Runtime instance variables for Objective-C classes

CRZRuntimeIvars.h

//
//  CRZRuntimeIvars.h
//  CRZRuntimeIvars
//
//  Copyright (c) 2013-2015 Elia Cereda
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in all
//  copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
//  SOFTWARE.
//

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

//
//  README
//
//  This is the workflow to create an instance variable at runtime:
//    1. Import this file
//
//    2. Create the ivar at the beginning of your .m file
//      CRZCreateIvar(class, name, associationPolicy)
//
//    3. Enjoy!
//      CRZIvarGetValue(class, object, name)
//      CRZIvarSetValue(class, object, name, value)
//
//  If you need the same ivar in more than one file, create it in one .m
//  file and load it in the others with:
//    CRZExternIvar(class, name)
//

// The values in this enum have the same meaning as the attributes of a @property.
typedef NS_ENUM(objc_AssociationPolicy, CRZAssociationPolicy) {
    CRZAssociationAssign = OBJC_ASSOCIATION_ASSIGN,
    
    CRZAssociationRetain = OBJC_ASSOCIATION_RETAIN,
    CRZAssociationRetainNonAtomic = OBJC_ASSOCIATION_RETAIN_NONATOMIC,
    
    CRZAssociationCopy = OBJC_ASSOCIATION_COPY,
    CRZAssociationCopyNonAtomic = OBJC_ASSOCIATION_COPY_NONATOMIC,
};

typedef struct {
    // No value is actually stored in this variable, the API just uses its memory location.
    char identifier;
    
    CRZAssociationPolicy associationPolicy;
} _CRZIvar;

#define CRZIvar(class, name)                            \
    (CRZIvar_ ## class ## _ ## name)

#define CRZCreateIvar(class, name, _associationPolicy)  \
    _CRZIvar CRZIvar(class, name) = {                   \
        .associationPolicy = _associationPolicy         \
    };

// Use this if you want to use an ivar created in another file
#define CRZExternIvar(class, name)                      \
    extern _CRZIvar CRZIvar(class, name);

#define CRZIvarGetValue(class, object, name)            \
    (objc_getAssociatedObject(                          \
        object, &CRZIvar(class, name).identifier        \
    ))

#define CRZIvarSetValue(class, object, name, value)     \
    (objc_setAssociatedObject(                          \
        object, &CRZIvar(class, name).identifier,       \
        value, CRZIvar(class, name).associationPolicy   \
    ))