圖片加載方式一：imageWithContentsOfFile利用這種方式加載圖片是很耗內(nèi)存的。因?yàn)樗菍?shí)時(shí)的加載圖片，當(dāng)加載的圖片較多時(shí)，且是在主線程中，每一幀加載相同大小的圖片，會(huì)造成卡頓的現(xiàn)象
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
self.view.backgroundColor = [UIColor lightGrayColor];
// self.layerView = [[UIView alloc] initWithFrame:[UIScreen mainScreen].bounds];
// [self.view addSubview:self.layerView];
self.imagePaths= [[NSBundle mainBundle] pathsForResourcesOfType:@".png" inDirectory:@""];
[self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cell"];
}
-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
return self.imagePaths.count;
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView
cellForItemAtIndexPath:(NSIndexPath *)indexPath{
//dequeue cell
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
//add image view
const NSInteger imageTag = 99;
UIImageView *imageView = (UIImageView *)[cell viewWithTag:imageTag];
if (!imageView) {
imageView = [[UIImageView alloc] initWithFrame: cell.contentView.bounds];
imageView.tag = imageTag;
[cell.contentView addSubview:imageView];
}
//set image
NSString *imagePath = self.imagePaths[indexPath.row];
imageView.image = [UIImage imageWithContentsOfFile:imagePath];
return cell;
}
第二種方式：開啟一個(gè)線程，將耗性能的代碼寫在一個(gè)線程里，在主線程中加載圖片。解決圖片卡頓，造成圖片延遲的加載。但對(duì)于圖片本身的加載來說，并沒有得到解決
//dequeue cell
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell"
forIndexPath:indexPath];
//add image view
const NSInteger imageTag = 99;
UIImageView *imageView = (UIImageView *)[cell viewWithTag:imageTag];
if (!imageView) {
imageView = [[UIImageView alloc] initWithFrame: cell.contentView.bounds];
imageView.tag = imageTag;
[cell.contentView addSubview:imageView];
}
//tag cell with index and clear current image
cell.tag = indexPath.row;
imageView.image = nil;
//switch to background thread
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
//load image
NSInteger index = indexPath.row;
NSString *imagePath = self.imagePaths[index];
UIImage *image = [UIImage imageWithContentsOfFile:imagePath];
//set image on main thread, but only if index still matches up
dispatch_async(dispatch_get_main_queue(), ^{
if (index == cell.tag) {
imageView.image = image; }
});
});
return cell;
第三種方式：ImageIO框架，使用kCGImageSourceShouldCache來創(chuàng)建圖片，強(qiáng)制圖片立刻解壓，然后在圖片的生命周期保留解壓后的版本
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView
cellForItemAtIndexPath:(NSIndexPath *)indexPath{
//dequeue cell
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell"
forIndexPath:indexPath];
//add image view
const NSInteger imageTag = 99;
UIImageView *imageView = (UIImageView *)[cell viewWithTag:imageTag];
if (!imageView) {
imageView = [[UIImageView alloc] initWithFrame: cell.contentView.bounds];
imageView.tag = imageTag;
[cell.contentView addSubview:imageView];
}
//tag cell with index and clear current image
cell.tag = indexPath.row;
imageView.image = nil;
//switch to background thread
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
NSInteger index = indexPath.row;
NSURL *imageURL = [NSURL fileURLWithPath:self.imagePaths[index]];
NSDictionary *options = @{(__bridge id)kCGImageSourceShouldCache: @YES};
CGImageSourceRef source = CGImageSourceCreateWithURL((__bridge CFURLRef)imageURL, NULL);
CGImageRef imageRef = CGImageSourceCreateImageAtIndex(source, 0,(__bridge CFDictionaryRef)options);
UIImage *image = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);
CFRelease(source);
//set image on main thread, but only if index still matches up
dispatch_async(dispatch_get_main_queue(), ^{
if (index == cell.tag) {
imageView.image = image; }
});
});
return cell;
}
第四種方式：實(shí)現(xiàn)圖片解壓，則把整張圖片繪制到CGContext可能是最佳的方式了。盡管你可能認(rèn)為多余的繪制相較別的解壓技術(shù)而言性能不是很高，但是新創(chuàng)建的圖片（在特定的設(shè)備上做過優(yōu)化）可能比原始圖片繪制的更快。
同樣，如果想顯示圖片到比原始尺寸小的容器中，那么一次性在后臺(tái)線程重新繪制到正確的尺寸會(huì)比每次顯示的時(shí)候都做縮放會(huì)更有效（盡管在這個(gè)例子中我們加載的圖片呈現(xiàn)正確的尺寸，所以不需要多余的優(yōu)化）
//load image
NSInteger index = indexPath.row;
NSString *imagePath = self.imagePaths[index];
UIImage *image = [UIImage imageWithContentsOfFile:imagePath];
//redraw image using device context
UIGraphicsBeginImageContextWithOptions(imageView.bounds.size, YES, 0);
[image drawInRect:imageView.bounds];
image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
//set image on main thread, but only if index still matches up
dispatch_async(dispatch_get_main_queue(), ^{
if (index == cell.tag) {
imageView.image = image;
}
第五種方式：CATiledLayer可以用來異步加載和顯示大型圖片，而不阻塞用戶輸入
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView
cellForItemAtIndexPath:(NSIndexPath *)indexPath{
//dequeue cell
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
//add the tiled layer
CATiledLayer *tileLayer = [cell.contentView.layer.sublayers lastObject];
if (!tileLayer) {
tileLayer = [CATiledLayer layer];
tileLayer.frame = cell.bounds;
tileLayer.contentsScale = [UIScreen mainScreen].scale;
tileLayer.tileSize = CGSizeMake(cell.bounds.size.width * [UIScreen mainScreen].scale, cell.bounds.size.height * [UIScreen mainScreen].scale);
tileLayer.delegate = self;
[tileLayer setValue:@(indexPath.row) forKey:@"index"];
[cell.contentView.layer addSublayer:tileLayer];
}
//tag the layer with the correct index and reload
tileLayer.contents = nil;
[tileLayer setValue:@(indexPath.row) forKey:@"index"];
[tileLayer setNeedsDisplay];
return cell;
}
- (void)drawLayer:(CATiledLayer *)layer inContext:(CGContextRef)ctx
{
//get image index
NSInteger index = [[layer valueForKey:@"index"] integerValue];
//load tile image
NSString *imagePath = self.imagePaths[index];
UIImage *tileImage = [UIImage imageWithContentsOfFile:imagePath];
//calculate image rect
CGFloat aspectRatio = tileImage.size.height / tileImage.size.width;
CGRect imageRect = CGRectZero;
imageRect.size.width = layer.bounds.size.width;
imageRect.size.height = layer.bounds.size.height * aspectRatio;
imageRect.origin.y = (layer.bounds.size.height - imageRect.size.height)/2;
//draw tile
UIGraphicsPushContext(ctx);
[tileImage drawInRect:imageRect];
UIGraphicsPopContext();
}
