創(chuàng)建NSFetchRequest實例，然后對其進行一些必要的配置，再把它傳遞給contex對象

1.初始化FetchRequest的四中方式

//1根據(jù)實體名稱創(chuàng)建實體 并用將其賦值給NSFetchRequest的實體
let fetchRequest = NSFetchRequest()
let entity =  NSEntityDescription.entityForName("Person",inManagedObjectContext: managedObjectContext)!
fetchRequest.entity = entity

//2直接用實體名稱進項初始化
let fetchRequest = NSFetchRequest(entityName: "Person")

//3檢索NSManagedObjectModel中的fetchRequest，可以重新設(shè)置之前使用過的fetchRequest
let fetchRequest = managedObjectModel.fetchRequestTemplateForName("data")

//4與上面的方法類似，substitutionVariab中的值用來刪選request的結(jié)果
let fetchRequest = managedObjectModel.fetchRequestFromTemplateWithName("data",substitutionVariables: ["NAME" :"Test"])

2.在CoreData中添加fetchRequest

屏幕快照 2016-01-11 下午3.50.05.png

注：點擊加號可以添加篩選條件

//上面的第三種初始化NSFetchRequest的方式，coreDataStack為CoreData(二)中創(chuàng)建的stack。
var fetchRequest: NSFetchRequest!
 var persons: [Person]!
let model = coreDataStack.context.persistentStoreCoordinator!.managedObjectModel
fetchRequest = model.fetchRequestTemplateForName("FetchRequest")
do { 
       //結(jié)果保存在persons中
       persons = try coreDataStack.context .executeFetchRequest(fetchRequest) as! [Persion]
        } catch let error as NSError {
            print("Could not fetch \(error), \(error.userInfo)")
        }

3.根據(jù)type返回不同的搜索結(jié)果

//NSManagedObjectResultType返回模型
//NSCountResultType返回滿足要求的結(jié)果的個數(shù)
//NSDictionaryResultType保存為字典類型
//NSManagedObjectIDResultType返回唯一的標識


//篩選條件。 NSPredicate支持keypath
lazy var PersonPredicate: NSPredicate = {
            var predicate =
            NSPredicate(format: "person.name == %@", "hehe")
            return predicate
        }()

let fetchRequest = NSFetchRequest(entityName: "Person")
//.CountResultType類型 返回滿足要求的結(jié)果的數(shù)目
fetchRequest.resultType = .CountResultType
fetchRequest.predicate = PersonPredicate
        do {
            let results =try coreDataStack.context
            .executeFetchRequest(fetchRequest) as! [NSNumber]
            
            } catch let error as NSError {
                
            print("Could not fetch \(error), \(error.userInfo)")
                
            }
    }

4.其他的 fetch requests

let fetchRequest = NSFetchRequest(entityName: "Person")
fetchRequest.resultType = .DictionaryResultType

let sumExpression = NSExpressionDescription()
sumExpression.name = "sumAges"

//求和："sum:"、最小值:"min:"、最大值："max:"、平均值："average:"等
sumExpression.expression = NSExpression(forFunction: "sum:",arguments:[NSExpression(forKeyPath: "age")])
sumExpression.expressionResultType = .Integer32AttributeType

fetchRequest.propertiesToFetch = [sumExpression]

do {
let results = try coreDataStack.context.executeFetchRequest(fetchRequest) as! [NSDictionary]
let resultDict = results.first!
} catch let error as NSError {
print("Could not fetch (error), (error.userInfo)")
}
}

//條件判斷
 lazy var agePredicate: NSPredicate = {
            var predicate = NSPredicate(format: "age > 0")
            return predicate
        }()