Refactor resizeImage method (#7236)

- Use URL.createObjectURL (replace from FileReader)
- Use HTMLCanvasElement.prototype.toBlob
  (replace from HTMLCanvasElement.prototype.toDataURL)
- Use Promise (replace callback interface)

app/javascript/mastodon/utils/resize_image.js

@@ -0,0 +1,66 @@
+const MAX_IMAGE_DIMENSION = 1280;
+
+const getImageUrl = inputFile => new Promise((resolve, reject) => {
+  if (window.URL && URL.createObjectURL) {
+    try {
+      resolve(URL.createObjectURL(inputFile));
+    } catch (error) {
+      reject(error);
+    }
+    return;
+  }
+
+  const reader = new FileReader();
+  reader.onerror = (...args) => reject(...args);
+  reader.onload  = ({ target }) => resolve(target.result);
+
+  reader.readAsDataURL(inputFile);
+});
+
+const loadImage = inputFile => new Promise((resolve, reject) => {
+  getImageUrl(inputFile).then(url => {
+    const img = new Image();
+
+    img.onerror = (...args) => reject(...args);
+    img.onload  = () => resolve(img);
+
+    img.src = url;
+  }).catch(reject);
+});
+
+export default inputFile => new Promise((resolve, reject) => {
+  if (!inputFile.type.match(/image.*/) || inputFile.type === 'image/gif') {
+    resolve(inputFile);
+    return;
+  }
+
+  loadImage(inputFile).then(img => {
+    const canvas = document.createElement('canvas');
+    const { width, height } = img;
+
+    let newWidth, newHeight;
+
+    if (width < MAX_IMAGE_DIMENSION && height < MAX_IMAGE_DIMENSION) {
+      resolve(inputFile);
+      return;
+    }
+
+    if (width > height) {
+      newHeight = height * MAX_IMAGE_DIMENSION / width;
+      newWidth  = MAX_IMAGE_DIMENSION;
+    } else if (height > width) {
+      newWidth  = width * MAX_IMAGE_DIMENSION / height;
+      newHeight = MAX_IMAGE_DIMENSION;
+    } else {
+      newWidth  = MAX_IMAGE_DIMENSION;
+      newHeight = MAX_IMAGE_DIMENSION;
+    }
+
+    canvas.width  = newWidth;
+    canvas.height = newHeight;
+
+    canvas.getContext('2d').drawImage(img, 0, 0, newWidth, newHeight);
+
+    canvas.toBlob(resolve, inputFile.type);
+  }).catch(reject);
+});