如何有一个A-Z索引与NSFetchedResultsController [英] How to have a A-Z index with a NSFetchedResultsController

问题描述

我使用NSFetchedResultsController作为我的表视图，我想有一个索引A-Z。

问题是，如果我使用我想要的部分的实体的属性，它会返回尽可能多的部分，因为这个属性有不同的名称。

所以我想在NSManagedObject子类中添加另一个属性。这个属性只会是另一个的第一个字母。

所以我发现这个主题，其中他们解释如何做，但：

• 这是一个很老的，所以我想知道如果没有任何更好的选择现在


• 它在Objective-C，我必须承认，我不真的掌握这种语言。

虽然我试图做他们说的做，但它不工作。

  class Currency：NSManagedObject {
 
 @NSManaged var country：String 
 @NSManaged var code：String 
 
 func firstLetterCodeForSection（） - >字符串{
 self.willAccessValueForKey（firstLetterCodeForSection）
 var tempString = self.valueForKey（code）！。uppercaseString 
 tempString.removeRange（Range（start：tempString.startIndex， （b）
 
 
 func firstLetterCountryForSection（） - > String {
 self.willAccessValueForKey（firstLetterCountryForSection）
 var tempString = self.valueForKey（country）！。uppercaseString 
 tempString.removeRange（Range（start：tempString.startIndex， （$ temp）
 
 
 
 
 
 
 
 
 
 
 
 / pre> 
 
 
我设置 firstLetterCountryForSection 和 firstLetterCodeForSection 在实体
 
 
 
这里是我的NSFetchedResultsController：
  fetchedResultsController = NSFetchedResultsController（fetchRequest：fetchRequest，managedObjectContext：managedContext，sectionNameKeyPath：firstLetterCountryForSection，cacheName：nil）
  
排序是基于代码还是国家。
 
 
 
 ... 
 
 
 
我错过了什么或做错了什么？
 
 
 
感谢您的帮助！
 
 
 
更新：
 
 
 
我发现问题，我是把第一个字母子字符串，我实际上保留除第一个字母之外的所有字母... 
 
 
 
我替换了
  tempString.removeRange（Range（start：tempString.startIndex，end：advance（tempString.startIndex，1）））
  
由
  tempString.removeRange（Range（start：advance tempString.startIndex，1），end：tempString.endIndex））
  
但现在我有另一个问题。我试过：
  func firstLetterCountryForSection（） - > String {} 
 
 var firstLetterCountryForSection：String {} 
 
动态var firstLetterCountryForSection：String？ {} 
  
，结果是一样的，它的工作（至少它看起来像）。有什么不同？ 
 
 
 
再次感谢
解决方案
当您应该使用动态变量时，尝试使用函数计算节：
  dynamic var section：String？ {
 let characters = Array（code.uppercaseString）
返回字符[0] 
} 
  
，然后您的 fetchedResultsController 应如下所示：
  fetchedResultsController = NSFetchedResultsController（fetchRequest：fetchRequest（），managedObjectContext：managedObjectContext !, sectionNameKeyPath：section，cacheName：root）
  
有关设置临时属性的详细信息，请参阅我的回答此处。
 
I'm using a NSFetchedResultsController for my table view and I would like to have an index A-Z. 

The problem is that if I use the entity's attribute for which I want to have section, it will return as many section as there are different names for this attribute. 

So I was thinking to add another attribute in my NSManagedObject subclass. This attribute would be only the first letter of the other one.

So I found out this topic where they explain how to do it but:


it's an pretty old one so I was wondering if there were not any better option now
it's in Objective-C and I have to confess that I don't really master this language.


Although I tried to do what they said to do but it didn't work.
class Currency: NSManagedObject {

    @NSManaged var country: String
    @NSManaged var code: String

    func firstLetterCodeForSection() -> String {
        self.willAccessValueForKey("firstLetterCodeForSection")
        var tempString = self.valueForKey("code")!.uppercaseString
        tempString.removeRange(Range(start: tempString.startIndex, end:advance(tempString.startIndex, 1)))
        self.didAccessValueForKey("firstLetterCodeForSection")
        return tempString
    }

    func firstLetterCountryForSection() -> String {
        self.willAccessValueForKey("firstLetterCountryForSection")
        var tempString = self.valueForKey("country")!.uppercaseString
        tempString.removeRange(Range(start: tempString.startIndex, end:advance(tempString.startIndex, 1)))
        self.didAccessValueForKey("firstLetterCountryForSection")
        return tempString
    }
}
I set firstLetterCountryForSection and firstLetterCodeForSection as transient in the entity

And here is my NSFetchedResultsController:
fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: managedContext, sectionNameKeyPath: "firstLetterCountryForSection", cacheName: nil)
The sort is based whether on code or on country.

And... it doesn't work...

Am I missing something or doing something wrong?

Thanks for your help!

UPDATE:

I found the problem, it came from the line where I was substringing the first letter, I was actually keeping all the letters except the first one...

I replaced 
tempString.removeRange(Range(start: tempString.startIndex, end:advance(tempString.startIndex, 1)))
by
tempString.removeRange(Range(start: advance(tempString.startIndex, 1), end:tempString.endIndex))
But now I have another questions. I tried it with:
func firstLetterCountryForSection() -> String { }

var firstLetterCountryForSection: String { }

dynamic var firstLetterCountryForSection: String? { }
and the result is the same, it's working (at least it looks like it). What's the difference? What's the best one to use?

Thanks again
解决方案
You're trying to use a function to calculate the section when you should be using a dynamic variable:
dynamic var section: String? {
    let characters = Array(code.uppercaseString)
    return characters[0]
}
and then your fetchedResultsController should look like this:
fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest(), managedObjectContext: managedObjectContext!, sectionNameKeyPath: "section", cacheName: "Root")
See my answer here for more info on setting up transient properties.

                        
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