PDF.js viewer pinch to zoom

PDF.js Viewer Pinch to Zoom

PDF.js Viewer is a powerful and versatile JavaScript library for rendering and interacting with PDF documents in web browsers. One of its essential features is the ability to zoom in and out of the PDF content, allowing users to examine details closely or view the document in its entirety.

Pinch to zoom, a widely-used gesture on touch-enabled devices, provides a seamless and intuitive way to navigate and zoom through PDF content. This article will guide you through implementing pinch to zoom functionality within PDF.js Viewer.

Understanding the Implementation

The pinch to zoom feature in PDF.js Viewer is primarily achieved through JavaScript events and the manipulation of the PDF.js Viewer’s internal API. Here’s a breakdown:

1. Event Handling

• **`touchstart` Event:** This event triggers when one or more fingers touch the screen. We capture the initial touch points and their distance.
• **`touchmove` Event:** As the fingers move, we track the change in distance between the touch points. This change indicates the zoom direction.
• **`touchend` Event:** When the fingers are lifted, the zoom operation is finalized.

2. PDF.js Viewer API

• **`PDFViewerApplication.setScale`:** This function is used to set the zoom level of the PDF.js Viewer. We dynamically adjust the zoom level based on the distance change detected during the pinch gesture.
• **`PDFViewerApplication.currentScaleValue`:** This property provides the current zoom scale of the Viewer.

Implementing Pinch to Zoom

Here’s a JavaScript snippet illustrating the implementation of pinch to zoom using PDF.js Viewer:

// Attach event listeners to the PDF.js Viewer canvas
const viewerCanvas = document.getElementById('viewerCanvas'); // Replace with the actual ID of your PDF.js Viewer canvas

let initialDistance = 0;
let currentScale = PDFViewerApplication.currentScaleValue;

viewerCanvas.addEventListener('touchstart', (event) => {
  if (event.touches.length === 2) {
    initialDistance = Math.hypot(event.touches[0].clientX - event.touches[1].clientX, event.touches[0].clientY - event.touches[1].clientY);
  }
});

viewerCanvas.addEventListener('touchmove', (event) => {
  if (event.touches.length === 2) {
    const currentDistance = Math.hypot(event.touches[0].clientX - event.touches[1].clientX, event.touches[0].clientY - event.touches[1].clientY);
    const scaleFactor = currentDistance / initialDistance;
    PDFViewerApplication.setScale(currentScale * scaleFactor);
    currentScale = PDFViewerApplication.currentScaleValue;
  }
});

viewerCanvas.addEventListener('touchend', () => {
  initialDistance = 0;
});

**Explanation:**

• The code attaches `touchstart`, `touchmove`, and `touchend` event listeners to the PDF.js Viewer canvas.
• In `touchstart`, we record the initial distance between two touch points.
• In `touchmove`, we calculate the distance change and use it to dynamically adjust the zoom scale using `PDFViewerApplication.setScale`.
• Finally, in `touchend`, we reset the `initialDistance` to prepare for potential future pinch gestures.

Considerations

• **Browser Compatibility:** Ensure compatibility across different browsers and touch devices.
• **Performance Optimization:** For large PDF documents or complex rendering scenarios, consider optimizing performance to prevent lag or stuttering.
• **Customizations:** You can further customize the pinch to zoom behavior, such as adding zoom limits, visual feedback, or handling edge cases.

Conclusion

By leveraging the power of JavaScript events and PDF.js Viewer’s API, you can effortlessly implement pinch to zoom functionality, providing users with a natural and intuitive way to navigate and explore PDF content on touch-enabled devices. This feature enhances the user experience and makes PDF viewing even more engaging.