如何在html 5中销毁/重新加载画布？ [英] How to destroy / reload the canvas in html 5?

问题描述

我正在开发一个电子书应用程序，我使用PDF.js在画布上绘制每个页面，问题是，当我点击按钮并转到其他页面时，我尝试再次在同一个画布上渲染，但是画布似乎移动到错误的位置或错误的大小。

I am working on an ebook application, I draw each page on canvas using PDF.js , the problem is , when I click on the button and turn to other page, I tried simply render on the same canvas again , but the canvas seems move to a wrong location or wrong size .

function renderPage(url) {
      canvas = document.getElementById('canvas');
      ctx = canvas.getContext('2d');
      //clearCanvasGrid('canvas');

      PDFJS.getDocument(url).then(function (pdf) {
          // Using promise to fetch the page
          pdf.getPage(1).then(function(page) {
            var viewport = page.getViewport(5); //scale 5

            canvas.height = viewport.height;
            canvas.width = viewport.width;

            // Render PDF page into canvas context
            var renderContext = {
              canvasContext: ctx,
              viewport: viewport
            };

            page.render(renderContext).then(function() {
                initialZoomPage(viewport.height,viewport.width);
            });
        });
    });
}

那么，在重绘页面之前，我需要做一些必要的步骤吗？另外，如果我要关闭页面，我怎么能破坏它呢？谢谢

So, are there any necessary step I need to do before redraw the page? Also , how can I destroy it if I would like to close the page? Thanks

更新：

function clearCanvasGrid(canvasID){
    canvas = document.getElementById(canvasID); //because we are looping //each location has its own canvas ID
    context = canvas.getContext('2d');
    //context.beginPath();

    // Store the current transformation matrix
    context.save();

    // Use the identity matrix while clearing the canvas
    context.setTransform(1, 0, 0, 1, 0, 0);
    context.clearRect(0, 0, canvas.width, canvas.height);

    // Restore the transform
    context.restore(); //CLEARS THE SPECIFIC CANVAS COMPLETELY FOR NEW DRAWING
}

我找到了一个清除功能画布但除了clearRect之外它还有.save，.setTransform和.restore，它们是否必要？谢谢

I found a function to clear the canvas but it has .save , .setTransform and .restore besides clearRect, are they necessary? thanks

推荐答案

一种方法是使用 context.clearRect（0,0，width，height）（参考）。

One way is to clear out the canvas using context.clearRect(0,0, width, height) (Reference).

或者，您可以在每次需要新页面时附加一个新的canvas元素（并可能删除旧的canvas元素，具体取决于您是否要再次显示它）。这样的事情应该这样做：

Alternatively, you can append a new canvas element (and possibly remove the old one, depending on whether you will want to display it again) each time you want a new page. Something like this should do it:

var oldcanv = document.getElementById('canvas');
document.removeChild(oldcanv)

var canv = document.createElement('canvas');
canv.id = 'canvas';
document.body.appendChild(canv);

请注意，如果您计划保留多个，则每个人必须拥有唯一的 id 而不只是 id =canvas（可能基于页码 - 类似 canvas- 1 ）。

Just note that if you plan to keep more than one, each one must have a unique id instead of just id="canvas" (perhaps based on the page number - something like canvas-1).

回答更新的问题：

保存， setTransform 和 restore 。我不知道PDF.js库是否在幕后进行任何转换，所以最好将它留在那里。

The save, setTransform, and restore are only necessary if you are doing (or somehow allowing users to do) transformation. I don't know if the PDF.js library does any transformation behind the scenes, so it may be best to leave it there.

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