一个简短例子来说明一下为什么 NSString @property 最好用 copy 而不是 retain:
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29 | #import <Foundation/Foundation.h>
@interface Person : NSObject
@property (nonatomic, retain) NSString *name;
@property (nonatomic, copy) NSString *school;
@end
@implementation Person
@synthesize name, school;
@end
int main(int argc, char *argv[]) {
@autoreleasepool {
Person *p = [[Person alloc] init];
NSMutableString *s1 = [NSMutableString stringWithString:@"fannheyward"];
NSMutableString *s2 = [NSMutableString stringWithString:@"hfut"];
p.name = s1;
p.school = s2;
NSLog(@"%@, %@", p.name, p.school); // fannheyward, hfut
[s1 appendString:@"---Heybot"];
[s2 appendString:@"---Heybot"];
NSLog(@"%@, %@", p.name, p.school); // fannheyward---Heybot, hfut
}
}
|
简单来说就是 NSString 可以通过 NSMutableString (isa NSString) 来进行修改，如果 @property 是 retain 的话就可以绕过 Person 实例来修改 name 值（因为 name 指向 s1），大部分时候这种情况都是不应该发生的，用 copy 就没有这个问题。
这样来说象 NSArray/NSDictionary 等可修改类型都应该用 copy。
For attributes whose type is an immutable value class that conforms to the NSCopying protocol, you almost always should specify copy in your @property declaration.
参考 NSString property: copy or retain?
