不常用属性

//默认选中第一个collectionViewCell

- (void)viewDidAppear:(BOOL)animated {
    NSInteger selectedIndex = 0;
    
    NSIndexPath *selectedIndexPath = [NSIndexPath indexPathForRow:selectedIndex inSection:0];
    
    [self.pageCollection selectItemAtIndexPath:selectedIndexPath animated:NO scrollPosition:UICollectionViewScrollPositionNone];
    //selectRowAtIndexPath:selectedIndexPath animated:NO scrollPosition:UITableViewScrollPositionNone];
    
    [super viewDidAppear:animated];
}

－. 布局

UICollectionViewFlowLayout流水布局

1. 通过UICollectionViewFlowLayout对象进行布局( 如果布局的尺寸是固定的，例如：item的尺寸大小都是固定，可以使用全局属性，即设置下文中layout )
   UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
2. // 1.设置列间距
   3. layout.minimumInteritemSpacing = 1;
3. // 2.设置行间距
   5. layout.minimumLineSpacing = 1;
4. // 3.设置每个item的大小
   layout.itemSize = CGSizeMake(50, 50);
5. // 4.设置Item的估计大小,用于动态设置item的大小，结合自动布局（self-sizing-cell）
   layout.estimatedItemSize = CGSizeMake(320, 60);
6. // 5.设置布局方向

UICollectionViewScrollDirectionVertical垂直(竖着布局)
UICollectionViewScrollDirectionHorizontal水平(横向布局)```
###约束属性
 设置头视图尺寸大小
 ` layout.headerReferenceSize = CGSizeMake(50, 50);`
设置尾视图尺寸大小
`  layout.footerReferenceSize = CGSizeMake(50, 50);`
 设置分区(组)的EdgeInset（四边距）
`layout.sectionInset = UIEdgeInsetsMake(10, 20, 30, 40);`
设置分区的头视图和尾视图是否始终固定在屏幕上边和下边
`layout.sectionFootersPinToVisibleBounds = YES;`
`layout.sectionHeadersPinToVisibleBounds = YES;`
/** 注册collectionView cell 的重用池*/
  ``` [_homePagecollection registerNib:[UINib nibWithNibName:@"HomePageCollectionViewCell" bundle:nil] forCellWithReuseIdentifier:registert];```
/** 注册collectionView 头视图的重用池*/
 ` [_homePagecollection registerClass:[UICollectionReusableView class] forSupplementaryViewOfKind:UICollectionElementKindSectionHeader withReuseIdentifier:@"header"];`

 通过遵守UICollectionViewDelegateFlowLayout实现代理方法来布局（非固定情况则需要通过数据源方法）
//动态设置每个分区的EdgeInsets
```- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout insetForSectionAtIndex:(NSInteger)section;```

//动态设置每行的间距大小
```- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout minimumLineSpacingForSectionAtIndex:(NSInteger)section;```

//动态设置每列的间距大小
```- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout minimumInteritemSpacingForSectionAtIndex:(NSInteger)section;```

//动态设置某组头视图大小
```- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section;```

//动态设置某组尾视图大小
```- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout referenceSizeForFooterInSection:(NSInteger)section;```
二. 数据源方法
//设置分区数（必须实现）
```- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {
    return 1;
}```

//设置每个分区的item个数
```- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section { 
    return 5;  
}```

//设置返回每个item的属性必须实现）
```- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewCell *cell = [self.collection dequeueReusableCellWithReuseIdentifier:ID forIndexPath:indexPath];
    return cell;
}```

//对头视图或者尾视图进行设置
```- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath {
    NSString *identifier;
    if ([kind isEqualToString:UICollectionElementKindSectionHeader]) {
        identifier = @"headerView";
    } else {
        identifier = @"footerView";
    }
    UICollectionReusableView *view = [collectionView dequeueReusableSupplementaryViewOfKind:kind withReuseIdentifier:identifier forIndexPath:indexPath];
    UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, 10, 120, 30)];
    [view addSubview:label];
    if (indexPath.section == 0) {
        label.text = @"section1";
    }else {
        label.text = @"section2";
    }
    if ([kind isEqualToString:UICollectionElementKindSectionHeader]) {
        view.backgroundColor = [UIColor redColor];
    } else {
        view.backgroundColor = [UIColor purpleColor];
    }
    return view;
}```

//是否允许移动Item
```- (BOOL)collectionView:(UICollectionView *)collectionView canMoveItemAtIndexPath:(NSIndexPath *)indexPath NS_AVAILABLE_IOS(9_0){
    return YES;
}```

//移动Item时触发的方法
```- (void)collectionView:(UICollectionView *)collectionView moveItemAtIndexPath:(NSIndexPath *)sourceIndexPath toIndexPath:(NSIndexPath*)destinationIndexPath NS_AVAILABLE_IOS(9_0); {
}```
三. 常用到的代理方法
//是否允许某个Item的高亮，返回NO，则不能进入高亮状态
```- (BOOL)collectionView:(UICollectionView *)collectionView shouldHighlightItemAtIndexPath:(NSIndexPath *)indexPath; {
    return YES;
}```

//当item高亮时触发的方法
```- (void)collectionView:(UICollectionView *)collectionView didHighlightItemAtIndexPath:(NSIndexPath *)indexPath; {
}```

//结束高亮状态时触发的方法
```- (void)collectionView:(UICollectionView *)collectionView didUnhighlightItemAtIndexPath:(NSIndexPath *)indexPath; {
}```

//是否可以选中某个Item，返回NO，则不能选中
```- (BOOL)collectionView:(UICollectionView *)collectionView shouldSelectItemAtIndexPath:(NSIndexPath *)indexPath; {
    return YES;
}```

//是否可以取消选中某个Item
```- (BOOL)collectionView:(UICollectionView *)collectionView shouldDeselectItemAtIndexPath:(NSIndexPath *)indexPath; {
    return YES;
}```

//已经选中某个item时触发的方法
```- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath; {
}```

//取消选中某个Item时触发的方法
```- (void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath; {
}```

//将要加载某个Item时调用的方法
```- (void)collectionView:(UICollectionView *)collectionView willDisplayCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath NS_AVAILABLE_IOS(8_0); {
}```

//将要加载头尾视图时调用的方法
```- (void)collectionView:(UICollectionView *)collectionView willDisplaySupplementaryView:(UICollectionReusableView *)view forElementKind:(NSString *)elementKind atIndexPath:(NSIndexPath *)indexPath NS_AVAILABLE_IOS(8_0); {
}```

//已经展示某个Item时触发的方法
```- (void)collectionView:(UICollectionView *)collectionView didEndDisplayingCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath; {
}```

//已经展示某个头尾视图时触发的方法
```- (void)collectionView:(UICollectionView *)collectionView didEndDisplayingSupplementaryView:(UICollectionReusableView *)view forElementOfKind:(NSString *)elementKind atIndexPath:(NSIndexPath *)indexPath; {
}```

//这个方法设置是否展示长按菜单
```- (BOOL)collectionView:(UICollectionView *)collectionView shouldShowMenuForItemAtIndexPath:(NSIndexPath *)indexPath; {
    return YES;
}```

//这个方法用于设置要展示的菜单选项
```- (BOOL)collectionView:(UICollectionView *)collectionView canPerformAction:(SEL)action forItemAtIndexPath:(NSIndexPath *)indexPath withSender:(nullable id)sender; {
    return YES;
}```

//这个方法用于实现点击菜单按钮后的触发方法,通过测试，只有copy，cut和paste三个方法可以使用
```- (void)collectionView:(UICollectionView *)collectionView performAction:(SEL)action forItemAtIndexPath:(NSIndexPath *)indexPath withSender:(nullable id)sender; {
    //通过下面的方式可以将点击按钮的方法名打印出来：
    NSLog(@"%@",NSStringFromSelector(action));
}```

//collectionView进行重新布局时调用的方法
```- (nonnull UICollectionViewTransitionLayout *)collectionView:(UICollectionView *)collectionView transitionLayoutForOldLayout:(UICollectionViewLayout *)fromLayout newLayout:(UICollectionViewLayout *)toLayout; {
}```