CALayer和Off-Screen渲染 [英] CALayer and Off-Screen Rendering

问题描述

我有一个Paging UIScrollView ，内容大小足以容纳一些小的UIScrollViews缩放， viewForZoomingInScrollView 是一个viewController，用于保存用于绘制PDF页面的CALayer。这使我可以通过PDF浏览，就像ibooks PDF阅读器。

绘制PDF（Tiled Layers）的代码位于：

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

只需在可见屏幕上添加页面即可自动调用此方法。当我更改页面时，即使已经创建了对象（页面），在所有图块都绘制之前仍存在一些延迟。

我想要做什么在用户滚动到它之前呈现下一页，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添加到scrollview不调用 drawLayer 。

这里有任何想法/常见问题吗？

我试过：

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

注意：这是复制ibooks的功能的事实是不相关的完整的应用程式。

解决方案

如上所述，如果CALAYERS在屏幕外， >

我最终不是直接将PDF绘制到图层，而是在需要时将PDF页面渲染为图像（渲染1页加减一个关注页面） / p>

这是渲染代码：

   - （UIImage *）renderPDFPageToImage ：（int）pageNumber // NSOPERATION？ 
 {
 //你可能不想永久（app life）保留文档ref 
 
 CGSize size = CGSizeMake（x，y）; 
 UIGraphicsBeginImageContext（size）; 
 CGContextRef context = UIGraphicsGetCurrentContext（）; 
 
 CGContextTranslateCTM（context，0，750）; 
 CGContextScaleCTM（context，1.0，-1.0）; 
 
 CGPDFPageRef页面; // Move to class member 
 
 page = CGPDFDocumentGetPage（myDocumentRef，pageNumber）; 
 CGContextDrawPDFPage（context，page）; 
 
 UIImage * pdfImage = UIGraphicsGetImageFromCurrentImageContext（）; // autoreleased 
 UIGraphicsEndImageContext（）; 
 return pdfImage; 
 
} 
  

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.

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

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

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.

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.

Any Ideas/common gotchas here?

I have tried:

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

NB: The fact that this is replicating the ibooks functionally is irrelevant within the context of the full app.

解决方案

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

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)

Here is the render code:

-(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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