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

据我所知，从 LLVM 3.1 开始，合成 ivars 的默认存储是 strong。我想我的问题是:为什么 codegen 更喜欢 header 中的声明而不是我在实现中的重新声明？其次，为什么当我重新声明为 copy 而不是 weak 或 assign 时它会提示？

Answer 1

我明白你的问题了...

• 您想在 MyObject 类成员方法中读写 myproperty。
• 但是，想只读其他类的 myproperty。

---

//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

---

//Ohter.m

#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).
}