如何使用 Core Data 在 UITableView 中维护显示顺序? [英] How can I maintain display order in UITableView using Core Data?

问题描述

在使用 UITableView 时，我在让我的核心数据实体正常播放和排序时遇到了一些麻烦.

I'm having some trouble getting my Core Data entities to play nice and order when using an UITableView.

我已经在 StackOverflow 上浏览了许多教程和其他问题，但似乎没有一种清晰或优雅的方法来做到这一点 - 我真的希望我错过了一些东西.

I've been through a number of tutorials and other questions here on StackOverflow, but there doesn't seem to be a clear or elegant way to do this - I'm really hoping I'm missing something.

我有一个核心数据实体，它有一个名为displayOrder"的 int16 属性.我使用已按displayOrder"排序的 NSFetchRequest 来返回 UITableView 的数据.除了重新排序之外，其他一切都受到尊重.这是我的(低效)moveRowAtIndePath 方法:

I have a single Core Data entity that has an int16 attribute on it called "displayOrder". I use an NSFetchRequest that has been sorted on "displayOrder" to return the data for my UITableView. Everything but reordering is being respected. Here is my (inefficient) moveRowAtIndePath method:

- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath {         

    NSUInteger fromIndex = fromIndexPath.row;  
    NSUInteger toIndex = toIndexPath.row;

    FFObject *affectedObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:fromIndex];  
    affectedObject.displayOrderValue = toIndex;

    [self FF_fetchResults];


    for (NSUInteger i = 0; i < [self.fetchedResultsController.fetchedObjects count]; i++) {  
        FFObject *otherObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:i];  
        NSLog(@"Updated %@ / %@ from %i to %i", otherObject.name, otherObject.state, otherObject.displayOrderValue, i);  
        otherObject.displayOrderValue = i;  
    }

    [self FF_fetchResults];  
}

谁能给我指出一些示例代码的方向，或者看看我做错了什么?tableview 显示更新正常，我可以通过我的日志消息看到 displayOrder 属性正在更新.它只是没有持续地保存和重新加载，而且这个实现的某些东西感觉非常偏离"(除了我所有 FFObject 的浪费性迭代).

Can anyone point me in the direction of a good bit of sample code, or see what I'm doing wrong? The tableview display updates OK, and I can see through my log messages that the displayOrder property is being updated. It's just not consistently saving and reloading, and something feels very "off" about this implementation (aside from the wasteful iteration of all of my FFObjects).

提前感谢您提供的任何建议.

Thanks in advance for any advice you can lend.

推荐答案

我查看了您的代码，这可能会更好:

I took a look at your code and this might work better:

- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath {         

    NSUInteger fromIndex = fromIndexPath.row;  
    NSUInteger toIndex = toIndexPath.row;

    if (fromIndex == toIndex) {
        return;
    }

    FFObject *affectedObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:fromIndex];  
    affectedObject.displayOrderValue = toIndex;

    NSUInteger start, end;
    int delta;

    if (fromIndex < toIndex) {
        // move was down, need to shift up
        delta = -1;
        start = fromIndex + 1;
        end = toIndex;
    } else { // fromIndex > toIndex
        // move was up, need to shift down
        delta = 1;
        start = toIndex;
        end = fromIndex - 1;
    }

    for (NSUInteger i = start; i <= end; i++) {
        FFObject *otherObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:i];  
        NSLog(@"Updated %@ / %@ from %i to %i", otherObject.name, otherObject.state, otherObject.displayOrderValue, otherObject.displayOrderValue + delta);  
        otherObject.displayOrderValue += delta;
    }

    [self FF_fetchResults];  
}

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