为什么我的变量超出范围?

我在AppDelegate类中定义了NSMutableArray:

NSMutableArray *devices;

我从didFinishLaunchingWIthOptions方法中的类填充数组:

Devices *devs = [[Devices alloc] init];
self.devices = [devs getDevices];

getDevices方法解析一个json字符串并创建一个设备对象,将其添加到数组中:

NSMutableArray *retDevices = [[NSMutableArray alloc] initWithCapacity:[jsonDevices count]];

for (NSDiectionary *s in jsonDevices) {
    Device *newDevice = [[Device alloc] init];
    device.deviceName = [s objectForKey:@"name"];

    [retDevices addObject: newDevice];
}

return retDevices;

AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
Device *d = (Device *)[appDelegate.devices objectAtIndex:indexPath.row];

cell.label.text = d.deviceName;

最初填充tableview时,它会工作。但是,当我滚动列表时,cellForRowAtIndexPath再次执行时,d.deviceName会抛出一个错误,因为它超出了范围。

提前谢谢。