目標C - 不能在一個方法設定值，並從另一個

Question

我嘗試做以下訪問：

1）用戶點擊按鈕開始，它運行startTimer方法，它設置了一個新[NSDate date] 。

2）用戶點擊按鈕停止它運行stopTimer方法，其中檢索[NSDate date]值。

我無法獲得第2步的工作。我已將其設置在.h文件中。如果我將start方法中的代碼複製到停止方法中，它將起作用。所以，我可以設置[NSDate date]。但是，這不是我想要的。我希望能夠在startTimer方法中設置它。我如何去做這件事？

.h文件中

@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不會保留，由您要訪問它的時候，它已經dealloc'd，因此你試圖訪問垃圾指針

最簡單的解決方案，你將是

@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]; 
}