iOS开发UICollectionView实现拖拽效果

一.介绍

iOS9提供API实现单元格排序功能，使用UICollectionView及其代理方法。iOS9之后有自带方法可以实现该效果，只需添加长按手势，实现手势方法和调用iOS9的API交换数据，iOS9之前需要自己写方法实现这效果，除了要添加长按手势，这里还需要利用截图替换原理，手动计算移动位置来处理视图交换和数据交换。

二.方法和步骤

1.创建工程项目和视图控制器,如下图

2.声明对象和设置代理和数据源代理

@interface ViewController ()<UICollectionViewDelegate,UICollectionViewDataSource,UICollectionViewDelegateFlowLayout>

@property (nonatomic, strong) NSMutableArray *dataArr;
@property (nonatomic, strong) UICollectionView *collectionView;
/**之前选中cell的NSIndexPath*/
@property (nonatomic, strong) NSIndexPath *oldIndexPath;
/**单元格的截图*/
@property (nonatomic, strong) UIView *snapshotView;
/**之前选中cell的NSIndexPath*/
@property (nonatomic, strong) NSIndexPath *moveIndexPath;

@end

3.初始化UICollectionView,并添加长按手势,在viewDidLoad中初始化

CGFloat SCREEN_WIDTH = self.view.frame.size.width;
  UICollectionViewFlowLayout *flowLayout = [[UICollectionViewFlowLayout alloc] init];
  flowLayout.itemSize = CGSizeMake((SCREEN_WIDTH-40.0)/3, (SCREEN_WIDTH-40.0)/3);
  UICollectionView *collectionView = [[UICollectionView alloc] initWithFrame:CGRectMake(0, 50.0, SCREEN_WIDTH, (SCREEN_WIDTH-40.0)/3+20.0) collectionViewLayout:flowLayout];
  collectionView.dataSource = self;
  collectionView.delegate = self;
  collectionView.backgroundColor = [UIColor whiteColor];
  [collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"uicollectionviewcell"];
  [self.view addSubview:self.collectionView = collectionView];

  // 添加长按手势
  UILongPressGestureRecognizer *longPress = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(handlelongGesture:)];
  [collectionView addGestureRecognizer:longPress];

4.实例化数据源,(50个随机颜色,透明度0.8),在viewDidLoad中初始化

self.dataArr = [[NSMutableArray alloc] init];
for (NSInteger index = 0; index < 50; index ++) {
    CGFloat hue = (arc4random()%256/256.0); //0.0 到 1.0
    CGFloat saturation = (arc4random()%128/256.0)+0.5; //0.5 到 1.0
    CGFloat brightness = (arc4random()%128/256.0)+0.5; //0.5 到 1.0
    UIColor *color = [UIColor colorWithHue:hue saturation:saturation brightness:brightness alpha:0.5];
    [self.dataArr addObject:color];
  }

5.实现UICollectionView的UICollectionViewDataSource的两个必须实现的方法

#pragma mark - UICollectionViewDataSource
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
  return self.dataArr.count;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
  UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"uicollectionviewcell" forIndexPath:indexPath];
  cell.backgroundColor = self.dataArr[indexPath.row];
  return cell;
}

6.重点来了,实现长按手势方法

#pragma mark - 长按手势
- (void)handlelongGesture:(UILongPressGestureRecognizer *)longPress
{
  if ([[[UIDevice currentDevice] systemVersion] floatValue] < 9.0) {
    [self action:longPress];
  } else {
    [self iOS9_Action:longPress];
  }
}

7.iOS9之后的实现

#pragma mark - iOS9 之后的方法
- (BOOL)collectionView:(UICollectionView *)collectionView canMoveItemAtIndexPath:(NSIndexPath *)indexPath
{
  // 返回YES允许row移动
  return YES;
}

- (void)collectionView:(UICollectionView *)collectionView moveItemAtIndexPath:(NSIndexPath *)sourceIndexPath toIndexPath:(NSIndexPath *)destinationIndexPath
{
  //取出移动row数据
  id color = self.dataArr[sourceIndexPath.row];
  //从数据源中移除该数据
  [self.dataArr removeObject:color];
  //将数据插入到数据源中的目标位置
  [self.dataArr insertObject:color atIndex:destinationIndexPath.row];
}

- (void)iOS9_Action:(UILongPressGestureRecognizer *)longPress
{
  switch (longPress.state) {
    case UIGestureRecognizerStateBegan:
    { //手势开始
      //判断手势落点位置是否在row上
      NSIndexPath *indexPath = [self.collectionView indexPathForItemAtPoint:[longPress locationInView:self.collectionView]];
      if (indexPath == nil) {
        break;
      }
      UICollectionViewCell *cell = [self.collectionView cellForItemAtIndexPath:indexPath];
      [self.view bringSubviewToFront:cell];
      //iOS9方法 移动cell
      [self.collectionView beginInteractiveMovementForItemAtIndexPath:indexPath];
    }
      break;
    case UIGestureRecognizerStateChanged:
    { // 手势改变
      // iOS9方法 移动过程中随时更新cell位置
      [self.collectionView updateInteractiveMovementTargetPosition:[longPress locationInView:self.collectionView]];
    }
      break;
    case UIGestureRecognizerStateEnded:
    { // 手势结束
      // iOS9方法 移动结束后关闭cell移动
      [self.collectionView endInteractiveMovement];
    }
      break;
    default: //手势其他状态
      [self.collectionView cancelInteractiveMovement];
      break;
  }
}

8.iOS9之前的实现

#pragma mark - iOS9 之前的方法
- (void)action:(UILongPressGestureRecognizer *)longPress
{
  switch (longPress.state) {
    case UIGestureRecognizerStateBegan:
    { // 手势开始
      //判断手势落点位置是否在row上
      NSIndexPath *indexPath = [self.collectionView indexPathForItemAtPoint:[longPress locationInView:self.collectionView]];
      self.oldIndexPath = indexPath;
      if (indexPath == nil) {
        break;
      }
      UICollectionViewCell *cell = [self.collectionView cellForItemAtIndexPath:indexPath];
      // 使用系统的截图功能,得到cell的截图视图
      UIView *snapshotView = [cell snapshotViewAfterScreenUpdates:NO];
      snapshotView.frame = cell.frame;
      [self.view addSubview:self.snapshotView = snapshotView];
      // 截图后隐藏当前cell
      cell.hidden = YES;

      CGPoint currentPoint = [longPress locationInView:self.collectionView];
      [UIView animateWithDuration:0.25 animations:^{
        snapshotView.transform = CGAffineTransformMakeScale(1.05, 1.05);
        snapshotView.center = currentPoint;
      }];
    }
      break;
    case UIGestureRecognizerStateChanged:
    { // 手势改变
      //当前手指位置 截图视图位置随着手指移动而移动
      CGPoint currentPoint = [longPress locationInView:self.collectionView];
      self.snapshotView.center = currentPoint;
      // 计算截图视图和哪个可见cell相交
      for (UICollectionViewCell *cell in self.collectionView.visibleCells) {
        // 当前隐藏的cell就不需要交换了,直接continue
        if ([self.collectionView indexPathForCell:cell] == self.oldIndexPath) {
          continue;
        }
        // 计算中心距
        CGFloat space = sqrtf(pow(self.snapshotView.center.x - cell.center.x, 2) + powf(self.snapshotView.center.y - cell.center.y, 2));
        // 如果相交一半就移动
        if (space <= self.snapshotView.bounds.size.width / 2) {
          self.moveIndexPath = [self.collectionView indexPathForCell:cell];
          //移动 会调用willMoveToIndexPath方法更新数据源
          [self.collectionView moveItemAtIndexPath:self.oldIndexPath toIndexPath:self.moveIndexPath];
          //设置移动后的起始indexPath
          self.oldIndexPath = self.moveIndexPath;
          break;
        }
      }
    }
      break;
    default:
    { // 手势结束和其他状态
      UICollectionViewCell *cell = [self.collectionView cellForItemAtIndexPath:self.oldIndexPath];
      // 结束动画过程中停止交互,防止出问题
      self.collectionView.userInteractionEnabled = NO;
      // 给截图视图一个动画移动到隐藏cell的新位置
      [UIView animateWithDuration:0.25 animations:^{
        self.snapshotView.center = cell.center;
        self.snapshotView.transform = CGAffineTransformMakeScale(1.0, 1.0);
      } completion:^(BOOL finished) {
        // 移除截图视图,显示隐藏的cell并开始交互
        [self.snapshotView removeFromSuperview];
        cell.hidden = NO;
        self.collectionView.userInteractionEnabled = YES;
      }];
    }
      break;
  }
}

三.iOS9之后添加的API如下

// Support for reordering
- (BOOL)beginInteractiveMovementForItemAtIndexPath:(NSIndexPath *)indexPath NS_AVAILABLE_IOS(9_0); // returns NO if reordering was prevented from beginning - otherwise YES
- (void)updateInteractiveMovementTargetPosition:(CGPoint)targetPosition NS_AVAILABLE_IOS(9_0);
- (void)endInteractiveMovement NS_AVAILABLE_IOS(9_0);
- (void)cancelInteractiveMovement NS_AVAILABLE_IOS(9_0);

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