CALayer 和屏幕外渲染 [英] CALayer and Off-Screen Rendering

问题描述

我有一个分页 UIScrollView，其 contentSize 足够大，可以容纳许多用于缩放的小型 UIScrollView，viewForZoomingInScrollView 是一个 viewController，它包含一个用于绘制 PDF 的 CALayer页到.这让我可以像 ibooks PDF 阅读器一样浏览 PDF.

I have a Paging UIScrollView with a contentSize large enough to hold a number of small UIScrollViews for zooming, The viewForZoomingInScrollView is a viewController that holds a CALayer for drawing a PDF page onto. This allows me to navigate through a PDF much like the ibooks PDF reader.

绘制PDF(Tiled Layers)的代码位于:

The code that draws the PDF (Tiled Layers) is located in:

- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx;

只需在可见屏幕上添加一个页面"即可自动调用此方法.当我更改页面时，在绘制所有图块之前会有一些延迟，即使对象(页面)已经创建.

And simply adding a 'page' to the visible screen calls this method automatically. When I change page there is some delay before all the tiles are drawn, even though the object (page) has already been created.

我想要做的是在用户滚动到下一页之前渲染下一页，从而防止可见的平铺效果.但是，我发现如果图层位于屏幕外，将其添加到滚动视图不会调用 drawLayer.

What i want to be able to do is render the next page before the user scrolls to it, thus preventing the visible tiling effect. However, i have found that if the layer is located offscreen adding it to the scrollview doesn't call the drawLayer.

这里有什么想法/常见的陷阱吗?

Any Ideas/common gotchas here?

我试过了:

[viewController.view.layer setNeedsLayout]; 
[viewController.view.layer setNeedsDisplay];

注意:这在功能上复制 ibook 的事实与整个应用程序的上下文无关.

推荐答案

正如我上面提到的，如果 CALayers 不在屏幕上，则不会渲染.

As i mentioned above, CALayers don't render if they are offscreen.

我最终没有将 PDF 直接绘制到图层上，而是在需要时将 PDF 页面呈现为图像(呈现 1 页加上和减去一个焦点页面)

I ended up not drawing the PDF directly to the layer but instead, rendered the PDF page to an image when i needed (renders 1 page plus and minus one of the focused page)

这里是渲染代码:

-(UIImage *)renderPDFPageToImage:(int)pageNumber//NSOPERATION?
{
 //you may not want to permanently (app life) retain doc ref

 CGSize size = CGSizeMake(x,y);     
 UIGraphicsBeginImageContext(size);
 CGContextRef context = UIGraphicsGetCurrentContext();

 CGContextTranslateCTM(context, 0, 750);
 CGContextScaleCTM(context, 1.0, -1.0);

 CGPDFPageRef page;  //Move to class member 

    page = CGPDFDocumentGetPage (myDocumentRef, pageNumber);
    CGContextDrawPDFPage (context, page);

 UIImage * pdfImage = UIGraphicsGetImageFromCurrentImageContext();//autoreleased
 UIGraphicsEndImageContext();
 return pdfImage;

}

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