自定义UICollectionViewFlowLayout，图

以前瀑布流的时候使用过UICollectionView，但是那时使用的是系统自带的UICollectionViewFlowLayout布局，今天看博客，看到UICollectionViewFlowLayout自定义相关的东西，于是动手写了一个简单图片浏览的demo，熟练一些UICollectionViewFlowLayout自定义布局。

#import <UIKit/UIKit.h>

@interface JWCollectionViewFlowLayout : UICollectionViewFlowLayout

@end

自定义UICollectionViewFlowLayout，首先继承UICollectionViewFlowLayout，实现一下几个方法

#define screenWidth [UIScreen mainScreen].bounds.size.width

#define MaxChangeRange 100

#import "JWCollectionViewFlowLayout.h"

@implementation JWCollectionViewFlowLayout

-(void)prepareLayout
{
    self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    self.itemSize = CGSizeMake(300, 500);
}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
{
    return YES;
}

- (nullable NSArray<__kindof UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSArray *array = [super layoutAttributesForElementsInRect:rect];
    
    CGRect visibleRect = CGRectMake(self.collectionView.contentOffset.x, 0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);
    for (UICollectionViewLayoutAttributes *attr in array)
    {
        if (CGRectIntersectsRect(attr.frame, rect)) {
            
            BOOL isAtRight = YES;
            CGFloat distance = (attr.center.x - CGRectGetMidX(visibleRect));
            if (distance<0) {
                distance = -distance;
                isAtRight = NO;
            }
            CGFloat precent ;
            if (distance < 180)
            {
                precent = 1.0;
            }
            else
            {
                precent = ((screenWidth / 2) - distance) / (screenWidth / 2);
            }
            CATransform3D transform = CATransform3DIdentity;
            transform.m34 = 1.0 / 600;
            
            if (precent < 0.5) {
                precent = 0.5;
            }
            transform =  CATransform3DScale(transform, 1, precent, 1);
            CGFloat p = isAtRight?M_PI_4:-M_PI_4;
            transform = CATransform3DRotate(transform, p * (1 - precent), 0, 1, 0);
            attr.transform3D = transform;
            attr.zIndex = 1;
            attr.alpha = precent;  
        }
    }
    
    return array;
}

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
    CGFloat offset = MAXFLOAT;
    
    CGFloat hCenter = proposedContentOffset.x + (CGRectGetWidth(self.collectionView.bounds) / 2.0);
    
    CGRect currentRect = CGRectMake(proposedContentOffset.x, 0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);
    
    NSArray* array = [super layoutAttributesForElementsInRect:currentRect];
    for (UICollectionViewLayoutAttributes* layoutAttributes in array)
    {
        CGFloat itemHorizontalCenter = layoutAttributes.center.x;
        if (ABS(itemHorizontalCenter - hCenter) < ABS(offset))
        {
            
            offset = itemHorizontalCenter - hCenter;
        }
    }
   
    return CGPointMake(proposedContentOffset.x + offset, proposedContentOffset.y);
}

使用

-(void)setupUI
{
    
    JWCollectionViewFlowLayout *flowLayout = [[JWCollectionViewFlowLayout alloc] init];
    UICollectionView *imgBrowseView = [[UICollectionView alloc] initWithFrame:self.view.bounds collectionViewLayout:flowLayout];
    imgBrowseView.dataSource = self;
    imgBrowseView.delegate = self;
    imgBrowseView.backgroundColor = [UIColor whiteColor];
    [self.view addSubview:imgBrowseView];
    _imgBrowseView = imgBrowseView;
    
    [self.imgBrowseView registerNib:[UINib nibWithNibName:@"CustumCollectionViewCell" bundle:nil] forCellWithReuseIdentifier:@"cell"];
}

demo:https://github.com/jiangtaidi/JWImageBrowseDemo.git 运行结果：

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