viewer: when loading a picture, displays a preview based on the thumbnail on slow connections

This works on Chrome, but FireFox presents some issues:
- the picture background is sometimes white instead of transparent, hidding the background
- image-orientation doesn't work for background pictures or for negative values

2 files changed, 55 insertions, 8 deletions

diff --git a/viewer/src/assets/scss/global.scss b/viewer/src/assets/scss/global.scss
index b418911..5de5946 100644
--- a/viewer/src/assets/scss/global.scss
+++ b/viewer/src/assets/scss/global.scss
@@ -96,10 +96,12 @@ img {
 
 // === Effect to apply on lazy-image loading
 
-.v-lazy-image {
-  opacity: 0;
-  transition: opacity 0.4s;
-}
-.v-lazy-image-loaded {
-  opacity: 1;
+img {
+  &.v-lazy-image {
+    opacity: 0;
+    transition: opacity 0.4s;
+  }
+  &.v-lazy-image-loaded {
+    opacity: 1;
+  }
 }
diff --git a/viewer/src/components/LdPicture.vue b/viewer/src/components/LdPicture.vue
index b6a719f..5425a00 100644
--- a/viewer/src/components/LdPicture.vue
+++ b/viewer/src/components/LdPicture.vue
@@ -25,7 +25,13 @@
     @click="onClick"
     @dragscrollstart="dragging=true"
   >
-    <v-lazy-image :src="pictureSrc" />
+    <v-lazy-image
+      :src="pictureSrc()"
+      :class="{'slow-loading': Boolean(slowLoadingStyle)}"
+      :style="slowLoadingStyle"
+      @load="clearTimer"
+    />
+    <b-loading :active="Boolean(slowLoadingStyle)" :is-full-page="false" class="ld-picture-loader" />
   </div>
 </template>
 
@@ -36,12 +42,35 @@ import { Component, Vue, Prop } from "vue-property-decorator";
 export default class LdPicture extends Vue {
   @Prop({ required: true }) readonly picture!: Gallery.Picture;
 
+  readonly SLOW_LOADING_TIMEOUT_MS: number = 1500;
+
   dragging: boolean = false;
+  slowLoadingStyle: string | null = null;
+  timer: NodeJS.Timeout | null = null;
+
+  mounted() {
+    if (this.picture.thumbnail) this.timer = setTimeout(this.generateSlowLoadingStyle, this.SLOW_LOADING_TIMEOUT_MS);
+  }
+
+  destroyed() {
+    this.clearTimer();
+  }
+
+  clearTimer() {
+    if (this.timer) clearTimeout(this.timer);
+    this.timer = null;
+    this.slowLoadingStyle = null;
+  }
 
-  get pictureSrc() {
+  pictureSrc() {
     return `${process.env.VUE_APP_DATA_URL}${this.picture.properties.resource}`;
   }
 
+  generateSlowLoadingStyle() {
+    this.clearTimer();
+    this.slowLoadingStyle = `background-image: url('${process.env.VUE_APP_DATA_URL}${this.picture.thumbnail}');`;
+  }
+
   onClick() {
     if (!this.dragging) this.$uiStore.toggleFullscreen();
     this.dragging = false;
@@ -50,6 +79,22 @@ export default class LdPicture extends Vue {
 </script>
 
 <style lang="scss">
+@import "@/assets/scss/theme.scss";
+
+.ld-picture-loader {
+  position: relative;
+  & .loading-background {
+    background: none !important;
+  }
+}
+img.slow-loading {
+  background-repeat: no-repeat;
+  background-position: center;
+  background-size: contain;
+  background-color: $content-bgcolor;
+  background-blend-mode: soft-light;
+  opacity: 1 !important;
+}
 .fit-to-screen {
   display: flex;
   justify-content: space-around;