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

我在使用UITableView时会遇到一些麻烦，让我的Core Data实体播放不错和顺序。

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

我已经完成了许多教程和其他问题在这里StackOverflow，但似乎没有一个明确或优雅的方式来做到这一点 - 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.

我有一个具有int16属性的单个Core Data实体，称为displayOrder。我使用一个NSFetchRequest已经排序在displayOrder返回我的UITableView的数据。除了重新排序之外的一切都受到尊重。这里是我的（无效）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];  
}

任何人都可以指向一个好的示例代码，看看我做错了什么？ tableview显示更新确定，我可以通过我的日志消息看到displayOrder属性正在更新。它只是不是一贯的保存和重新加载，并且有些东西感觉非常关闭这个实现（除了所有的FFObjects的浪费的迭代）。

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).

推荐答案

我看了你的代码，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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