class ChromeDownloads extends ChromeApi {
 static final JsObject _downloads = chrome['downloads'];

 ChromeDownloads._();

 bool get available => _downloads != null;

 /**
  * Download a URL. If the URL uses the HTTP[S] protocol, then the request will
  * include all cookies currently set for its hostname. If both `filename` and
  * `saveAs` are specified, then the Save As dialog will be displayed,
  * pre-populated with the specified `filename`. If the download started
  * successfully, `callback` will be called with the new [DownloadItem]'s
  * `downloadId`. If there was an error starting the download, then `callback`
  * will be called with `downloadId=undefined` and [runtime.lastError] will
  * contain a descriptive string. The error strings are not guaranteed to
  * remain backwards compatible between releases. Extensions must not parse it.
  * [options]: What to download and how.
  * [callback]: Called with the id of the new [DownloadItem].
  */
 Future<int> download(DownloadOptions options) {
   if (_downloads == null) _throwNotAvailable();

   var completer = new ChromeCompleter<int>.oneArg();
   _downloads.callMethod('download', [jsify(options), completer.callback]);
   return completer.future;
 }

 /**
  * Find [DownloadItem]. Set `query` to the empty object to get all
  * [DownloadItem]. To get a specific [DownloadItem], set only the `id` field.
  * To page through a large number of items, set `orderBy: ['-startTime']`, set
  * `limit` to the number of items per page, and set `startedAfter` to the
  * `startTime` of the last item from the last page.
  */
 Future<List<DownloadItem>> search(DownloadQuery query) {
   if (_downloads == null) _throwNotAvailable();

   var completer = new ChromeCompleter<List<DownloadItem>>.oneArg((e) => listify(e, _createDownloadItem));
   _downloads.callMethod('search', [jsify(query), completer.callback]);
   return completer.future;
 }

 /**
  * Pause the download. If the request was successful the download is in a
  * paused state. Otherwise [runtime.lastError] contains an error message. The
  * request will fail if the download is not active.
  * [downloadId]: The id of the download to pause.
  * [callback]: Called when the pause request is completed.
  */
 Future pause(int downloadId) {
   if (_downloads == null) _throwNotAvailable();

   var completer = new ChromeCompleter.noArgs();
   _downloads.callMethod('pause', [downloadId, completer.callback]);
   return completer.future;
 }

 /**
  * Resume a paused download. If the request was successful the download is in
  * progress and unpaused. Otherwise [runtime.lastError] contains an error
  * message. The request will fail if the download is not active.
  * [downloadId]: The id of the download to resume.
  * [callback]: Called when the resume request is completed.
  */
 Future resume(int downloadId) {
   if (_downloads == null) _throwNotAvailable();

   var completer = new ChromeCompleter.noArgs();
   _downloads.callMethod('resume', [downloadId, completer.callback]);
   return completer.future;
 }

 /**
  * Cancel a download. When `callback` is run, the download is cancelled,
  * completed, interrupted or doesn't exist anymore.
  * [downloadId]: The id of the download to cancel.
  * [callback]: Called when the cancel request is completed.
  */
 Future cancel(int downloadId) {
   if (_downloads == null) _throwNotAvailable();

   var completer = new ChromeCompleter.noArgs();
   _downloads.callMethod('cancel', [downloadId, completer.callback]);
   return completer.future;
 }

 /**
  * Retrieve an icon for the specified download. For new downloads, file icons
  * are available after the [onCreated] event has been received. The image
  * returned by this function while a download is in progress may be different
  * from the image returned after the download is complete. Icon retrieval is
  * done by querying the underlying operating system or toolkit depending on
  * the platform. The icon that is returned will therefore depend on a number
  * of factors including state of the download, platform, registered file types
  * and visual theme. If a file icon cannot be determined, [runtime.lastError]
  * will contain an error message.
  * [downloadId]: The identifier for the download.
  * [callback]: A URL to an image that represents the download.
  */
 Future<String> getFileIcon(int downloadId, [GetFileIconOptions options]) {
   if (_downloads == null) _throwNotAvailable();

   var completer = new ChromeCompleter<String>.oneArg();
   _downloads.callMethod('getFileIcon', [downloadId, jsify(options), completer.callback]);
   return completer.future;
 }

 /**
  * Open the downloaded file now if the [DownloadItem] is complete; otherwise
  * returns an error through [runtime.lastError]. Requires the
  * `"downloads.open"` permission in addition to the `"downloads"` permission.
  * An [onChanged] event will fire when the item is opened for the first time.
  * [downloadId]: The identifier for the downloaded file.
  */
 void open(int downloadId) {
   if (_downloads == null) _throwNotAvailable();

   _downloads.callMethod('open', [downloadId]);
 }

 /**
  * Show the downloaded file in its folder in a file manager.
  * [downloadId]: The identifier for the downloaded file.
  */
 void show(int downloadId) {
   if (_downloads == null) _throwNotAvailable();

   _downloads.callMethod('show', [downloadId]);
 }

 /**
  * Show the default Downloads folder in a file manager.
  */
 void showDefaultFolder() {
   if (_downloads == null) _throwNotAvailable();

   _downloads.callMethod('showDefaultFolder');
 }

 /**
  * Erase matching [DownloadItem] from history without deleting the downloaded
  * file. An [onErased] event will fire for each [DownloadItem] that matches
  * `query`, then `callback` will be called.
  */
 Future<List<int>> erase(DownloadQuery query) {
   if (_downloads == null) _throwNotAvailable();

   var completer = new ChromeCompleter<List<int>>.oneArg(listify);
   _downloads.callMethod('erase', [jsify(query), completer.callback]);
   return completer.future;
 }

 /**
  * Remove the downloaded file if it exists and the [DownloadItem] is complete;
  * otherwise return an error through [runtime.lastError].
  */
 Future removeFile(int downloadId) {
   if (_downloads == null) _throwNotAvailable();

   var completer = new ChromeCompleter.noArgs();
   _downloads.callMethod('removeFile', [downloadId, completer.callback]);
   return completer.future;
 }

 /**
  * Prompt the user to accept a dangerous download. Does not automatically
  * accept dangerous downloads. If the download is accepted, then an
  * [onChanged] event will fire, otherwise nothing will happen. When all the
  * data is fetched into a temporary file and either the download is not
  * dangerous or the danger has been accepted, then the temporary file is
  * renamed to the target filename, the [state] changes to 'complete', and
  * [onChanged] fires.
  * [downloadId]: The identifier for the [DownloadItem].
  * [callback]: Called when the danger prompt dialog closes.
  */
 Future acceptDanger(int downloadId) {
   if (_downloads == null) _throwNotAvailable();

   var completer = new ChromeCompleter.noArgs();
   _downloads.callMethod('acceptDanger', [downloadId, completer.callback]);
   return completer.future;
 }

 /**
  * Initiate dragging the downloaded file to another application. Call in a
  * javascript `ondragstart` handler.
  */
 void drag(int downloadId) {
   if (_downloads == null) _throwNotAvailable();

   _downloads.callMethod('drag', [downloadId]);
 }

 /**
  * Enable or disable the gray shelf at the bottom of every window associated
  * with the current browser profile. The shelf will be disabled as long as at
  * least one extension has disabled it. Enabling the shelf while at least one
  * other extension has disabled it will return an error through
  * [runtime.lastError]. Requires the `"downloads.shelf"` permission in
  * addition to the `"downloads"` permission.
  */
 void setShelfEnabled(bool enabled) {
   if (_downloads == null) _throwNotAvailable();

   _downloads.callMethod('setShelfEnabled', [enabled]);
 }

 Stream<DownloadItem> get onCreated => _onCreated.stream;

 final ChromeStreamController<DownloadItem> _onCreated =
     new ChromeStreamController<DownloadItem>.oneArg(_downloads, 'onCreated', _createDownloadItem);

 Stream<int> get onErased => _onErased.stream;

 final ChromeStreamController<int> _onErased =
     new ChromeStreamController<int>.oneArg(_downloads, 'onErased', selfConverter);

 Stream<DownloadDelta> get onChanged => _onChanged.stream;

 final ChromeStreamController<DownloadDelta> _onChanged =
     new ChromeStreamController<DownloadDelta>.oneArg(_downloads, 'onChanged', _createDownloadDelta);

 Stream<OnDeterminingFilenameEvent> get onDeterminingFilename => _onDeterminingFilename.stream;

 final ChromeStreamController<OnDeterminingFilenameEvent> _onDeterminingFilename =
     new ChromeStreamController<OnDeterminingFilenameEvent>.twoArgs(_downloads, 'onDeterminingFilename', _createOnDeterminingFilenameEvent);

 void _throwNotAvailable() {
   throw new UnsupportedError("'chrome.downloads' is not available");
 }
}