *_names = [NSMutableSet set];

    // wraps a value and a reference to a value. used over std::reference_wrapper,
    // as that doesn't actually copy in the value passed. That is required for what
    // we are doing, as we cannot be assigning to constants.
    template<typename T>
    class Wrapper {
    private:
        // private value wrapped by this object.
        T _value;
        // private reference wrapped by this object. should always point to _value.
        T& _ref;

    public:
        // default constructor. assumes 'T' has a valid 0-argument constructor
        Wrapper() : _value(), _ref(_value) { }

        // argument constructor. makes sure that value is initialized properly
        Wrapper(T val) : _value(val), _ref(_value) { }

        // returns the reference wrapped by this object
        operator T& () {
            return _ref;
        }

        T& get() {
            return _ref;
        }
    };

    // interns a name. because objc_getAssociatedObject works only by comparing
    // pointers (and +stringWithFormat: isn't guaranteed to return the same pointer),
    // we have to make sure that we maintain a list of all valid associated object
    // names. these are NOT linked to specific objects, which allows us to reuse some
    // memory
    inline NSString *name_intern(NSString *name)
    {
        // intern the value. first check if the object has been interned already,
        // and if it is, return that interned value
        if (id tmpName = [_names member:name])
        {
            name = tmpName;
        }

        // if we haven't interned this value before, then add it to the list and return it.
        else
        {
            [_names addObject:name];
        }

        return name;
    }

    // check and see if the requested iVar has been set yet. used for default value setting
    BOOL IS_SET(id target, NSString *name)
    {
        // first intern the name
        name = name_intern(name);

        // check if the object has this property. objc_getAssociatedObject will ALWAYS
        // return NULL if the object doesn't exist. Note the bridged cast. This is because
        // objc_getAssociatedObject doesn't care what you throw into the second parameter,
        // as long as it is a pointer. That gives us the flexibility at a later date, to,
        // for example, just pass a pointer to a single byte, and pull out the value that
        // way. However, we pass in a NSString pointer, because it makes it easy for us to
        // use and to re-use later.
        id val = objc_getAssociatedObject(target, (__bridge const void *) name);

        return val != nil;
    }

    // the actual implementation for setting the iVar. luckily this code isn't too hacky,
    // but it is a bit confusing.
    template<typename T>
    Wrapper<T>& IMPL(id target, NSString *name)
    {
        // first intern the name
        name = name_intern(name);

        // define a reference. we use pointers & new here, because C++ memory managment is
        // weird at best. Most of the time, you should be using RAII, but when dealing with
        // templates & objective-c interpolation, it is almost required that you use pointers
        // with new.
        Wrapper<T> *reference = nullptr;

        // check and see if the object already contains this property, if so, return that value
        NSValue *result = objc_getAssociatedObject(target, (__bridge const void *) name);
        if (result == nil)
        {
            // at this point, we need to create a new iVar, with the default constructor for the type.
            // for objective-c objects this is 'nil', for integers and floating point values this is 0,
            // for C++ structs and classes, this calls the default constructor. If one doesn't exist,
            // you WILL get a compile error.
            reference = new Wrapper<T>();

            // we now set up the object that will hold this wrapper. This is an extension on NSValue
            // which allows us to store a generic pointer (in this case a C++ object), and run desired
            // code on -dealloc (which will be called at the time the parent object is destroyed), in
            // this case, free the memory used by our wrapper.
            result = [NSValue valueWithCppObject:reference onDealloc:^(void *) {
                delete reference;
            }];

            // finally, set the associated object to the target, and now we are good to go.
            // We use OBJC_ASSOCIATION_RETAIN, so that our NSValue is properly freed when done.
            objc_setAssociatedObject(target, (__bridge const void *) name, result, OBJC_ASSOCIATION_RETAIN);
        }

        // from result, we cast it's -cppObjectValue to a Wrapper, to pull out the value.
        reference = static_cast<Wrapper<T> *>([result cppObjectValue]);

        // finally, return the pointer as a reference, not a pointer
        return *reference;
    }

    // this is pretty much the same as the other IMPL, but it has specific code for default values.
    // I will ignore everything that is the same about the two functions, and only focus on the
    // differences, which are few, but mandatory.
    template<typename T>
    Wrapper<T>& IMPL(id target, NSString *name, const T& defVal)
    {
        name = name_intern(name);

        Wrapper<T> *reference = nullptr; // asign to be the default constructor for 'T'

        NSValue *result = objc_getAssociatedObject(target, (__bridge const void *) name);
        if (result == nil)
        {
            // this is the only difference. Instead of constructing with the default constructor,
            // simply pass in our new default value as a copy.
            reference = new Wrapper<T>(defVal);
            result = [NSValue valueWithCppObject:reference onDealloc:^(void *) {
                delete reference;
            }];

            objc_setAssociatedObject(target, (__bridge const void *) name, result, OBJC_ASSOCIATION_RETAIN);
        }

        reference = static_cast<Wrapper<T> *>([result cppObjectValue]);
        return *reference;
    }
}

#endif // OBJC_IVAR_HPP

NSValue+CppObject.h

//
//  NSValue+CppObject.h
//  TestProj
//
//  Created by Richard Ross on 8/17/12.
//  Copyright (c) 2012 Ultimate Computer Services, Inc. All rights reserved.
//

#import <Foundation/Foundation.h>

// Extension on NSValue to add C++ object support. Because of the difficulty
// involved in templates, I took the easy way out and simply passed in a block
// of code to be run at dealloc.
@interface NSValue (CppObject)

// create a new NSValue instance that holds ptr, and calls 'deallocBlock' on destruction.
+(id) valueWithCppObject:(void *) ptr onDealloc:(void (^)(void *)) deallocBlock;
-(id) initWithCppObject:(void *)  ptr onDealloc:(void (^)(void *)) deallocBlock;

// get the held pointer of this object. I called it -cppObjectValue, so
// there was no confusion with -pointerValue.
-(void *) cppObjectValue;

@end

NSValue+CppObject.m

//
//  NSValue+CppObject.m
//  TestProj
//
//  Created by Richard Ross on 8/17/12.
//  Copyright (c) 2012 Ultimate Computer Services, Inc. All rights reserved.
//

#import "NSValue+CppObject.h"

// the concrete NSValue subclass for supporting C++ objects. Pretty straight-forward interface.
@interface ConcreteCppObject : NSValue
{
    // the underlying object that is being pointed to
    void *_object;
    // the block that is called on -dealloc
    void (^_deallocBlock)(void *);
}

@end

@implementation ConcreteCppObject

// object initialization
+(id) valueWithCppObject:(void *)ptr onDealloc:(void (^)(void *))deallocBlock
{
    return [[self alloc] initWithCppObject:ptr onDealloc:deallocBlock];
}

-(id) initWithCppObject:(void *)ptr onDealloc:(void (^)(void *))deallocBlock
{
    if (self = [super init])
    {
        _object = ptr;
        _deallocBlock = deallocBlock;
    }

    return self;
}

// required methods for subclassing NSValue
-(const char *) objCType
{
    return @encode(void *);
}

-(void) getValue:(void *)value
{
    *((void **) value) = _object;
}

// comparison
-(BOOL) isEqual:(id)compare
{
    if (![compare isKindOfClass:[self class]])
        return NO;

    return [compare cppObjectValue] == [self cppObjectValue];
}

// cleanup
-(void) dealloc
{
    // this should manage cleanup for us
    _deallocBlock(_object);
}

// value access
-(void *) cppObjectValue
{
    return _object;
}


@end

// NSValue additions for creating the concrete instances
@implementation NSValue (CppObject)

// object initialization
+(id) valueWithCppObject:(void *)ptr onDealloc:(void (^)(void *))deallocBlock
{
    return [[ConcreteCppObject alloc] initWithCppObject:ptr onDealloc:deallocBlock];
}

-(id) initWithCppObject:(void *)ptr onDealloc:(void (^)(void *))deallocBlock
{
    return [[self class] valueWithCppObject:ptr onDealloc:deallocBlock];
}

// unless the NSValue IS a ConcreteCppObject, then we shouldn't do anything here
-(void *) cppObjectValue
{
    [self doesNotRecognizeSelector:_cmd];

    return nil;
}

@end

示例用法:

#import "OBJC_IVAR.hpp"

@interface SomeObject : NSObject

-(void) doSomething;

@end

@implementation SomeObject

-(void) doSomething
{
    OBJC_IVAR(__strong id, test, @"Hello World!");
    OBJC_IVAR(int, test2, 15);

    NSLog(@"%@", test);
    NSLog(@"%i", test2 += 7);

    // new scope
    {
        OBJC_IVAR(int, test, 100);

        NSLog(@"%i", ++test);
    }

    [self somethingElse];
}

-(void) somethingElse
{
    OBJC_IVAR(int, newVar, 7);

    NSLog(@"%i", newVar++);
}

@end

int main()
{
    SomeObject *obj = [SomeObject new];

    [obj doSomething];
    [obj doSomething];
    [obj doSomething];
}