viewer/LdPicture: implement mousewheel zoom

GitHub: closes #153

1 files changed, 92 insertions, 0 deletions

diff --git a/viewer/src/services/ldzoom.ts b/viewer/src/services/ldzoom.ts
new file mode 100644
index 0000000..f001805
--- /dev/null
+++ b/viewer/src/services/ldzoom.ts
@@ -0,0 +1,92 @@
+/* ldgallery - A static generator which turns a collection of tagged
+--             pictures into a searchable web gallery.
+--
+-- Copyright (C) 2020  Pacien TRAN-GIRARD
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- This program is distributed in the hope that it will be useful,
+-- but WITHOUT ANY WARRANTY; without even the implied warranty of
+-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+-- GNU Affero General Public License for more details.
+--
+-- You should have received a copy of the GNU Affero General Public License
+-- along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+
+// polyfill still required for IE and Safari, see https://caniuse.com/#feat=resizeobserver
+import ResizeObserver from 'resize-observer-polyfill';
+
+/**
+ * Mousewheel picture zoom helper.
+ */
+export default class LdZoom {
+  readonly containerElement: HTMLDivElement;
+  readonly imageElement: HTMLImageElement;
+  readonly maxScaleFactor: number;
+  readonly zoomSpeed: number;
+  scaleFactor: number;
+
+  constructor(
+    containerElement: HTMLDivElement, imageElement: HTMLImageElement,
+    maxScaleFactor: number, zoomSpeed: number
+  ) {
+    this.containerElement = containerElement;
+    this.imageElement = imageElement;
+    this.maxScaleFactor = maxScaleFactor;
+    this.zoomSpeed = zoomSpeed;
+    this.scaleFactor = imageElement.clientWidth / imageElement.naturalWidth;
+  }
+
+  public install() {
+    new ResizeObserver(() => {
+      this.setImageScale(this.scaleFactor);
+      this.recenterImageElement();
+    }).observe(this.containerElement);
+
+    this.containerElement.addEventListener('wheel', wheelEvent => {
+      wheelEvent.preventDefault();
+      this.zoom(wheelEvent);
+    });
+
+    // TODO: handle pinch-to-zoom.
+
+    this.recenterImageElement();
+  }
+
+  /**
+   * Centers the image element inside its container if it fits, or stick to the top and left borders otherwise.
+   * It's depressingly hard to do in pure CSS…
+   */
+  private recenterImageElement() {
+    const marginLeft = Math.max((this.containerElement.clientWidth - this.imageElement.clientWidth) / 2, 0);
+    const marginTop = Math.max((this.containerElement.clientHeight - this.imageElement.clientHeight) / 2, 0);
+    this.imageElement.style.marginLeft = `${marginLeft}px`;
+    this.imageElement.style.marginTop = `${marginTop}px`;
+  }
+
+  private zoom(wheelEvent: WheelEvent) {
+    const ratioX = wheelEvent.offsetX / this.imageElement.clientWidth;
+    const ratioY = wheelEvent.offsetY / this.imageElement.clientHeight;
+
+    const zoomDelta = -Math.sign(wheelEvent.deltaY) * this.zoomSpeed;
+    this.setImageScale(Math.min(this.scaleFactor + zoomDelta, this.maxScaleFactor));
+
+    this.containerElement.scrollLeft -= wheelEvent.offsetX - ratioX * this.imageElement.clientWidth;
+    this.containerElement.scrollTop -= wheelEvent.offsetY - ratioY * this.imageElement.clientHeight;
+  }
+
+  private setImageScale(newScaleFactor: number) {
+    const horizontalFillRatio = this.containerElement.clientWidth / this.imageElement.naturalWidth;
+    const verticalFillRatio = this.containerElement.clientHeight / this.imageElement.naturalHeight;
+    const minScaleFactor = Math.min(horizontalFillRatio, verticalFillRatio, 1.0);
+    this.scaleFactor = Math.max(newScaleFactor, minScaleFactor);
+
+    this.imageElement.width = this.scaleFactor * this.imageElement.naturalWidth;
+    this.imageElement.height = this.scaleFactor * this.imageElement.naturalHeight;
+    this.recenterImageElement();
+  }
+}