NSDate基本使用错误

Question

在显示滑块的值的标签旁边，我想显示与开始日期对应的日期+滑块值单位中的天数。

当我移动滑块时，应用程序在模拟器中崩溃。据我可以发现，当我尝试创建一个新的NSDate字段和我的startDate对象“超出范围”时，会发生这种情况。

阅读了很多关于保留和发布NSDate文档的内容，直到现在还没有帮助我，因为文档太抽象了，以至于在我看来，我无法在文档中找到相关部分。我希望通过示例来了解更多。

那么，下面的代码有什么问题（以及初学者在文档中从哪里学到的）？

#import <UIKit/UIKit.h> 

@interface SliderDateViewController : UIViewController { 
    UILabel *dateLabel; 
    UILabel *sliderValueLabel; 
    UISlider *slider; 
    NSDate *startDate; 

} 

@property (nonatomic, retain) IBOutlet UILabel *dateLabel; 
@property (nonatomic, retain) IBOutlet UILabel *sliderValueLabel; 
@property (nonatomic, retain) IBOutlet UISlider *slider; 
@property (nonatomic, retain) NSDate *startDate; 

-(IBAction)sliderValueChanged:(UISlider *)theSlider; 

@end 


#import "SliderDateViewController.h" 

@implementation SliderDateViewController 

@synthesize dateLabel; 
@synthesize sliderValueLabel; 
@synthesize slider; 
@synthesize startDate; 

- (void)viewDidLoad { 

    [super viewDidLoad]; 

    dateLabel.text = @"01.01.2010"; 

    sliderValueLabel.text = @"0"; 

    NSDateFormatter *inputFormatter = [[NSDateFormatter alloc] init]; 
    [inputFormatter setDateFormat:@"dd.MM.yyyy"]; 

    self.startDate = [inputFormatter dateFromString:dateLabel.text]; 

    NSLog(@"startDate: ", self.startDate.description); 

    [inputFormatter release]; 

} 

-(IBAction)sliderValueChanged:(UISlider *)theSlider { 

    int sliderValueAsInt = (int)(theSlider.value + 0.5f); 
    NSString *newText = [[NSString alloc] initWithFormat:@"%d", sliderValueAsInt]; 
    sliderValueLabel.text = newText; 
    [newText release]; 

    // Next line following the comment crashed with 
    // *** -[CFDate release]: message sent to deallocated instance 0x3b07d10 
    NSDate *newDate = [startDate addTimeInterval:86400 * sliderValueAsInt]; 

    NSDateFormatter *outputFormatter = [[NSDateFormatter alloc] init]; 
    [outputFormatter setDateFormat:@"dd.MM.yyyy"]; 

    dateLabel.text = [outputFormatter stringFromDate:newDate]; 
    [outputFormatter release]; 
    // [newDate release] // <- this was the error 
} 

- (void)didReceiveMemoryWarning { 
    [super didReceiveMemoryWarning]; 
} 

- (void)viewDidUnload { 
} 


- (void)dealloc { 
    [dateLabel release]; 
    [sliderValueLabel release]; 
    [slider release]; 
    [startDate release]; 
    [super dealloc]; 
} 

@end 

Answer 1

首先，-addTimeInterval:已弃用。改为使用-dateByAddingTimeInterval:。

---

NSDate *newDate = [startDate addTimeInterval:86400 * sliderValueAsInt]; 
[startDate release]; 
startDate = newDate; 
[newDate release]; 

由于-addTimeInterval:不是+alloc/-copy/-create法，newDate是没有人拥有，你不应该-release它。为了获得可以方便地改写为

self.startDate = [startDate addTimeInterval:86400 * sliderValueAsInt]; 

---

从代码逻辑来看，startDate应保持不变，你需要

NSDate *newDate = [startDate addTimeInterval:86400 * sliderValueAsInt]; 
if (newDate != startDate) { 
    [startDate release]; 
    startDate = [newDate retain]; 
} 

所有权。这意味着startDate = newDate;的东西不应该出现。删除NSDate *newDate = [startDate ...];以下的3行，代码应该正常工作。