ios - 使用谓词查询核心数据
<p><p>所以我正在使用 CoreData 构建这个应用程序。</p>
<p>我拥有的两个实体是列表和项目。它们有一对多的关系，即一个列表可以有多个项目。</p>
<p>例如:List1 有 Items: item1, item2</p>
<p>我已经编写了将项目存储在特定列表中的代码，但我很难弄清楚如何从特定列表中获取和处理项目。</p>
<p>到目前为止我所做的如下</p>
<pre><code>func getItemsOnList(){
let app = UIApplication.shared.delegate as! AppDelegate
let context = app.persistentContainer.viewContext
//fetchRequest to get the List
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "List")
let predicate = NSPredicate(format: "title == %@", listName)
fetchRequest.returnsObjectsAsFaults = false
fetchRequest.predicate = predicate
if let fetchResults = try? context.fetch(fetchRequest){
if fetchResults.count > 0 {
for listEntity in fetchResults {
let list = listEntity as! List
print(list.title as Any)
itemsOnList = list.contains!
print(itemsOnList)
print("The list with name:\(list.title)has \(itemsOnList.count) items")
}
}
}
}
</code></pre>
<p>此函数返回一个 NSSet，它假定包含该特定列表中的所有项目。</p>
<p>我的数据模型是:</p>
<p> <a href="/image/2cVrL.png" rel="noreferrer noopener nofollow"><img src="/image/2cVrL.png" alt="Data model"/></a> </p>
<p>我的问题是:</p>
<p>A.我编写 getItemsOnList() 函数的方式是否正确？还是我做错了什么。</p>
<p>B.鉴于代码是正确的，并且我得到的 NSSet 对所有项目都是正确的，我将如何获取该 NSSet 中的每个项目以便将其放在 TableView 上。</p></p>
<br><hr><h1><strong>Best Answer-推荐答案</ strong></h1><br>
<p><pre><code> func getItemsWithFilter(filterQuery:NSPredicate,sortBy:String?,order:Bool) -> Array<Items> {
var fetchedResults:Array<Items> = Array<Items>()
let fetchRequest = NSFetchRequest(entityName: "Items")
fetchRequest.predicate = filterQuery
if sortBy != nil{
let sortDescriptor = NSSortDescriptor(key:sortBy! ,
ascending:order )
let sortDescriptors =
fetchRequest.sortDescriptors = sortDescriptors
}
//Execute Fetch request you can go with your approach to
do {
fetchedResults = try self.mainContextInstance.executeFetchRequest(fetchRequest) as!
} catch let fetchError as NSError {
print("retrieveById error: \(fetchError.localizedDescription)")
fetchedResults = Array<Items>()
}catch {
fetchedResults = Array<Items>()
}
return fetchedResults
</code></pre>
<p>}</p>
<p>要调用此方法，您可以将谓词中的列表项作为查询传递给 <strong>fetch Items in which List.id == XXX</strong></p>
<pre><code> let predicate = NSPredicate(format: "ANY list.name in %@", name)
let myResult = self.getItemsWithFilter(predicate,sortBy:nil,order:false)
</code></pre></p>
<p style="font-size: 20px;">关于ios - 使用谓词查询核心数据，我们在Stack Overflow上找到一个类似的问题：
<a href="https://stackoverflow.com/questions/41257278/" rel="noreferrer noopener nofollow" style="color: red;">
https://stackoverflow.com/questions/41257278/
</a>
</p>
页:
[1]