stringFromDate未将NSDate转换为字符串

Question

我有一些简单的代码应该转换我从CoreData检索的NSDate。由于某些原因，当相同的代码在其他视图中工作时，它不能在此类中工作。我究竟做错了什么？以下是有问题的代码和我日志的屏幕截图......

NSFetchRequest *request26 = [[NSFetchRequest alloc] init];
[request26 setEntity:entityDiscription];
[request26 setResultType:NSDictionaryResultType];
NSExpression *keyPathExpression26 = [NSExpression expressionForKeyPath:@"date"];
NSExpression *swimLast750Expression = [NSExpression expressionForFunction:@"max:" arguments:[NSArray arrayWithObject:keyPathExpression26]];
NSExpressionDescription *expressionDescription26 = [[NSExpressionDescription alloc] init];
[expressionDescription26 setName:@"swimLast750"];
[expressionDescription26 setExpression:swimLast750Expression];
[expressionDescription26 setExpressionResultType:NSInteger16AttributeType];
[request26 setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription26]];
NSPredicate *pred26 = [NSPredicate predicateWithFormat:@"(date >= %@) AND (sport like %@) AND (sessiontype like %@)", swimSinceDateAsDate, sportTypeSwim, sessType1];
[request26 setPredicate:pred26];

NSError *error26;
NSArray *objects26 = [context executeFetchRequest:request26 error:&error26];
if (objects26 == nil) {
    NSLog(@"The fetch request returned an array == nil");
} else {
    NSLog(@"Contents of Array Object26 is:%@", objects26);
    NSDate *sessDate = [[objects26 objectAtIndex:0] valueForKey:@"swimLast750"];
    NSLog(@"NSDate is: %@", sessDate);
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"dd-MMM-yyyy"];
    NSLog(@"DATE FORMATTER: %@", [formatter stringFromDate:[[objects26 objectAtIndex:0] valueForKey:@"swimLast750"]]);
    NSString *dateString = [formatter stringFromDate:sessDate];
    //NSString *dateString = [formatter stringFromDate:[[objects26 objectAtIndex:0] valueForKey:@"swimLast750"]];
    NSLog(@"dateString is: %@", dateString);
    sw750Last = dateString;
}

Answer 1

您已设置

[expressionDescription26 setExpressionResultType:NSInteger16AttributeType];

导致获取请求将日期作为NSNumber返回。 这个数字是自2001年1月1日格林尼治标准时间“参考日期”以来的秒数， 所以你可以将其转换为NSDate

NSNumber *sessDateAsNumber = [[objects26 objectAtIndex:0] valueForKey:@"swimLast750"];
NSDate *sessDate = [NSDate dateWithTimeIntervalSinceReferenceDate:[sessDateAsNumber doubleValue]];

但更好的解决方案可能是使用

[expressionDescription26 setExpressionResultType:NSDateAttributeType];

在表达式描述中。 然后，日期将作为NSDate对象返回，stringFromDate按预期工作。

Answer 2

你必须使用下面的代码将时间戳转换为日期。我想你会把它作为NSNumber得到它，所以你必须在之前将它转换成双值

NSTimeInterval interval = [sessDate doubleValue];
NSDate *swimLast750=[NSDate dateWithTimeIntervalSince1970:interval];

Answer 3

首先，您需要更改到目前为止的时间间隔，然后设置指定的格式。请尝试以下代码： -

NSDate *date=[NSDate dateWithTimeIntervalSinceReferenceDate:402663864];
NSDateFormatter *formatter=[[NSDateFormatter alloc]init];
[formatter setDateFormat:@"yyyy-MM-dd"];
NSLog(@"%@",date);
NSString *dateString=[formatter stringFromDate:date];
NSLog(@"dateString=%@",dateString);