ObjectiveC out of scope问题

Objective-C有个很脑残的问题

描述如下：

ASIFormRequest类是ASIHTTPRequest类的子类。在函数中，对另一地方返回的ASIFormRequest指针进行retain,出了这个函数，retain的指针可以访问，但是所有从父类继承的属性、方法，都out of scope，一使用就程序崩溃。

解决办法很简单，retain时不要retain子类指针，retain父类的指针既可。

-(ASIHTTPRequest *) _getSinaRequest{
    ASIHTTPRequest *request = [sinaApi uploadWithImage:selectedPhoto status:text.text];
    request.delegate = self;
    request.showAccurateProgress = YES;
    request.uploadProgressDelegate = sinaPostProgress;
    [request retain];
    return request;

}

- (void)uploadPhoto
{
    if ([sinaApi isAuthorized]) {
        sinaPosted = NO;
        sinaPostRequest = [self _getSinaRequest];
        [sinaPostRequest startAsynchronous];
    }
    else {
        sinaPosted = YES;
    }
        //... ...

}

- (void)cancel: (id)sender{

    if(sinaPostRequest){
        if(![sinaPostRequest isFinished]){ // 如果retain的是ASIFormRequest指针，这里马上死掉
            [sinaPostRequest clearDelegatesAndCancel];
        }
        [sinaPostRequest release];
    }
    sinaPostRequest = nil;
    [progressWindow hideImmediately];

}