ImageCommand API for async image processing

[brendan-duncan]

Part of the major 4.0 update, I am addressing the much asked for async functionality. Image loading and processing is a heavy process, particularly in a language like Dart that lacks things like shared memory or SIMD instructions.

Coming up with a way to do image processing async, in a way that works with Dart's Isolate memory model, was tricky. I came up with an ImageCommand API I think will work.

A sequence of image loading / processing / saving functions can be defined in an ImageCommand. To load a jpeg image, resize it, and save it to a png file, you could do the following:

final cmd = ImageCommand()
  // Read a jpeg file from a file named 'test.jpg'
  ..decodeJpgFile('test.jpg')
  // Resize the image so its width is 120 and its height maintains the aspect ratio
  ..copyResize(width: 120)
  // Save the resized image to a png file named 'thumbnail.png'.
  ..encodePngFile('thumbnail.png');

Nothing has actually been run yet, this just recorded the commands into cmd. You can execute the commands asynchronously with

await cmd.executeAsync();

Or you can execute the commands synchronously with

cmd.execute();

You can access the resulting image from the execution results with

final image = cmd.getImage(); // Returns an Image?

This will be the resized Image.

If the commands had encoded an Image to a format, which happened with the encodePngFile command, you can access the encoded bytes with:

final bytes = cmd.getBytes(); // Returns a Uint8List?

getImage and getBytes will execute the commands if they haven't been executed yet. As such, there are also getImageAsync and getBytesAsync variants of the functions.

[g-apparence]

Hi, that would be easier for developpers to get this info on readme.
I used to try getting result with "outputImage" which seems not working.