iOS开发UICollectionView实现拖拽效果
iOS开发中，UICollectionView是非常常用的控件之一，用于实现列表、网格等视图。今天，我们将讨论如何在UICollectionView中实现拖拽效果。
一、iOS9之前实现拖拽效果
在iOS9之前，实现拖拽效果需要我们自己写方法来实现。这其中最重要的步骤是添加长按手势，捕捉用户的长按事件，并在手势方法中实现拖拽效果。
我们需要添加长按手势到UICollectionView中：
```objective-c
UILongPressGestureRecognizer *longPress = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(handlelongGesture:)];
[collectionView addGestureRecognizer:longPress];
```
然后，在handlelongGesture：方法中，我们需要获取当前选中的cell的IndexPath，并将其截图保存起来，用于后续的视图交换：
```objective-c
- (void)handlelongGesture:(UILongPressGestureRecognizer *)gesture {
NSIndexPath *indexPath = [self.collectionView indexPathForItemAtPoint:[gesture locationInView:self.collectionView]];
// 获取当前选中的cell的截图
UIView *snapshotView = [self createSnapshotView:indexPath];
// 保存截图
self.snapshotView = snapshotView;
// 保存当前选中的IndexPath
self.oldIndexPath = indexPath;
}
```
接着，我们需要在UICollectionView的代理方法中实现拖拽效果。在collectionView：moveItemAtIndexPath：toIndexPath：方法中，我们需要交换数据源，并更新UICollectionView的显示：
```objective-c
- (void)collectionView:(UICollectionView *)collectionView moveItemAtIndexPath:(NSIndexPath *)sourceIndexPath toIndexPath:(NSIndexPath *)destinationIndexPath {
// 交换数据源
[self.dataArr exchangeObjectAtIndex:sourceIndexPath.item withObjectAtIndex:destinationIndexPath.item];
// 更新UICollectionView的显示
[collectionView moveItemAtIndexPath:sourceIndexPath toIndexPath:destinationIndexPath];
}
```
二、iOS9之后实现拖拽效果
在iOS9之后，苹果提供了自带的API来实现拖拽效果，我们可以使用UICollectionView的built-in方法来实现拖拽效果。
我们需要添加长按手势到UICollectionView中：
```objective-c
UILongPressGestureRecognizer *longPress = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(handlelongGesture:)];
[collectionView addGestureRecognizer:longPress];
```
然后，在handlelongGesture：方法中，我们可以直接调用iOS9的API来实现拖拽效果：
```objective-c
- (void)handlelongGesture:(UILongPressGestureRecognizer *)gesture {
[collectionView beginInteractiveMovementForItemAtIndexPath:[self.collectionView indexPathForItemAtPoint:[gesture locationInView:self.collectionView]]];
}
```
三、完整实现代码
下面是完整的实现代码：
```objective-c
#import <UIKit/UIKit.h>
@interface ViewController () <UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout>
@property (nonatomic, strong) NSMutableArray *dataArr;
@property (nonatomic, strong) UICollectionView *collectionView;
@property (nonatomic, strong) NSIndexPath *oldIndexPath;
@property (nonatomic, strong) UIView *snapshotView;
@property (nonatomic, strong) NSIndexPath *moveIndexPath;
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
// 初始化UICollectionView
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];
// 初始化数据源
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:0.8];
[self.dataArr addObject:color];
}
}
- (void)handlelongGesture:(UILongPressGestureRecognizer *)gesture {
NSIndexPath *indexPath = [self.collectionView indexPathForItemAtPoint:[gesture locationInView:self.collectionView]];
UIView *snapshotView = [self createSnapshotView:indexPath];
self.snapshotView = snapshotView;
self.oldIndexPath = indexPath;
}
- (UIView *)createSnapshotView:(NSIndexPath *)indexPath {
UICollectionViewCell *cell = [self.collectionView cellForItemAtIndexPath:indexPath];
UIView *snapshotView = [cell snapshotViewAfterScreenUpdates:YES];
return snapshotView;
}
- (void)collectionView:(UICollectionView *)collectionView moveItemAtIndexPath:(NSIndexPath *)sourceIndexPath toIndexPath:(NSIndexPath *)destinationIndexPath {
[self.dataArr exchangeObjectAtIndex:sourceIndexPath.item withObjectAtIndex:destinationIndexPath.item];
[collectionView moveItemAtIndexPath:sourceIndexPath toIndexPath:destinationIndexPath];
}
@end
```
实现UICollectionView的拖拽效果需要添加长按手势，捕捉用户的长按事件，并在手势方法中实现拖拽效果。在iOS9之前，我们需要自己写方法来实现拖拽效果，而在iOS9之后，我们可以使用苹果提供的API来实现拖拽效果。
