Pinch zoom implementation for PDF.js viewer

viewer.html

<!-- Goes into viewer.html just before ending </body> -->
<script>
    let pinchZoomEnabled = false;
    function enablePinchZoom(pdfViewer) {
        let startX = 0, startY = 0;
        let initialPinchDistance = 0;        
        let pinchScale = 1;    
        const viewer = document.getElementById("viewer");
        const container = document.getElementById("viewerContainer");
        const reset = () => { startX = startY = initialPinchDistance = 0; pinchScale = 1; };

        // Prevent native iOS page zoom
        document.addEventListener("touchmove", (e) => { if (e.scale !== 1) { e.preventDefault(); } }, { passive: false });

        viewer.addEventListener("touchstart", (e) => {
            if (e.touches.length > 1) {
                startX = (e.touches[0].pageX + e.touches[1].pageX) / 2;
                startY = (e.touches[0].pageY + e.touches[1].pageY) / 2;
                initialPinchDistance = Math.hypot((e.touches[1].pageX - e.touches[0].pageX), (e.touches[1].pageY - e.touches[0].pageY));
            } else {
                initialPinchDistance = 0;
            }
        });

        viewer.addEventListener("touchmove", (e) => {
            if (initialPinchDistance <= 0 || e.touches.length < 2) { return; }

            const pinchDistance = Math.hypot((e.touches[1].pageX - e.touches[0].pageX), (e.touches[1].pageY - e.touches[0].pageY));
            const originX = startX + container.scrollLeft;
            const originY = startY + container.scrollTop;
            pinchScale = pinchDistance / initialPinchDistance;

            viewer.style.transform = `scale(${pinchScale})`;
            viewer.style.transformOrigin = `${originX}px ${originY}px`;
        });

        viewer.addEventListener("touchend", (e) => {
            if (initialPinchDistance <= 0) { return; }

            viewer.style.transform = `none`;
            viewer.style.transformOrigin = `unset`;

            PDFViewerApplication.pdfViewer.currentScale *= pinchScale;
            const rect = container.getBoundingClientRect();
            const dx = startX - rect.left;
            const dy = startY - rect.top;
            container.scrollLeft += dx * (pinchScale - 1);
            container.scrollTop += dy * (pinchScale - 1);

            reset();
        });
    }

    document.addEventListener('webviewerloaded', () => {
        if (!pinchZoomEnabled) {
            pinchZoomEnabled = true;
            enablePinchZoom();
        }
    });
</script>

@larsneo thank you so so so much its working prefectly now

<script> let pinchZoomEnabled = false; function enablePinchZoom(pdfViewer) { let startX = 0, startY = 0; let initialPinchDistance = 0; let pinchScale = 1; const viewer = document.getElementById("viewer"); const container = document.getElementById("viewerContainer"); const reset = () => { startX = startY = initialPinchDistance = 0; pinchScale = 1; }; // Prevent native iOS page zoom document.addEventListener("touchstart", (e) => { if (e.touches.length > 1) { startX = (e.touches[0].pageX + e.touches[1].pageX) / 2; startY = (e.touches[0].pageY + e.touches[1].pageY) / 2; initialPinchDistance = Math.hypot((e.touches[1].pageX - e.touches[0].pageX), (e.touches[1].pageY - e.touches[0].pageY)); } else { initialPinchDistance = 0; } }); document.addEventListener("touchmove", (e) => { if (initialPinchDistance <= 0 || e.touches.length < 2) { return; } if (e.scale !== 1) { e.preventDefault(); } const pinchDistance = Math.hypot((e.touches[1].pageX - e.touches[0].pageX), (e.touches[1].pageY - e.touches[0].pageY)); const originX = startX + container.scrollLeft; const originY = startY + container.scrollTop; pinchScale = pinchDistance / initialPinchDistance; viewer.style.transform = `scale(${pinchScale})`; viewer.style.transformOrigin = `${originX}px ${originY}px`; }, { passive: false }); document.addEventListener("touchend", (e) => { if (initialPinchDistance <= 0) { return; } viewer.style.transform = `none`; viewer.style.transformOrigin = `unset`; PDFViewerApplication.pdfViewer.currentScale *= pinchScale; const rect = container.getBoundingClientRect(); const dx = startX - rect.left; const dy = startY - rect.top; container.scrollLeft += dx * (pinchScale - 1); container.scrollTop += dy * (pinchScale - 1); reset(); }); } document.addEventListener('webviewerloaded', () => { if (!pinchZoomEnabled) { pinchZoomEnabled = true; enablePinchZoom(); } }); </script>

my pleasure, complete code also forked at https://gist.github.com/larsneo/bb75616e9426ae589f50e8c8411020f6

This works well for zooming, but seems to disable the normal drag-on-finger-to-pan action because of line 13. If I change the condition to be e.touches.length > 1 && e.scale !== 1, then it seems to work without problem on Android.