Objective-C ARC屬性重新聲明混淆

Question

我有以下代碼：

// 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

這會生成以下編譯器警告和錯誤：

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，如果我將類continuation的屬性重新聲明更改為具有weak的存儲限定符，則不會生成警告或錯誤。 但是，（令人擔憂的是）生成的代碼-[MyObject setProperty:]調用objc_storeStrong而不是我預期的objc_storeWeak 。

據我所知，從LLVM 3.1開始，合成ivars的默認存儲空間strong 。 我想我的問題是：為什么codegen在實現中重新聲明時更喜歡標題中的聲明？ 其次，為什么當我重新宣布copy ，它會抱怨，但不是weak或assign ？

Answer 1

我明白你的問題...

• 您想在MyObject類成員方法中讀寫myproperty。
• 但是，想要從其他班級只讀我的財產。

---

// 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).
}