This post shows how to read and write an instance var in another class.
Suppose we have a class 'ReadOnly'
@interface ReadOnly : NSObject{
@private int value;
}
@property (readonly) int data;
@end
So, if we want to set data or value in other class, we can take advantage of objc runtime feature.
ReadOnly * obj = [[ReadOnly alloc] init];
Ivar ivar1 = class_getInstanceVariable(object_getClass(obj), "value");
int *ivarPtr1 = (int *)((uint8_t *)obj + ivar_getOffset(ivar1));
*ivarPtr1 = -1;
// obj.value = -1;
Ivar ivar2 = class_getInstanceVariable(object_getClass(obj), "_data");
int *ivarPtr2 = (int *)((uint8_t *)obj + ivar_getOffset(ivar2));
*ivarPtr2 = 1;
// obj.data = 1;
实际上并没有修改啊，给obj加个方法，输出data的值，发现并没有改变「」；其次这句话编译不过：
int *ivarPtr1 = (int *)((uint8_t *)obj + ivar_getOffset(ivar1));
首先需要
#import <objc/runtime.h>
其次，在Build Settings里要设置
Objective-C Automatic Reference Counting为No
输出：
2016-06-28 13:16:07.997394 ivar[92542:3806138] value -1
2016-06-28 13:16:07.997597 ivar[92542:3806138] data 1
2016-06-28 13:16:07.997617 ivar[92542:3806138] 1
Program ended with exit code: 0
代码：
#import <Foundation/Foundation.h> #import <objc/runtime.h> @interface ReadOnly : NSObject { @private int value; } @property (readonly) int data; @end @implementation ReadOnly - (void)print { NSLog(@"value %d", value); NSLog(@"data %d", [self getData]); } - (int)getData { return _data; } @end int main(int argc, const char * argv[]) { @autoreleasepool { ReadOnly * obj = [[ReadOnly alloc] init]; Ivar ivar1 = class_getInstanceVariable(object_getClass(obj), "value"); int *ivarPtr1 = (int *)((uint8_t *)obj + ivar_getOffset(ivar1)); *ivarPtr1 = -1; // obj.value = -1; Ivar ivar2 = class_getInstanceVariable(object_getClass(obj), "_data"); int *ivarPtr2 = (int *)((uint8_t *)obj + ivar_getOffset(ivar2)); *ivarPtr2 = 1; // obj.data = 1; [obj print]; NSLog(@"%d", obj.data); } return 0; }那如果在arc下呢，现在基本都是arc的
原来的(uint8_t *)obj改为(uint8_t *)(long)obj即可
这句话没看懂唉，为啥啊