Objective-C ARC属性重新声明混淆

Question

我有以下代码：

// MyObject.h
#import <Foundation/Foundation.h>

@interface MyObject : NSObject
@property (nonatomic, readonly) id property;
@end

// MyObject.m
#import "MyObject.h"

@interface MyObject ()
@property (nonatomic, copy, readwrite) id property;
@end

@implementation MyObject
@synthesize property = _property;
@end

这会生成以下编译器警告和错误：

warning: property attribute in continuation class does not match the primary class
@property (nonatomic, copy, readwrite) id property;
^
note: property declared here
@property (nonatomic, readonly) id property;
                                   ^
error: ARC forbids synthesizing a property of an Objective-C object with unspecified ownership or storage attribute

但是，如果我将类continuation的属性重新声明更改为具有weak的存储限定符，则不会生成警告或错误。 但是，（令人担忧的是）生成的代码-[MyObject setProperty:]调用objc_storeStrong而不是我预期的objc_storeWeak 。

据我所知，从LLVM 3.1开始，合成ivars的默认存储空间strong 。 我想我的问题是：为什么codegen在实现中重新声明时更喜欢标题中的声明？ 其次，为什么当我重新宣布copy ，它会抱怨，但不是weak或assign ？

Answer 1

我明白你的问题...

• 您想在MyObject类成员方法中读写myproperty。
• 但是，想要从其他班级只读我的财产。

---

// MyObject.h

@interface MyObject : NSObject
{
@private
    id _myproperty;
}
@property (nonatomic, copy, readonly) id myproperty;
@end

---

// MyObject.m

#import "MyObject.h"

@interface MyObject ()
// @property (nonatomic, copy, readwrite) id myproperty; // No needs
@end

@implementation MyObject
@synthesize myproperty = _myproperty;


- (void)aMethod
{
    _myproperty = [NSString new]; // You can read & write in this class.
}

@end

---

//哦

#import "MyObject.h"

void main()
{
   MyObject *o = [MyObject new];
   o.myproperty = [NSString new]; // Error!! Can't write.
   o._myproperty = [NSString new]; // Error!! Can't acsess by objective c rule.
   NSString *tmp = o.myproperty; // Success readonly. (but myproperty value is nil).
}