yongxu-app-ios-v1/Temperature/Tools/CollectionLayout/RDSCollectionViewFlowLayout.m

//
//  RDSCollectionViewFlowLayout.m
//  LampRibbon
//
//  Created by coderYK on 2017/8/9.
//  Copyright © 2017年 RD-iOS. All rights reserved.
//

#import "RDSCollectionViewFlowLayout.h"

@implementation RDSCollectionViewFlowLayout

//设置 cell 布局
//返回一定范围内的 cell 的布局
//可以一次性返回全部的 cell
//返回的数组是 cell 的布局信息
- (nullable NSArray<__kindof UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect
{
    /*
     实现思路:在指定 rect 中,距离中心点越近的 cell 缩放比例越大,越远的,缩放比例越小
     实时距离  ->  缩放比例
     */
    NSArray *attrs = [self getCopyOfAttributes:[super layoutAttributesForElementsInRect: self.collectionView.bounds]];
    
    CGFloat distance;
    CGFloat scale;
    for (UICollectionViewLayoutAttributes *attr in attrs) {
        
        distance = attr.center.x - (self.collectionView.contentOffset.x + SCREEN_WIDTH * 0.5);
        
        scale = 1 - fabs(distance) / SCREEN_WIDTH * 0.5;
        
        attr.transform = CGAffineTransformMakeScale(scale, scale);
    }
    
    return attrs;
}


- (NSArray *)getCopyOfAttributes:(NSArray *)attributes {
    NSMutableArray *copyArr = [NSMutableArray new];
    for (UICollectionViewLayoutAttributes *attribute in attributes) {
        [copyArr addObject:[attribute copy]];
    }
    return copyArr;
}


//作用:决定UICollectionView最终偏移量
//什么时候调用:只要当用户停止拖动时才调用
//可以在这个方法中定位 cell
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
    //获得最终的显示区域
    CGRect targetR = CGRectMake(proposedContentOffset.x, 0, SCREEN_WIDTH, MAXFLOAT);
    
    NSArray *attrs = [super layoutAttributesForElementsInRect:targetR];
    
    //对距离中心点最近的 cell平移
    CGFloat minDistance = MAXFLOAT;
    CGFloat distance;
    for (UICollectionViewLayoutAttributes *attr in attrs) {
        
        distance = attr.center.x - (proposedContentOffset.x + SCREEN_WIDTH * 0.5);
        if (fabs(distance) < fabs(minDistance)) {
            minDistance = distance;
        }
    }
    
    proposedContentOffset.x += minDistance;
    
    if (proposedContentOffset.x < 0) {
        proposedContentOffset.x = 0;
    }
    /*
     快速拖动: 最终偏移量 != 手指离开时的偏移量
     */
    
    return proposedContentOffset;
}


//Invalidate:刷新
//是否允许刷新布局,当内容改变的时候
- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
{
    return YES;
}


@end