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Canvas

Canvas

The Canvas project

This is a client-server system to represent an ASCII art drawing canvas. It has two parts:

  • API to create a canvas and draw on it
  • Read-only web client to present and observe given canvas

API

API allows to create a canvas and draw on it.

To start the API server:

  • Install dependencies with mix deps.get
  • Create and migrate your database with mix ecto.setup
  • Start Phoenix endpoint with mix phx.server or inside IEx with iex -S mix phx.server

First step would be to create a canvas. You can do this by invoking POST /canvas method.

For example from the terminal: curl -X POST http://localhost:4000/canvas

It will create default 50x50 canvas and will return its ID. The response is formatted as JSON:

{"content":["                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  ","                                                  "],"id":1}

You can also pass your own height and width of the canvas: curl -X POST --header "Content-Type: application/json" -d '{"canvas": {"width": "4", "height": "4"}}' http://localhost:4000/canvas:

{"content":["    ","    ","    ","    "],"id":2}

Now you can draw on your canvas!

It can be done by invoking POST /draw endpoint.

That enpoint accepts following parameters:

  • operation (required) - what to draw. Two values are possible: Rectangle (draw a rectangle), Flood fill (fill empty space starting from a given point)
  • x and y (required) - cordinates where to start drawing,
  • canvas_id (required) - ID of the canvas created in the previous step,
  • width and height - required for Rectangle - how big the rectangle should be,
  • fill_char - required for Flood fill - a single character that will be used to fill the canvas or the rectangle,
  • outline_char - a single character that will be used to draw an outline of the Rectangle. One of the fill_char or outline_char is required for Rectangle.

Examples:

  1. On canvas with ID=1 draw a rectangle starting from point 5, 5 with width and height equal 5 and outline "," (so without fill).
curl -H "Content-Type: application/json" \
  -X POST \
  -d '{"draw": {"operation": "Rectangle", "x": "5", "y": "5", "width": "5", "height": "5", "outline_char": ",", "canvas_id": "1"}}' \
  http://localhost:4000/draw
  1. Fill canvas with ID=1 with characted "O".
curl -H "Content-Type: application/json" \
  -X POST \
  -d '{"draw": {"operation": "Flood fill", "x": "0", "y": "0",  "canvas_id": "1"}}' \
  http://localhost:4000/draw

Errors:

If your API request is missing something, it will be reported in the response. For example:

curl -H "Content-Type: application/json" -X POST -d '{"draw": {"operation": "Flood fill", "x": "0", "y": "0",  "canvas_id": "1"}}' http://localhost:4000/draw

will produce:

{"errors":{"fill_char":["Fill char has to be provided."]}}

Read-only web client to present and observe given canvas

You can also observe how the canvas is created in the browser. All in real-time.

Assuming you have the server running (see above) go to http://localhost:4000/?id=CANVAS_ID, wheras CANVAS_ID is ID of the canvas created above with API endpoint.

Once API endpoints to draw rectangles or fill shapes are invoked you will observe updated canvas without any need to refresh.

Author

Copyright, Krzysztof Kempiński