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  • core data操作

     

        AppDelegate *appDelegate =

    [[UIApplicationsharedApplication] delegate];

        NSManagedObjectContext *context = [appDelegate managedObjectContext];

       

       

        //classes

        NSEntityDescription *entityDescriptionForClass = [NSEntityDescription

      entityForName:@"ClassEntity"

      inManagedObjectContext:context];

        NSFetchRequest *requestForClass = [[NSFetchRequestalloc] init];

        [requestForClass setEntity:entityDescriptionForClass];

        NSError *error;

        NSArray *objectsForClass = [context executeFetchRequest:requestForClass error:&error];

        if (objectsForClass == nil) {

            NSLog(@"There was an error in my Zander App!");

            // Do whatever error handling is appropriate 可以在这里填写 发生存取错误的时候 处理的内容

        }

    //初始化一个entity对象 

     StudentEntity *student1= [[StudentEntity alloc]initWithEntity:entityDescription insertIntoManagedObjectContext:context];

     student1.name = @"student1_1";

     student1.student_class = class1;

     

    //将set 转化成 可变数组mutablearray

     NSSet *set = selectedClass.class_student;

        NSSortDescriptor *sort = [NSSortDescriptorsortDescriptorWithKey:@"index"ascending:YES];

        

        NSArray *array = [set sortedArrayUsingDescriptors:[NSArray  arrayWithObject:sort]];    

        

        NSMutableArray *studentsArray = [NSMutableArrayarrayWithArray:array] ;

     

      

     

     

    //降序取数据的 NSFetchRequest 声明

        NSFetchRequest *fetchRequest2 =[[NSFetchRequestalloc]init];

        NSEntityDescription *entity = [NSEntityDescriptionentityForName:@"EntityName" inManagedObjectContext:context];

        [fetchRequest2 setEntity:entity];

        [fetchRequest2 setSortDescriptors:[NSArrayarrayWithObject:[NSSortDescriptorsortDescriptorWithKey:@"displayOrder"ascending:NO]]];

     

     

    主要用的就是这个 [fetchRequest2 setSortDescriptors:[NSArrayarrayWithObject:[NSSortDescriptorsortDescriptorWithKey:@"displayOrder"ascending:NO]]];

    设置这个数据库取数据的请求 1. 按照那个字段  (displayOrder)   2. 排序:升序还是降序  YES (升序) NO(降序)

        

     

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  • 原文地址:https://www.cnblogs.com/zander/p/2591133.html
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