CoreAnimation之CATiledLayer

CATiledLayer可以用来显示大尺寸的图片或者PDF文件，我们使用imageNamed或者contentsOfFile加载一张大尺寸的图片的时候，需要等待一段时间才能显示，看起来有一些延迟。CATiledLayer可以将大尺寸的图片分割为小块小块的区域，异步分别加载需要显示的区域，只有在滑动加载查看新区域的时候，即将显示的新区域会有短暂的空白加载等待，避免了用户长时间等待。
CATiledLayer在制作PDF阅读器或者地图类应用中肯定会有用武之地的。

创建一个基于UIView的TiledView，这些为主要代码

#import "TiledView.h"
@interface TiledView ()
@property (nonatomic ,assign) CGPDFPageRef page;
@end
@implementation TiledView
+(Class)layerClass {
    return [CATiledLayer class];
}
-(instancetype)initWithFrame:(CGRect)frame path:(NSString *)path completion:(void(^)(CGSize size))completion {
    self = [super initWithFrame:frame];
    if (self) {
        NSURL *docURL = [NSURL fileURLWithPath:path];
        CGPDFDocumentRef pdf = CGPDFDocumentCreateWithURL((CFURLRef)docURL);
        CGPDFPageRef page = CGPDFDocumentGetPage(pdf, 1);
        self.page = page;
        CGRect pageRect = CGPDFPageGetBoxRect(page, kCGPDFCropBox);
        CATiledLayer * layer = (CATiledLayer *)self.layer;
        layer.levelsOfDetailBias = 5;
        
        layer.bounds = pageRect;
        int w = pageRect.size.width;
        int h = pageRect.size.height;
        
        int levels = 1;
        while (w > 1 && h > 1) {
            levels++;
            w = w >> 1;
            h = h >> 1;
        }
        layer.levelsOfDetail = levels;
        [self setFrame:pageRect];
        if (completion) {
            completion(pageRect.size);
        }
    }
    return self;
}
-(void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx {
    CGContextDrawPDFPage(ctx, self.page);
}

使用过程：

#import "TiledViewController.h"
#import "TiledView.h"

@interface TiledViewController ()

@end

@implementation TiledViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    UIScrollView * scr = [[UIScrollView alloc] initWithFrame:[UIScreen mainScreen].bounds];
    [self.view addSubview:scr];
    
    NSString *path = [[NSBundle mainBundle] pathForResource:@"sf_muni" ofType:@"pdf"];
    NSLog(@"%@", path);
    TiledView * view = [[TiledView alloc] initWithFrame:self.view.bounds path:path completion:^(CGSize size) {
        scr.contentSize = size;
    }];
    [scr addSubview:view];
}

tiledView.gif

参考链接：
https://developer.apple.com/documentation/quartzcore/catiledlayer
http://www.jianshu.com/p/29cbc1744153
http://www.cocoachina.com/bbs/read.php?tid-31201-page-1.html
https://zhuanlan.zhihu.com/p/31323034