Obj-C- 更新 UICollectionView 数据而不重新加载整个 collectionView？

Question

I have a UICollectionView where when cells are selected, they change to a color selected by the user. To paint the full picture: The color is selected by the user from a color wheel (UIImageView) with a tap gesture attached to it.

That said, when the user taps a new color, say purple (and resets the defined rString, bString & gString...) after selecting 3 cells and making them green, I want to reload the color they're using without wiping the initial 3 selected green cells from the Collection View. How can I accomplish this?

See code below.

ViewController.m

  @interface ViewController () {
        
        CGPoint lastPoint;
        NSInteger rString;
        NSInteger bString;
        NSInteger gString;
        UIColor *colour;
        
    }
    
    @property (strong, nonatomic, nullable) NSIndexPath *trackingCellIndexPath;
    
    
    @end
    
    @implementation ViewController
    
    
    
    - (void)viewDidLoad {
        [super viewDidLoad];
      
        self.ringCollectionView.allowsMultipleSelection = YES;
    
    UITapGestureRecognizer * tapRecognizer = [[UITapGestureRecognizer alloc] 
     initWithTarget:self action:@selector(tapGesture:)];
     [self.colorWheel addGestureRecognizer:tapRecognizer];
     self.colorWheel.userInteractionEnabled = YES;

    }
    
    - (__kindof UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
    {
        
        UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"RingCollectionViewCell" forIndexPath:indexPath];
     
        if (!cell.selectedBackgroundView) {
            cell.selectedBackgroundView = [[UIView alloc] initWithFrame:cell.bounds];
                   cell.selectedBackgroundView.backgroundColor = [UIColor grayColor];
        } else {
            cell.selectedBackgroundView = [[UIView alloc] initWithFrame:cell.bounds];
            cell.selectedBackgroundView.backgroundColor = [UIColor colorWithRed:rString/255.0 green:gString/255.0 blue:bString/255.0 alpha:1.0];
            
        }
      
        
        if ((indexPath.row >=9 && indexPath.row <=14) || ((indexPath.row >=17 && indexPath.row < 23) || (indexPath.row >=25 && indexPath.row <=30) ||  (indexPath.row >=33 && indexPath.row <=38))) {
            
            NSLog(@"NOT AVAILABLE SORRY");
            cell.backgroundColor = [UIColor whiteColor];
            
            [cell setUserInteractionEnabled:NO];
            
        }
    
            return cell;
       
    
    }
    
    
    -(void)tapGesture:(UITapGestureRecognizer *)recognizer  {
        
        CGPoint location = [recognizer locationInView:recognizer.view];
        CGPoint p = { round(location.x), round(location.y) };
        _colorView.backgroundColor = [self colorInViewAtPoint:p];
        
       
        UIColor *mylovelycolor = [self colorInViewAtPoint:p];
         
      
           const CGFloat *components = CGColorGetComponents(mylovelycolor.CGColor);
           NSLog(@"Red: %f", components[0]);
           NSLog(@"Green: %f", components[1]);
           NSLog(@"Blue: %f", components[2]);
           NSLog(@"Alpha: %f", CGColorGetAlpha(mylovelycolor.CGColor));
           
    
               int red = components[0] * 255;
               int green = components[1] * 255;
               int blue = components[2] * 255;
           
               NSString *red1 = [@(red) stringValue];
               NSString *green1 = [@(green) stringValue];
               NSString *blue1 = [@(blue) stringValue];
               
               
               NSInteger redInt = [red1 integerValue];
               NSInteger greenInt = [green1 integerValue];
               NSInteger blueInt = [blue1 integerValue];
               
                 rString = [red1 integerValue];
                 bString = [blue1 integerValue];
                 gString =  [green1 integerValue];
        
    
                self.redValue.text = red1;
                self.greenValue.text = green1;
                self.blueValue.text = blue1;
        
                
                        NSMutableString *str1 = [NSMutableString string];
                        for(NSInteger numberCopy = redInt; numberCopy > 0; numberCopy >>= 1)
                        {
                         
                            [str1 insertString:((numberCopy & 1) ? @"1" : @"0") atIndex:0];
                        }
    
                      
                        NSMutableString *str2 = [NSMutableString string];
                        for(NSInteger numberCopy = greenInt; numberCopy > 0; numberCopy >>= 1)
                        {
                         
                            [str2 insertString:((numberCopy & 1) ? @"1" : @"0") atIndex:0];
                        }
    
                     
                        NSMutableString *str3 = [NSMutableString string];
                        for(NSInteger numberCopy = blueInt; numberCopy > 0; numberCopy >>= 1)
                        {
                           
                            [str3 insertString:((numberCopy & 1) ? @"1" : @"0") atIndex:0];
                        }
    
        
                        self.binaryString = [NSString stringWithFormat:@" %@ %@ %@", str1, str2, str3];
        
    }

Answer 1

You need to track your user-selected colors in your data model.

In cellForItemAtIndexPath you want to set the cell's background color (or whatever element you're using) to the data element color.

When the user has one or more cells selected, and taps your "colorWheel," update your data model and then either set the cell elements directly or reload those cells.

Here is a very simple example...

MyDataObject.h

//
//  MyDataObject.h
//

#import <UIKit/UIKit.h>

NS_ASSUME_NONNULL_BEGIN

@interface MyDataObject : NSObject
@property (strong, nonatomic) UIColor *userColor;
@end

NS_ASSUME_NONNULL_END

MyDataObject.m

//
//  MyDataObject.m
//

#import "MyDataObject.h"

@implementation MyDataObject

@end

MyCollectionViewCell.h

//
//  MyCollectionViewCell.h
//

#import <UIKit/UIKit.h>

NS_ASSUME_NONNULL_BEGIN

@interface MyCollectionViewCell : UICollectionViewCell
@property (strong, nonatomic) UILabel *label;
@end

NS_ASSUME_NONNULL_END

MyCollectionViewCell.m

//
//  MyCollectionViewCell.m
//

#import "MyCollectionViewCell.h"

@implementation MyCollectionViewCell

- (instancetype)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        [self commonInit];
    }
    return self;
}
- (instancetype)initWithCoder:(NSCoder *)coder
{
    self = [super initWithCoder:coder];
    if (self) {
        [self commonInit];
    }
    return self;
}
- (void)commonInit {
    _label = [UILabel new];
    _label.textAlignment = NSTextAlignmentCenter;
    _label.backgroundColor = [UIColor colorWithWhite:0.9 alpha:1.0];
    _label.translatesAutoresizingMaskIntoConstraints = NO;
    [self.contentView addSubview:_label];

    UILayoutGuide *g = [self.contentView layoutMarginsGuide];
    [NSLayoutConstraint activateConstraints:@[
        [_label.widthAnchor constraintEqualToAnchor:g.widthAnchor multiplier:0.8],
        [_label.heightAnchor constraintEqualToAnchor:g.heightAnchor multiplier:0.8],
        [_label.centerXAnchor constraintEqualToAnchor:g.centerXAnchor],
        [_label.centerYAnchor constraintEqualToAnchor:g.centerYAnchor],
    ]];
    self.contentView.layer.borderColor = [UIColor yellowColor].CGColor;
}
- (void)setSelected:(BOOL)selected {
    [super setSelected:selected];
    self.contentView.layer.borderWidth = selected ? 2 : 0;
}
@end

MyTestViewController.h

//
//  MyTestViewController.h
//

#import <UIKit/UIKit.h>

NS_ASSUME_NONNULL_BEGIN

@interface MyTestViewController : UIViewController <UICollectionViewDataSource, UICollectionViewDelegate>

@end

NS_ASSUME_NONNULL_END

MyTestViewController.m

//
//  MyTestViewController.m
//

#import "MyTestViewController.h"
#import "MyCollectionViewCell.h"
#import "MyDataObject.h"

@interface MyTestViewController ()
{
    NSMutableArray <MyDataObject *>*myCellData;
    UICollectionView *collectionView;
}
@end

@implementation MyTestViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    UICollectionViewFlowLayout *fl = [UICollectionViewFlowLayout new];
    fl.itemSize = CGSizeMake(50, 50);
    fl.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    
    collectionView = [[UICollectionView alloc] initWithFrame:CGRectZero collectionViewLayout:fl];
    collectionView.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:collectionView];
    
    UILayoutGuide *g = [self.view safeAreaLayoutGuide];
    [NSLayoutConstraint activateConstraints:@[
        [collectionView.leadingAnchor constraintEqualToAnchor:g.leadingAnchor constant:20.0],
        [collectionView.trailingAnchor constraintEqualToAnchor:g.trailingAnchor constant:-20.0],
        [collectionView.heightAnchor constraintEqualToConstant:240.0],
        [collectionView.centerYAnchor constraintEqualToAnchor:g.centerYAnchor],
    ]];
    
    // a few color views to tap, and an
    //  "Instructions" label
    UILabel *label = [UILabel new];
    label.text = @"Tap a color to change the selected cells:";
    
    UIStackView *stack = [UIStackView new];
    
    NSArray *colors = @[
        [UIColor redColor],
        [UIColor greenColor],
        [UIColor blueColor],
        [UIColor systemYellowColor],
        [UIColor systemTealColor],
    ];
    
    for (UIColor *c in colors) {
        UIView *v = [UIView new];
        v.backgroundColor = c;
        UITapGestureRecognizer *t = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(gotTap:)];
        [v addGestureRecognizer:t];
        [stack addArrangedSubview:v];
    }
    
    stack.spacing = 20.0;
    stack.distribution = UIStackViewDistributionFillEqually;
    
    label.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:label];
    
    stack.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:stack];
    
    [NSLayoutConstraint activateConstraints:@[
        [stack.leadingAnchor constraintEqualToAnchor:g.leadingAnchor constant:20.0],
        [stack.trailingAnchor constraintEqualToAnchor:g.trailingAnchor constant:-20.0],
        [stack.bottomAnchor constraintEqualToAnchor:g.bottomAnchor constant:-20.0],
        [stack.heightAnchor constraintEqualToConstant:40.0],

        [label.leadingAnchor constraintEqualToAnchor:g.leadingAnchor constant:20.0],
        [label.trailingAnchor constraintEqualToAnchor:g.trailingAnchor constant:-20.0],
        [label.bottomAnchor constraintEqualToAnchor:stack.topAnchor constant:-4.0],
    ]];

    collectionView.dataSource = self;
    collectionView.delegate = self;
    
    collectionView.allowsMultipleSelection = YES;
    
    [collectionView registerClass:MyCollectionViewCell.class forCellWithReuseIdentifier:@"c"];
    
    // create 50 objects for our data
    myCellData = [NSMutableArray new];
    for (int i = 0; i < 50; i++) {
        MyDataObject *obj = [MyDataObject new];
        obj.userColor = [UIColor redColor];
        [myCellData addObject:obj];
    }
    
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
    return myCellData.count;
}
- (__kindof UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    
    // dequeue a cell
    MyCollectionViewCell *c = (MyCollectionViewCell *)[collectionView dequeueReusableCellWithReuseIdentifier:@"c" forIndexPath:indexPath];
    
    // get the data object
    MyDataObject *obj = (MyDataObject *)[myCellData objectAtIndex:indexPath.item];
    
    // set cell's contentView.backgroundColor to the data object's .userColor
    c.contentView.backgroundColor = obj.userColor;
    
    // set the cell's label text
    c.label.text = [NSString stringWithFormat:@"%ld", indexPath.item];
    
    return c;

}

- (void)gotTap:(UITapGestureRecognizer *)g {
    
    if (collectionView.indexPathsForSelectedItems.count == 0) {
        UIAlertController * alert = [UIAlertController
                                     alertControllerWithTitle:@"Error"
                                     message:@"No cells are selected!"
                                     preferredStyle:UIAlertControllerStyleAlert];
        
        UIAlertAction* okButton = [UIAlertAction
                                   actionWithTitle:@"OK"
                                   style:UIAlertActionStyleDefault
                                   handler:^(UIAlertAction * action) {
        }];
        
        [alert addAction:okButton];
        
        [self presentViewController:alert animated:YES completion:nil];

        return;
    }
    
    UIView *v = g.view;
    if (v) {
        // get the background color from the tapped view
        UIColor *color = v.backgroundColor;
        
        // loop through selected cells
        for (NSIndexPath *p in collectionView.indexPathsForSelectedItems) {
            
            // update the object in our data
            [myCellData objectAtIndex:p.item].userColor = color;
            
            // get a reference to the cell
            MyCollectionViewCell *c = (MyCollectionViewCell *)[collectionView cellForItemAtIndexPath:p];
            
            // set its background color
            c.contentView.backgroundColor = color;
            
            // if we want to auto-deselect the cells
            [collectionView deselectItemAtIndexPath:p animated:YES];
        }
    }
}

@end

So,

• our data object has just a single property: userColor
• our cell class has a centered label
• our controller
  • creates a horizontal scrolling collection view
  • creates an array of 50 data objects, with default userColor of red
  • adds 5 color views to select from

When a cell is selected, it will be outlined in yellow. When a color view is tapped, we:

• update the data model for the currently selected cells
• set the background color of the contentView of the currently selected cells
• deselect the currently selected cells

It looks like this:

then we select cells 5, 9, 14:

tap on the Green view:

then we select cells 16, 17, 18:

tap on the Blue view:

then we scroll a little and select cells 17, 21, 24, 25, 26:

tap on the Yellow view:

and so on.