Product
Docs
Resources
Log InSign Up
Courier Live Header
COURIER

Designing Slack Notifications using Block Kit and Jsonnet

Aydrian Howard

September 02, 2020

Earlier this summer, Riley Napier from our engineering team joined me for our June 24th Courier Live to help me build a Slack Slash Command to display estimated departure times for BART Stations. We created a Glitch ExpressJS app to accept the commands and Courier to handle the responses. We designed the messages by dynamically generating Block Kit using Jsonnet.

Check out the video below to watch us:

  • Walk through setting up a Glitch App that handles Slack Slash Commands
  • Create a notification for Slack
  • Use the Jsonnet Block to dynamically create Block Kit elements

Find the full project code on the Courier Slack Slash Bart Glitch App.

Be sure to Like the video and Subscribe to our YouTube channel.

What is Jsonnet?

Jsonnet is a powerful data templating language for JSON. It has a Python-like syntax that allows you to build JSON output using variables, functions, conditionals, etc. This comes in handy when dynamically creating Block Kit elements for Slack. Courier provides the following functions that allow you to grab data and profile information.

1
# Grab values by JSON path from data passed during send
2
local data = data("path", "default");
3
4
# Grab values by JSON path from merged recipient profile
5
local profile =  profile("path", "default");

For an overview of the basics of Jsonnet syntax, check out Learn Jsonnet in Y minutes.

Displaying BART Station Departure Times

To display the Departure Times passed to the notification, we used variables, functions, and list comprehensions to generate the resulting Block Kit sections. We started with the following data originating from the BART API:

1
{
2
  "data": {
3
    "date": "09/01/2020",
4
    "time": "06:50:01 AM PDT",
5
    "station": {
6
      "name": "Powell St.",
7
      "abbr": "POWL",
8
      "etd": [
9
        {
10
          "destination": "Antioch",
11
          "abbreviation": "ANTC",
12
          "limited": "0",
13
          "estimate": [
14
            {
15
              "minutes": "7",
16
              "platform": "2",
17
              "direction": "North",
18
              "length": "10",
19
              "color": "YELLOW",
20
              "hexcolor": "#ffff33",
21
              "bikeflag": "1",
22
              "delay": "0"
23
            },
24
            {
25
              "minutes": "37",
26
              "platform": "2",
27
              "direction": "North",
28
              "length": "10",
29
              "color": "YELLOW",
30
              "hexcolor": "#ffff33",
31
              "bikeflag": "1",
32
              "delay": "0"
33
            },
34
            {
35
              "minutes": "66",
36
              "platform": "2",
37
              "direction": "North",
38
              "length": "10",
39
              "color": "YELLOW",
40
              "hexcolor": "#ffff33",
41
              "bikeflag": "1",
42
              "delay": "0"
43
            }
44
          ]
45
        },
46
        {
47
          "destination": "Berryessa",
48
          "abbreviation": "BERY",
49
          "limited": "0",
50
          "estimate": [
51
            {
52
              "minutes": "9",
53
              "platform": "2",
54
              "direction": "North",
55
              "length": "10",
56
              "color": "GREEN",
57
              "hexcolor": "#339933",
58
              "bikeflag": "1",
59
              "delay": "0"
60
            },
61
            {
62
              "minutes": "39",
63
              "platform": "2",
64
              "direction": "North",
65
              "length": "10",
66
              "color": "GREEN",
67
              "hexcolor": "#339933",
68
              "bikeflag": "1",
69
              "delay": "0"
70
            }
71
          ]
72
        }
73
      ]
74
    },
75
    "message": ""
76
  } 
77
}

Using a Jsonnet block, we used the following Jsonnet code to render the Block Kit Sections

1
local station = data("station");
2
local get_image(color, direction) = 
3
"https://dummyimage.com/100x100/%s/000000.png&text=%s" % [color[1:], direction];
4
5
local get_cars(length) = std.join("", [ ":train:" for x in std.range(1, std.parseInt(length))]);
6
7
local is_bike(flag) = 
8
if flag == "1" then "\n:bike:" else "";
9
10
local get_estimate(estimate) = 
11
{
12
     "type": "section",
13
        "text": {
14
         "type": "mrkdwn",
15
            "text": ":clock4: *%s min*\nPlatform %s\n%s %s" % [estimate.minutes, estimate.platform, get_cars(estimate.length), is_bike(estimate.bikeflag)]
16
        },
17
        "accessory": {
18
         "type": "image",
19
            "image_url": get_image(estimate.hexcolor, estimate.direction),
20
            "alt_text": "%s %s" % [estimate.direction, estimate.color]
21
        }
22
    };
23
24
local get_estimates(estimates) = 
25
[
26
     get_estimate(estimate)
27
    for estimate in estimates];
28
29
local get_destinations(etd) = 
30
[ 
31
     {
32
"type": "section",
33
"text": {
34
"type": "mrkdwn",
35
"text": "*%s*" % etd.destination
36
}
37
}
38
    ] + get_estimates(etd.estimate) +
39
    [
40
        {
41
        "type": "divider"
42
        }
43
    ];
44
45
std.flattenArrays([
46
get_destinations(etd)
47
for etd in station.etd
48
])

This results in Block Kit that looks like the following:

Bart API Slack Blocks

Feel free to remix our Glitch app and create your own Slack Slash Command. Be sure to let us know what you create.

Is there something you’d like to see us do using Courier? Let us know and it might be the subject of our next Courier Live. We stream a new Courier Live every Wednesday at noon Pacific. Follow us on Twitch to be notified when we go live.

-Aydrian

More from Courier

data-thumbnail
COURIER

Tools and Techniques to Establish Your Data Team Early

How tools like Segment, Metabase, Snowflake, Census, and others, can help establish a data team from the very early stages in a startup.

Raymond See

Raymond See

February 16, 2023

QOTD Thumbnail
ENGINEERINGCOURIER

Develop a Motivational QOTD with Courier and GPT2

Courier and OpenGPT2 in action: build a service that sends friends and family an AI generated motivational quote of the day.

Prakhar Srivastav

Prakhar Srivastav

February 09, 2023

Build your first notification in minutes

Send up to 10,000 notifications every month, for free.

Get started for free

Email & push notification

Build your first notification in minutes

Send up to 10,000 notifications every month, for free.

Get started for free

Email & push notification

Product

Pricing

Integrations

Changelog

Developers

Documentation

API

Libraries

Status

© 2023 Courier. All rights reserved.