iphone - @property 中的内存损坏

Question

我有下一段代码，一个具有此属性的 iVar 保留并在它的类 dealloc 方法中释放。 iVar 用于 2 种方法并不断更改值，但有时，当我使用该值时，该值已损坏。这是为什么？

.h

@interface ChatController : NSObject <ASIHTTPRequestDelegate>{
NSTimer *timer;
NSString *_idLastMessageFromServer;
}

@property(nonatomic, retain)NSString *idLastMessageFromServer;
@end

.m

@implementation ChatController

@synthesize idLastMessageFromServer = _idLastMessageFromServer;

- (void)initLoopTimer{
timer = [NSTimer timerWithTimeInterval:5 target:self selector:@selector(update:) userInfo:nil repeats:YES];
[[NSRunLoop mainRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];
}

- (void)update:(id)sender{
ASIFormDataRequest *request = [ASIFormDataRequest requestWithURL:CONSTANT_YYYY];
[request setDelegate:self];
[request addPostValue:_idLastMessageFromServer forKey:CONSTANT_XXX];
[request setDidFinishSelector:@selector(requestUpdateFinish:)];
[request startAsynchronous];
}

- (void)requestUpdateFinish:(ASIHTTPRequest *)request{
NSString *response = [request responseString];
if(response && response.length){
    if(![response isEqualToString:CHAT_RESPONSE_NO_MESSAGES]){
        NSArray *array = [response componentsSeparatedByString:CHAT_PARSE_RESPONSE];
        if(array && [array count] == 2){
            **_idLastMessageFromServer = [array objectAtIndex:0];**
        }
     }
   }
}

但是当循环调用方法update:时，就在这行代码中崩溃了

[request addPostValue:_idLastMessageFromServer forKey:CONSTANT_XXX];

带有 EXC_BAD_ACCESS 消息，但为什么呢？

Answer 1

这一行:

_idLastMessageFromServer = [array objectAtIndex:0];

应该是

self.idLastMessageFromServer = [array objectAtIndex:0];

这将直接访问属性而不是变量，从而在赋值期间触发保留/释放。否则，指针被赋值，但从数组中取出的对象并没有被保留，它可能很快就会失效，剩下一个指向已释放对象的指针。