objective-c - Objective C - 无法在一种方法中设置值并从另一种方法访问它

Question

我正在尝试执行以下操作:

1) 用户点击按钮开始，它运行startTimer方法，设置一个新的[NSDate date]。

2) 用户点击按钮 stop 它运行 stopTimer 方法，它检索 [NSDate date] 值。

我无法执行第 2 步。我已经在 .h 文件中设置了它。如果我将代码从 start 方法复制到 stop 方法，它就可以工作。所以，我可以设置 [NSDate date]。但是，这不是我想要的。我希望能够在 startTimer 方法中设置它。我该怎么做？

.h文件

@interface StartTest : UIViewController {
    IBOutlet UILabel *timer;

    NSDate *startNSDate;
    NSDate *start;



}

- (IBAction)startTimer;
- (IBAction)stopTimer;

- (NSDate *)setStart;
- (NSDate *)getStart;


@end

.m 文件:

@implementation Ash

- (IBAction)startTimer {

    startNSDate = [NSDate date];

}


- (IBAction)stopTimer{

    start = [NSDate date];


    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"yyyy"];


    NSString *stringFromDate = [formatter stringFromDate:startNSDate]; // <<< this is where it fails

    NSLog(@"stringfromdate: %@", stringFromDate);
}

Answer 1

startNSDate 没有被保留，当你想访问它时，它已经被释放，因此你正在尝试访问垃圾指针

最简单的解决方案是

@interface StartTest : UIViewController {
    IBOutlet UILabel *timer;

    NSDate *startNSDate;
    NSDate *start;



}
@property (nonatomic, strong) NSDate* startNSDate;

- (IBAction)startTimer;
- (IBAction)stopTimer;

- (NSDate *)setStart;
- (NSDate *)getStart;


@end




@implementation Ash

- (IBAction)startTimer {

    self.startNSDate = [NSDate date];

}


- (IBAction)stopTimer{

    start = [NSDate date];


    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"yyyy"];

    // this is where it fails, the reason being that startNSDate isn't retained
    // and by this line, it's already dealloc'd, hence you're trying to access
    // garbage pointer
    NSString *stringFromDate = [formatter self.startNSDate]; 

    NSLog(@"stringfromdate: %@", stringFromDate);
}

- (void)dealloc
{
    self.startNSDate = nil;
    [super dealloc];
}