在iOS10中，苹果为UICollectionViewCell引入了Pre-Fetching预加载机制用于提升它的性能。主要引入了一个新的数据源协议UICollectionViewDataSourcePrefetching，包含两个方法：
@protocol UICollectionViewDataSourcePrefetching <NSObject>
@required
// 预加载数据
- (void)collectionView:(UICollectionView *)collectionView prefetchItemsAtIndexPaths:(NSArray<NSIndexPath *> *)indexPaths NS_AVAILABLE_IOS(10_0);
@optional
// 取消提前加载数据
- (void)collectionView:(UICollectionView *)collectionView cancelPrefetchingForItemsAtIndexPaths:(NSArray<NSIndexPath *> *)indexPaths NS_AVAILABLE_IOS(10_0);
@end
网上搜了一大圈，讲述原理的（翻译文档）的文章很多，有干货的Demo很少，于是乎自己摸索了一下，写了一个简单的案例，在此记录并分享一下。
运行环境：Xcode 8.2.1 + iOS 10.2
核心步骤：
1、遵从 UICollectionViewDataSourcePrefetching 协议
2、实现 collectionView:prefetchItemsAtIndexPaths 方法和collectionView:cancelPrefetchItemsAtIndexPaths 方法（可选）
3、将第1步中遵从协议的类设置为 UICollectionView 的 prefetchDataSource 属性
实现
一、创建UICollectionViewFlowLayout
自己写一个类继承自UICollectionViewFlowLayout
@implementation MyCollectionViewFlowLayout
-(void)prepareLayout{
self.minimumLineSpacing = 1;//垂直间距
self.minimumInteritemSpacing = 0;//水平间距
self.sectionInset = UIEdgeInsetsMake(0, 0, 8, 0);//分组间距
}
@end
二、用XIB定义一个
里面就一个UIImageView，然后拽线设置一个IBOutlet
@property (weak, nonatomic) IBOutlet UIImageView *imgView;
三、控制器
注释很详细
#import "ViewController.h"
#import "MyCollectionViewFlowLayout.h"
#import "ImgCollectionViewCell.h"
#define ScreenW [UIScreen mainScreen].bounds.size.width
//重用标识
static NSString *cellId = @"imgCell";
//遵守协议
@interface ViewController ()<UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDataSourcePrefetching>
//下载图片任务
@property(nonatomic, strong) NSMutableDictionary<NSURL *, dispatch_queue_t> *tasks;
//图片地址
@property(nonatomic, copy) NSMutableArray<NSURL *> *imgURLArray;
//下载的图片
@property(nonatomic, copy) NSMutableDictionary<NSURL *, UIImage *> *imgs;
//UICollectionView
@property(nonatomic, weak) UICollectionView *collectionView;
@end
@implementation ViewController
//懒加载imgURLArray
-(NSMutableArray<NSURL *> *)imgURLArray{
if (_imgURLArray == nil) {
_imgURLArray = [NSMutableArray array];
for (int i = 0; i < 30; i++) {
NSURL *imgURL = [NSURL URLWithString:@"https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1494499175005&di=1d8d40ac84f4a71cb26d7bf4a5a845ec&imgtype=0&src=http%3A%2F%2Fimg10.360buyimg.com%2Fyixun_zdm%2Fjfs%2Ft2830%2F11%2F2310606472%2F165925%2F962fa94a%2F575f7664Nfd743845.jpg"];
[_imgURLArray addObject:imgURL];
}
}
return _imgURLArray;
}
//懒加载imgs
-(NSMutableDictionary<NSURL *,UIImage *> *)imgs{
if (_imgs == nil) {
_imgs = [NSMutableDictionary dictionary];
}
return _imgs;
}
//懒加载tasks
-(NSMutableDictionary<NSURL *,dispatch_queue_t> *)tasks{
if (_tasks == nil) {
_tasks = [NSMutableDictionary dictionary];
}
return _tasks;
}
- (void)viewDidLoad {
[super viewDidLoad];
//创建UICollectionView
//创建布局
UICollectionViewLayout *layout = [[MyCollectionViewFlowLayout alloc]init];
//初始化一个UICollectionView
UICollectionView *collection = [[UICollectionView alloc]initWithFrame:[UIScreen mainScreen].bounds collectionViewLayout:layout];
//设置背景色
collection.backgroundColor = [UIColor groupTableViewBackgroundColor];
//设置代理
collection.dataSource = self;
collection.delegate = self;
collection.prefetchDataSource = self;
//注册Cell
UINib *nib = [UINib nibWithNibName:@"ImgCollectionViewCell" bundle:nil];
[collection registerNib:nib forCellWithReuseIdentifier:cellId];
[self.view addSubview:collection];
self.collectionView = collection;
}
-(void)loadImage:(NSIndexPath *)indexPath{
NSURL *currentURL = [self.imgURLArray objectAtIndex:indexPath.row];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
__weak typeof(self) weakSelf = self;
//异步下载图片
dispatch_async(queue, ^{
NSData *imageData = [NSData dataWithContentsOfURL:currentURL];
UIImage *image = [UIImage imageWithData:imageData];
weakSelf.imgs[currentURL] = image;
//更新UI
dispatch_async(dispatch_get_main_queue(), ^{
ImgCollectionViewCell *cell = (ImgCollectionViewCell *)[weakSelf.collectionView cellForItemAtIndexPath:indexPath];
cell.imgView.image = image;
});
});
//为了取消任务
self.tasks[currentURL] = queue;
}
#pragma mark <UICollectionViewDataSource>
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
return self.imgURLArray.count;
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
ImgCollectionViewCell * cell = [collectionView dequeueReusableCellWithReuseIdentifier:cellId forIndexPath:indexPath];
// 获取URL
NSURL *imgURL = self.imgURLArray[indexPath.row];
//对应的URL的图片已经存在
if (self.imgs[imgURL]) {
cell.imgView.image = self.imgs[imgURL];
}
//不存在
else{
[self loadImage:indexPath];
}
return cell;
}
#pragma mark <UICollectionViewDelegate>
//定义每个Item 的大小
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
CGFloat W = (ScreenW - 1) / 2;
return CGSizeMake(W, 100);
}
#pragma mark <UICollectionViewDataSourcePrefetching>
- (void)collectionView:(UICollectionView *)collectionView prefetchItemsAtIndexPaths:(NSArray<NSIndexPath *> *)indexPaths {
for (NSIndexPath * indexPath in indexPaths) {
NSURL *currentURL = [self.imgURLArray objectAtIndex:indexPath.row];
//不存在就请求
if (!self.imgs[currentURL]) {
[self loadImage:indexPath];
}
}
}
- (void)collectionView:(UICollectionView *)collectionView cancelPrefetchingForItemsAtIndexPaths:(NSArray<NSIndexPath *> *)indexPaths {
for (NSIndexPath * indexPath in indexPaths){
NSURL *currentURL = [self.imgURLArray objectAtIndex:indexPath.row];
//当前任务存在
if (self.tasks[currentURL]) {
dispatch_queue_t queue = self.tasks[currentURL];
dispatch_suspend(queue);
self.tasks[currentURL] = nil;
}
}
}
@end
效果
写在后面的话
1、这个新特性仍然需要探究
2、遇到的一个坑：细心看的话可以发现我的字典是懒加载的，如果直接在viewDidLoad中初始化会在 weakSelf.imgs[currentURL] = image; 一行报错，why？烦请知道的告知。
