NSMutableArray addObject返回nil

Question

StackOverflow上已经存在一些类似的问题.我确实检查了它们,并且在大多数情况下它返回nil,因为它NSMutableArray尚未初始化.但在我的情况下,我做了初始化.

这是我的代码的一部分:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    self.storedTimeZones = [[NSMutableArray alloc] init];
    NSData *storedData = [[NSUserDefaults standardUserDefaults] dataForKey:@"timeZones"];

    if (storedData != nil ) {
        self.storedTimeZones = [NSKeyedUnarchiver unarchiveObjectWithData:storedData];
    }

    NSString *str = [self.smallRegionArray[indexPath.row] stringByReplacingOccurrencesOfString:@"_" withString:@" "];
    [self.storedTimeZones addObject: str];
    NSLog(str); //the string was printed successfully.
    NSLog(@"%lu", (unsigned long)self.storedTimeZones.count); //'0' was printed
}

更新 @Caleb是对的,[NSKeyedUnarchiver unarchiveObjectWithData:storedData返回零.我解决了这个问题:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    self.storedTimeZones = [[NSMutableArray alloc] init];
    NSData *storedData = [[NSUserDefaults standardUserDefaults] dataForKey:@"timeZones"];
    NSMutableArray *ary = [NSKeyedUnarchiver unarchiveObjectWithData:storedData];

    if (ary != nil ) {
        self.storedTimeZones = ary;
    }

    NSString *str = [self.smallRegionArray[indexPath.row] stringByReplacingOccurrencesOfString:@"_" withString:@" "];
    [self.storedTimeZones addObject: str];
    NSLog(str);
    NSLog(@"%lu", (unsigned long)self.storedTimeZones.count); //Now it prints `1`
}

Answer 1

但在我的情况下,我做了初始化.

你做了,但后来你取而代之.这是初始化行:

self.storedTimeZones = [[NSMutableArray alloc] init];

但是你分配了一个不同的值:

self.storedTimeZones = [NSKeyedUnarchiver unarchiveObjectWithData:storedData];

此时,self.storedTimeZones可以是不可变的实例,也可以是NSArray完全不同的对象,或者nil.我猜它是后者,因为你打电话时没有抛出异常-addObject:.那count是0也是有道理的,如果self.storedTimeZones是nil,因为消息的结果nil是nil或0或NO,这取决于你所期望的类型.

您真正需要做的就是正确诊断问题,就是检查self.storedTimeZones您最近的NSLog陈述.设置断点并查看该属性的内容.有用的命令是:

po self.storedTimeZones
po [self.storedTimeZones class]