6 use cases it's desirable for downloads to continue even when your app is in the
8 send commands to the service to add, remove and control the downloads. The
18 * `DownloadManager`: Manages multiple downloads, loading (and storing) their
19   states from (and to) a `DownloadIndex`, starting and stopping downloads based
23 * `DownloadIndex`: Persists the states of the downloads.
32   service when requirements needed for pending downloads to progress are met.
80 // already have an executor for background downloads may wish to reuse their existing executor.
138 ## Removing downloads ##
155 ## Starting and stopping downloads ##
160 * Downloads aren't paused.
161 * The requirements for downloads to progress are met. Requirements can specify
164 * The maximum number of parallel downloads is not exceeded.
171 It's possible to set a reason for one or all downloads being stopped:
194 multiple reasons for stopping downloads can use different values to keep track
196 downloads works the same way as setting and clearing the stop reason for a
208 #### Pausing and resuming all downloads ####  argument
210 All downloads can be paused and resumed as follows:
213 // Pause all downloads.
219 // Resume all downloads.
227 When downloads are paused, they will be in the `Download.STATE_QUEUED` state.
231 #### Setting the requirements for downloads to progress ####  argument
234 downloads to proceed. The requirements can be set by calling
253 #### Setting the maximum number of parallel downloads ####  argument
255 The maximum number of parallel downloads can be set by calling
259 When a download cannot proceed because the maximum number of parallel downloads
262 ## Querying downloads ##
265 downloads, including those that have completed or failed. The `DownloadIndex`
267 iterates over all downloads can then be obtained by calling
272 returns the state of current (i.e. not completed or failed) downloads only. This
274 the progress and status of current downloads.
276 ## Listening to downloads ##
279 downloads change state: