screenshot method Null safety

Future<List<int>> screenshot()

Take a screenshot.

The image will be returned as a PNG.

Warning: This is not reliable.

There is a two-second artificial delay before screenshotting. The delay here is to deal with a race between the driver script and the raster thread (formerly known as the GPU thread). The issue is that the driver API synchronizes with the framework based on transient callbacks, which are out of sync with the raster thread.

Here's the timeline of events in ASCII art:

---------------------------------------------------------------
Without this delay:
---------------------------------------------------------------
UI    : <-- build -->
Raster:               <-- rasterize -->
Gap   :              | random |
Driver:                        <-- screenshot -->

In the diagram above, the gap is the time between the last driver action taken, such as a tap(), and the subsequent call to screenshot(). The gap is random because it is determined by the unpredictable communication channel between the driver process and the application. If this gap is too short, which it typically will be, the screenshot is taken before the raster thread is done rasterizing the frame, so the screenshot of the previous frame is taken, which is not what is intended.

---------------------------------------------------------------
With this delay, if we're lucky:
---------------------------------------------------------------
UI    : <-- build -->
Raster:               <-- rasterize -->
Gap   :              |    2 seconds or more   |
Driver:                                        <-- screenshot -->

The two-second gap should be long enough for the raster thread to finish rasterizing the frame, but not longer than necessary to keep driver tests as fast a possible.

---------------------------------------------------------------
With this delay, if we're not lucky:
---------------------------------------------------------------
UI    : <-- build -->
Raster:               <-- rasterize randomly slow today -->
Gap   :              |    2 seconds or more   |
Driver:                                        <-- screenshot -->

In practice, sometimes the device gets really busy for a while and even two seconds isn't enough, which means that this is still racy and a source of flakes.

Implementation

Future<List<int>> screenshot() async {
  throw UnimplementedError();
}