Pdf.js: 在多个 render() 操作期间不能使用相同的画布

使用最新的 pdfjs 版本（2.0.550）我收到以下错误：

在多个 render() 操作期间不能使用相同的画布。

我用于渲染的代码如下：

function renderPage(pageNumber, renderOptions) {
                var documentId = renderOptions.documentId;
                var pdfDocument = renderOptions.pdfDocument;
                var scale = renderOptions.scale;
                var rotate = renderOptions.rotate;
                // Load the page and annotations
                return Promise.all([pdfDocument.getPage(pageNumber), _PDFJSAnnotate2.default.getAnnotations(documentId, pageNumber)]).then(function (_ref) {
                    var _ref2 = _slicedToArray(_ref, 2);
                    var pdfPage = _ref2[0];
                    var annotations = _ref2[1];
                    var page = document.getElementById('pageContainer' + pageNumber);
                    var svg = page.querySelector('.annotationLayer');
                    var canvas = page.querySelector('.canvasWrapper canvas');
                    var canvasContext = canvas.getContext('2d', {alpha: false});
                    var viewport = pdfPage.getViewport(scale, rotate);
                    var transform = scalePage(pageNumber, viewport, canvasContext);
                    var renderContext = {
                        canvasContext: canvasContext,
                        viewport: viewport,
                        transform: transform
                    };
                    // Render the page
                    return Promise.all([pdfPage.render(renderContext), _PDFJSAnnotate2.default.render(svg, viewport, annotations)]).then(function () {
                        // Text content is needed for a11y, but is also necessary for creating
                        // highlight and strikeout annotations which require selecting text.
                        return pdfPage.getTextContent({normalizeWhitespace: true}).then(function (textContent) {
                            return new Promise(function (resolve, reject) {// Render text layer for a11y of text content
                                var textLayer = page.querySelector('.textLayer');
                                var textLayerFactory = new pdfjsViewer.DefaultTextLayerFactory();
                                var textLayerBuilder = textLayerFactory.createTextLayerBuilder(textLayer, pageNumber - 1, viewport);
                                textLayerBuilder.setTextContent(textContent);
                                textLayerBuilder.render();// Enable a11y for annotations
                                // Timeout is needed to wait for `textLayerBuilder.render`
                                setTimeout(function () {
                                    try {
                                        (0, _renderScreenReaderHints2.default)(annotations.annotations);
                                        resolve();
                                    } catch (e) {
                                        reject(e);
                                    }
                                });
                            });
                        });
                    }).then(function () {// Indicate that the page was loaded
                        page.setAttribute('data-loaded', 'true');
                        return[pdfPage, annotations];
                    });
                });
            }

那部分代码来自注释js项目：
https://github.com/instructure/pdf-annotate.js/blob/master/docs/index.js

正如您还可以看到每个页面都有自己的画布。 所以，我不知道为什么我仍然会收到这样的错误。

出了什么问题以及我需要调查的地方？