大图片的分块加载实现

当在app中需要加载超大分辨率，比如现在的5K图片时，性能的优化就凸显出来了。那么通过什么方法来解决这个问题呢？我们可以采取UIScrollView和CATileLayer来实现，实现的效果类似于百度地图的分块加载

首先我们通过代码将大图裁剪成若干小图，MacOS代码如下所示。运行前将需要裁剪图片的地址添加到Edit Scheme中

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // 处理错误异常
        if (argc < 2) {
            NSLog(@"TileCutter arguments: inputfile");
            return 0;
        }

        //文件输入
        NSString *inputFile = [NSString stringWithCString:argv[1] encoding:NSUTF8StringEncoding];
        CGFloat tileSize = 256;
        //文件输出
        NSString *outputPath = [inputFile stringByDeletingPathExtension];
        NSImage *image = [[NSImage alloc]initWithContentsOfFile:inputFile];
        NSSize size = [image size];
        NSArray *representations = [image representations];
        if ([representations count]) {
            NSBitmapImageRep *representation = representations[0];
            size.width = [representation pixelsWide];
            size.height = [representation pixelsHigh];
        }
        NSRect rect = NSMakeRect(0.0, 0.0, size.width, size.height);
        CGImageRef imageRef = [image CGImageForProposedRect:&rect context:nil hints:nil];

        NSInteger rows = ceil(size.height / tileSize);
        NSInteger cols = ceil(size.width / tileSize);

        for (int y = 0; y < rows; ++y) {
            for (int x = 0; x < cols; ++x) {
                CGRect tileRect = CGRectMake(x * tileSize, y * tileSize, tileSize, tileSize);
                CGImageRef tileImage = CGImageCreateWithImageInRect(imageRef, tileRect);

                NSBitmapImageRep *imageRep = [[NSBitmapImageRep alloc] initWithCGImage:tileImage];
                NSData *data = [imageRep representationUsingType:NSJPEGFileType properties:nil];

                CGImageRelease(tileImage);

                NSString *path = [outputPath stringByAppendingFormat:@"_%02i_%02i.jpg",x,y];
                [data writeToFile:path atomically:NO];
            }
        }
    }
    return 0;
}

利用CATileLayer来实现的代码

- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.

    CATiledLayer *tileLayer = [CATiledLayer layer];
    tileLayer.frame = CGRectMake(0, 0, 2048, 2048);
    tileLayer.delegate = self;
    [self.scrollerView.layer addSublayer:tileLayer];

    self.scrollerView.contentSize = tileLayer.frame.size;
    
    [tileLayer setNeedsDisplay];
}

-(void)drawLayer:(CATiledLayer *)layer inContext:(CGContextRef)ctx
{
    CGRect bounds = CGContextGetClipBoundingBox(ctx);
    NSInteger x = floor(bounds.origin.x / layer.tileSize.width);
    NSInteger y = floor(bounds.origin.y / layer.tileSize.height);

    NSString *imageName = [NSString stringWithFormat:@"ai_%02i_%02i",x,y];
    NSString *imagePath = [[NSBundle mainBundle] pathForResource:imageName ofType:@"jpg"];

    UIImage *tileImage = [UIImage imageWithContentsOfFile:imagePath];

    UIGraphicsPushContext(ctx);
    [tileImage drawInRect:bounds];
    UIGraphicsPopContext();
}