sclass JVideoLibDownloader is Swingable {
  transient SingleComponentPanel scp;
  
  selfType forward(IF0<JComponent> forward) {
    this.forward = forward;
    this;
  }
  
  swappable JComponent forward() {
    ret jcenteredlabel("Video extensions enabled");
  }
  
  S libID() {
    ret ffmpegLibraryIDForPlatform();
  }
  
  bool videoSupported() { ret libID() != null; }
  
  bool libDownloaded() {
    ret fileExists(DiskSnippetCache_getLibrary(libID()));
  }
  
  bool hasLib() {
    bool has = currentProgramHasLibrary(DiskSnippetCache_getLibrary(libID()));
    print("Video library ID: " + libID() + ", loaded: " + has);
    ret has;
  }
  
  void addLib() {
    if (videoSupported())
      if (addLibraryToCurrentProgram(libID()))
        print("Video library loaded: " + libID());
  }
  
  visualize {
    if (scp == null)
      scp = singleComponentPanel(makeComponent());
    ret scp;
  }
  
  JComponent makeComponent() {
    if (!videoSupported())
      ret jcenteredlabel("Video playback is not yet supported on your platform");
      
    if (libDownloaded()) {
      addLib();
      ret forward();
    } else
      ret jfullcenter(centerAndEastWithMargin(
        jcenteredlabel("Download video player for " + platformName() + "(ca. 20 MB)?"),
        jbuttonWithDisable("OK", r addLibWithProgressBar)));
  }
  
  void addLibWithProgressBar {
    scp.setComponent(jcenteredlabel("Downloading extension..."));
    try {
      addLib();
      scp.setComponent(forward());
    } catch print e {
      scp.setComponent(jErrorView(e));
    }
  }
}

Began life as a copy of #1034410