使用 NSPredicate 从 NSManagedObjectContect 获取准确的 NSDate

发布于 2024-10-03 18:37:03 字数 898 浏览 4 评论 0原文

我正在尝试获取实体“MeterReading”的结果，该实体有两个属性：“时间戳”和“读数”。 “时间戳”是一个 NSDate。我现在正在尝试获取具有确切日期的对象。

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"MeterReading" inManagedObjectContext:context];
[request setEntity:entity];
NSLog(@"%f", [self.timestamp timeIntervalSince1970]);

NSPredicate *pre = [NSPredicate predicateWithFormat:@"timestamp == %@", self.timestamp];
NSLog(@"%@", pre);
[request setPredicate:pre];

现在，self.timestamp 被预先传递给另一个 ViewController，NSLog 显示：

1290264372.210091

NSPredicate 日志

时间戳 == CAST(311957172.210091, "NSDate")

第一个问题：为什么两个数字不一样？

第二个也是更重要的问题：在 ManagedContext 中，我有四个带有日期的条目。如果我使用“<=”而不是“==”，我确实会得到日期小于我通过的日期的结果，包括我想要的日期。为什么我无法使用“==”运算符获取单个日期？这可能与我的日期精度有关吗？

谢谢！

原文

I'm trying to fetch results of my entity "MeterReading", which has two properties, "timestamp" and "reading". "timestamp" is an NSDate. I'm now trying to fetch an object with an exact date.

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"MeterReading" inManagedObjectContext:context];
[request setEntity:entity];
NSLog(@"%f", [self.timestamp timeIntervalSince1970]);

NSPredicate *pre = [NSPredicate predicateWithFormat:@"timestamp == %@", self.timestamp];
NSLog(@"%@", pre);
[request setPredicate:pre];

Now, self.timestamp is passed to another ViewController beforehand, the NSLog shows:

1290264372.210091

The NSPredicate logs

timestamp == CAST(311957172.210091, "NSDate")

First question: Why aren't the two numbers the same?

Second and more important question: In the ManagedContext, I have four entries with dates. If I use "<=" instead of "==", I do get results with a date smaller than the one I passed, including the one I want to have. Why can't I get the one single date with the "==" operator? Can this be related to precision of my dates?

Thanks!

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临走之时 2024-10-10 18:37:03

这与浮点相等检查本质上不安全的问题相同。当浮点值被传递、转换、用于算术等时，它们会一点一点地失去准确性。您可能必须使用更复杂的谓词来检查特定容差内的日期；例如，

NSArray *timestamps = [NSArray arrayWithObjects:
    [self.timestamp dateByAddingTimeInterval:-1],
    [self.timestamp dateByAddingTimeInterval:1],
    nil
];

NSPredicate *pre = [NSPredicate predicateWithFormat:@"timestamp BETWEEN %@", timestamps];

这将为您提供与您的日期加或减一秒匹配的任何对象。

This is the same issue as floating-point equality checks being inherently unsafe. As floating-point values get passed around, converted, used in arithmetic, etc., they lose accuracy bit by bit. You may have to use a more complex predicate that instead checks for dates within a certain tolerance; for instance,

NSArray *timestamps = [NSArray arrayWithObjects:
    [self.timestamp dateByAddingTimeInterval:-1],
    [self.timestamp dateByAddingTimeInterval:1],
    nil
];

NSPredicate *pre = [NSPredicate predicateWithFormat:@"timestamp BETWEEN %@", timestamps];

which would get you any objects matching your date plus or minus one second.

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