Option to load text tracks on demand vs preload

Reimplementation of #2192
on current code. Seems to work but has not been carefully tested,
especially on conditions such as slow networks and complex tracks.

For #5252

A `preloadTextTracks` tech option is added, set to true by default,
to keep current behavior intact. Alternate behavior can be enabled
by setting this to false.

This delays loading of the VTT cue files until they are selected.
For sites like Wikipedia that tend to have large numbers of
crowdsourced subtitles and can show many files together on one
page, this saves a lot of unnecessary network transfer and API
hits.

Does mean there may be dropped cues while switching to a track
that requires on-demand loading.

Example usage:

videojs(element, {
  html5: {
    preloadTextTracks: false
  }
};

docs/guides/options.md

@@ -58,6 +58,7 @@
     * [nativeAudioTracks](#nativeaudiotracks)
     * [nativeTextTracks](#nativetexttracks)
     * [nativeVideoTracks](#nativevideotracks)
+    * [preloadTextTracks](#preloadtexttracks)
 
 ## Standard `<video>` Element Options
 
@@ -607,6 +608,14 @@ Can be set to `false` to force emulation of text tracks instead of native suppor
 
 Can be set to `false` to disable native video track support. Most commonly used with [videojs-contrib-hls][videojs-contrib-hls].
 
+#### `preloadTextTracks`
+
+> Type: `boolean`
+
+Can be set to `false` to delay loading of non-active text tracks until use. This can cause a short delay when switching captions during which there may be missing captions.
+
+The default behavior is to preload all text tracks.
+
 [plugins]: /docs/guides/plugins.md
 
 [languages]: /docs/guides/languages.md

src/js/tech/tech.js

@@ -141,6 +141,8 @@ class Tech extends Component {
       this.emulateTextTracks();
     }
 
+    this.preloadTextTracks = options.preloadTextTracks !== false;
+
     this.autoRemoteTextTracks_ = new TRACK_TYPES.ALL.text.ListClass();
 
     this.initTrackListeners();

src/js/tracks/text-track.js

@@ -172,6 +172,8 @@ class TextTrack extends Track {
     this.cues_ = [];
     this.activeCues_ = [];
 
+    this.preload_ = this.tech_.preloadTextTracks !== false;
+
     const cues = new TextTrackCueList(this.cues_);
     const activeCues = new TextTrackCueList(this.activeCues_);
     let changed = false;
@@ -229,6 +231,10 @@ class TextTrack extends Track {
             return;
           }
           mode = newMode;
+          if (!this.preload_ && mode !== 'disabled' && this.cues.length === 0) {
+            // On-demand load.
+            loadTrack(this.src, this);
+          }
           if (mode !== 'disabled') {
             this.tech_.ready(() => {
               this.tech_.on('timeupdate', timeupdateHandler);
@@ -324,7 +330,14 @@ class TextTrack extends Track {
 
     if (settings.src) {
       this.src = settings.src;
-      loadTrack(settings.src, this);
+      if (!this.preload_) {
+        // Tracks will load on-demand.
+        // Act like we're loaded for other purposes.
+        this.loaded_ = true;
+      }
+      if (this.preload_ || default_ || (settings.kind !== 'subtitles' && settings.kind !== 'captions')) {
+        loadTrack(this.src, this);
+      }
     } else {
       this.loaded_ = true;
     }