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iOS 7 Programming Cookbook by Vandad Nahavandipoor

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4.7. Moving Cells and Sections in Table Views

Problem

You want to move and shuffle cells and sections inside a table view, with smooth and intuitive animations.

Solution

Use the moveSection:toSection: method of the table view to move a section to a new position. You can also use the moveRowAtIndexPath:toIndexPath: method to move a table view cell from its current place to a new place.

Discussion

Moving table view cells and sections differs from exchanging them. Let’s have a look at an example that will make this easier to understand. Let’s say you have three sections in your table view: sections A, B, and C. If you move Section A to Section C, the table view will notice this move and will then shift Section B to the previous position of Section A, and will move Section C to the previous position of Section B. However, if Section B is moved to Section C, the table view will not have to move Section A at all, as it is sitting on top and doesn’t interfere with the repositioning of Section B and C. In this case, Section B will be moved to Section C and Section C to Section B. The same logic will be used by the table view when moving cells.

To demonstrate this, let’s create a table view and preload it with three sections, each of which contains three cells of its own. Let’s start with the implementation file of our view controller:

#import "ViewController.h"

static NSString *CellIdentifier = @"CellIdentifier";

@interface ViewController () <UITableViewDelegate, UITableViewDataSource>
@property (nonatomic, strong) UITableView *myTableView;
@property (nonatomic, strong) NSMutableArray *arrayOfSections;
@end

Our view controller will become the data source of the table view. The table view has sections, and each section has rows. We will keep an array of arrays; the first array is our array of sections, which will itself contain other arrays that contain our cells. The arrayOfSections defined on top of the implementation file of our view controller will bear that responsibility. Let’s go ahead and populate this array:

- (NSMutableArray *) newSectionWithIndex:(NSUInteger)paramIndex
                               cellCount:(NSUInteger)paramCellCount{

    NSMutableArray *result = [[NSMutableArray alloc] init];

    NSUInteger counter = 0;
    for (counter = 0;
         counter < paramCellCount;
         counter++){

        [result addObject:[[NSString alloc] initWithFormat:@"Section %lu Cell %lu",
                           (unsigned long)paramIndex,
                           (unsigned long)counter+1]];

    }

    return result;

}

- (NSMutableArray *) arrayOfSections{
    if (_arrayOfSections == nil){
        NSMutableArray *section1 = [self newSectionWithIndex:1
                                                   cellCount:3];
        NSMutableArray *section2 = [self newSectionWithIndex:2
                                                   cellCount:3];
        NSMutableArray *section3 = [self newSectionWithIndex:3
                                                   cellCount:3];

        _arrayOfSections = [[NSMutableArray alloc] initWithArray:@[
                                                                   section1,
                                                                   section2,
                                                                   section3
                                                                   ]
                            ];
    }
    return _arrayOfSections;
}

We shall then instantiate our table view and implement the necessary methods in the UITableViewDataSource protocol to populate our table view with data:

- (NSInteger) numberOfSectionsInTableView:(UITableView *)tableView{

    return self.arrayOfSections.count;

}

- (NSInteger) tableView:(UITableView *)tableView
  numberOfRowsInSection:(NSInteger)section{

    NSMutableArray *sectionArray = self.arrayOfSections[section];
    return sectionArray.count;

}

- (UITableViewCell *)tableView:(UITableView *)tableView
         cellForRowAtIndexPath:(NSIndexPath *)indexPath{

    UITableViewCell *cell = nil;


    cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier
                                           forIndexPath:indexPath];


    NSMutableArray *sectionArray = self.arrayOfSections[indexPath.section];

    cell.textLabel.text = sectionArray[indexPath.row];

    return cell;

}

- (void)viewDidLoad{
    [super viewDidLoad];

    self.myTableView =
    [[UITableView alloc] initWithFrame:self.view.bounds
                                 style:UITableViewStyleGrouped];

    [self.myTableView registerClass:[UITableViewCell class]
             forCellReuseIdentifier:CellIdentifier];

    self.myTableView.autoresizingMask =
        UIViewAutoresizingFlexibleWidth |
        UIViewAutoresizingFlexibleHeight;

    self.myTableView.delegate = self;
    self.myTableView.dataSource = self;

    [self.view addSubview:self.myTableView];

}

Showtime! Shall we first have a look at how sections can be moved to a new position? Let’s write a method that will move Section 1 to Section 3:

- (void) moveSection1ToSection3{

    NSMutableArray *section1 = self.arrayOfSections[0];
    [self.arrayOfSections removeObject:section1];
    [self.arrayOfSections addObject:section1];

    [self.myTableView moveSection:0
                        toSection:2];

}

I will leave it up to you to decide when you would like to invoke this method, as we don’t have a button on our UI at the moment. You can simply create a navigation controller, place a navigation button on it, and then invoke this method.

Once you run the app normally, you will see the sections lined up from 1 to 3, as in Figure 4-12.

A table view with three sections, each containing three cells

Figure 4-12. A table view with three sections, each containing three cells

After you invoke the moveSection1ToSection3 method, you will see that Section 1 gets moved to Section 3, Section 3 moves to Section 2’s previous position, and finally Section 2 moves to Section 1’s previous position (Figure 4-13).

Section 1 is moved to Section 3, and other sections are subsequently moved as well

Figure 4-13. Section 1 is moved to Section 3, and other sections are subsequently moved as well

Moving cells is very similar to moving sections. To move cells, all we have to do is use the moveRowAtIndexPath:toIndexPath: method. Remember that you can move a cell from one section to the same section, or to a new section. Let’s make it easy and move Cell 1 in Section 1 to Cell 2 in the same section and see what happens:

- (void) moveCell1InSection1ToCell2InSection1{

    NSMutableArray *section1 = self.arrayOfSections[0];
    NSString *cell1InSection1 = section1[0];

    [section1 removeObject:cell1InSection1];

    [section1 insertObject:cell1InSection1
                   atIndex:1];

    NSIndexPath *sourceIndexPath = [NSIndexPath indexPathForRow:0
                                                      inSection:0];
    NSIndexPath *destinationIndexPath = [NSIndexPath indexPathForRow:1
                                                           inSection:0];

    [self.myTableView moveRowAtIndexPath:sourceIndexPath
                             toIndexPath:destinationIndexPath];

}

So what is going on in this code? Well, we need to make sure our data source holds the correct data that needs to be displayed in our table view after we have moved the cells around, so we remove Cell 1 in Section 1 first. That moves Cell 2 to Cell 1, and Cell 3 to Cell 2, with a total of 2 cells in the array. Then we will insert Cell 1 into Index 1 (second object) of the array. That will make our array contain Cell 2, Cell 1, and then Cell 3. After that is done, we have actually moved the cells in our table view.

Let’s make this a bit more difficult. How about moving Cell 2 in Section 1 to Cell 1 in Section 2?

- (void) moveCell2InSection1ToCell1InSection2{

    NSMutableArray *section1 = self.arrayOfSections[0];
    NSMutableArray *section2 = self.arrayOfSections[1];

    NSString *cell2InSection1 = section1[1];
    [section1 removeObject:cell2InSection1];

    [section2 insertObject:cell2InSection1
                   atIndex:0];

    NSIndexPath *sourceIndexPath = [NSIndexPath indexPathForRow:1
                                                      inSection:0];
    NSIndexPath *destinationIndexPath = [NSIndexPath indexPathForRow:0
                                                           inSection:1];

    [self.myTableView moveRowAtIndexPath:sourceIndexPath
                             toIndexPath:destinationIndexPath];

}

The results of this transition are shown in Figure 4-14.

Cell 2 in Section 1 is moved to Cell 1 in Section 2

Figure 4-14. Cell 2 in Section 1 is moved to Cell 1 in Section 2

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