How to destroy / reload the canvas in html 5?

问题:

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);
            });
        });
    });
}

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

Update:

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
}

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

回答1:

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

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);

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).

---

Answer to updated question:

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.

回答2:

try using clearRect(), like:

canvas = document.getElementById('canvas');
ctx = canvas.getContext('2d');
ctx.clearRect(0, 0, canvas.width, canvas.height);