如何使用 NSPredicate 获取今天的 NSDate 对象？答案

【问题标题】：How fetch objects with NSDate of today using NSPredicate?如何使用 NSPredicate 获取今天的 NSDate 对象？
【发布时间】：2016-02-28 22:12:54
【问题描述】：

我的核心数据模型和涉及日期的获取请求遇到问题。

我在具有 NSDate 属性的实体中有一些对象； 我需要提取今天日期的对象，但我总是从这段代码中得到 nil：

public func getObjectsOfToday() -> Array<myObject>?
{
    let entityDescription = NSEntityDescription.entityForName("Objects", inManagedObjectContext: DataAccess.sharedInstance.managedObjectContext)

    let request = NSFetchRequest()
    request.entity = entityDescription
    request.returnsObjectsAsFaults = false

    let today = NSDate()
    request.predicate = NSPredicate(format: "(dateStart => %@) AND (dateStart <= %@)", today, today)

    var objects: [AnyObject]?
    do
    {
        objects = try DataAccess.sharedInstance.managedObjectContext.executeFetchRequest(request)
    }
    catch let error as NSError
    {
        print(error)
        objects = nil
    }

    return objects as? Array<Objects>
}

我认为是 NSPredicate 的问题，因为它还考虑了小时、分钟和秒。如果我今天打印是这样的：

今日打印说明：2016-02-28 22:02:01 +0000

但是我想获取具有相同日期的对象，忽略小时、分钟和秒。我需要做什么？

我还尝试使用组件创建另一个 NSDate：

let components = cal.components([.Day , .Month, .Year ], fromDate: today)
let newDate = cal.dateFromComponents(components)

但结果是一样的。我做错了什么？

【问题讨论】：

标签： ios swift core-data nsdate nspredicate

【解决方案1】：

我所做的是将它与一天的开始和结束进行比较，并使用几个辅助函数来计算它们：

class DateHelper{
    internal class func startOfDay(day: NSDate) -> NSDate {
        let gregorian = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)
        let unitFlags: NSCalendarUnit = [.Minute, .Hour, .Day, .Month, .Year]
        let todayComponents = gregorian!.components(unitFlags, fromDate: day)
        todayComponents.hour = 0
        todayComponents.minute = 0
        return (gregorian?.dateFromComponents(todayComponents))!
    }

    internal class func endOfDay(day: NSDate) -> NSDate {
        let gregorian = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)
        let unitFlags: NSCalendarUnit = [.Minute, .Hour, .Day, .Month, .Year]
        let todayComponents = gregorian!.components(unitFlags, fromDate: day)
        todayComponents.hour = 23
        todayComponents.minute = 59
        return (gregorian?.dateFromComponents(todayComponents))!
    }
}

所以在你的代码中，你会调用：

request.predicate = NSPredicate(format: "(dateStart => %@) AND (dateStart <= %@)", DateHelper.startOfDay(today), DateHelper.endOfDay(today))

【讨论】：

由于此代码不处理秒、纳秒和时代，因此在边缘情况下它可能会失败。此类方法更自然的地方是 NSDate 上的扩展而不是辅助类。
这两点都很好——如果你有改进的地方，请随意！

【解决方案2】：

创建一个开始日期，获取一天的长度（间隔），将间隔添加到开始日期以获得第二天的开始。

var startOfToday: NSDate?
var interval: NSTimeInterval = 0

NSCalendar.currentCalendar().rangeOfUnit(.Day, startDate: &startOfToday, interval: &interval, forDate: NSDate())
let startOfTomorrow = startOfToday!.dateByAddingTimeInterval(interval)

创建谓词

let predicate = NSPredicate(format: "dateStart >= %@ AND dateStart < %@", startOfToday, startOfTomorrow)

我使用以下测试代码没有核心数据的麻烦

import Foundation

let dates:[NSDate] = {
    var dates:[NSDate] = []

    dates.append({
        let c = NSDateComponents()
        c.year = 2016
        c.month = 2
        c.day = 1
        return NSCalendar.currentCalendar().dateFromComponents(c)!
    }())

    dates.append({
        let c = NSDateComponents()
        c.year = 2016
        c.month = 2
        c.day = 3
        return NSCalendar.currentCalendar().dateFromComponents(c)!
        }())

    dates.append({
        let c = NSDateComponents()
        c.year = 2016
        c.month = 3
        c.day = 1
        return NSCalendar.currentCalendar().dateFromComponents(c)!
        }())
    dates.append({
        let c = NSDateComponents()
        c.year = 2016
        c.month = 2
        c.day = 28
        c.hour = 12
        c.minute = 30
        return NSCalendar.currentCalendar().dateFromComponents(c)!
        }())
    dates.append({
        let c = NSDateComponents()
        c.year = 2016
        c.month = 2
        c.day = 28
        c.hour = 11
        c.minute = 15
        return NSCalendar.currentCalendar().dateFromComponents(c)!
        }())
    return dates
}()


var startOfToday: NSDate?
var interval: NSTimeInterval = 0

NSCalendar.currentCalendar().rangeOfUnit(.Day, startDate: &startOfToday, interval: &interval, forDate: NSDate())
if let startOfToday = startOfToday  {

    let startOfTomorrow = startOfToday.dateByAddingTimeInterval(interval)

    let predicate = NSPredicate(format: "self >= %@ AND self < %@", startOfToday, startOfTomorrow)

    let filteredArray = dates.filter({predicate.evaluateWithObject($0)})
    print(filteredArray)
}

结果：

[2016-02-28 11:30:00 +0000, 2016-02-28 10:15:00 +0000]

【讨论】：