For the most part the Slack API does not provide any official means to attach custom data / meta data to messages. But with some simple "hacks" it is still possible. Here is how:
Approach
The basic approach is to use an existing field of the message as container for your data. Obviously you want to pick a field that is not directly linked to Slack functionality.
Some field are not always needed, so you can just use that field as data container. Or if its needed, you can include the functional value of that field along with your custom data in the data container.
For example for message buttons you could use the value field of a button and structure your code in a way that you do not need it in its original function. Usually its sufficient to know which button the user client (via the name field), so the value field is free to be used for your custom data. Or you can include the functional value of your button along with the custom data in a data container (e.g. a JSON string) in that field.
Serialization
All messages are transported through HTTP and mostly encoded as UTF-8 in JSON. So you want to serialize / de-serialize your data accordingly, especially if its binary data. If possible I would recommend to use JSON.
Length
The maximum allowed length of most fields is documented in the official Slack API documentation. e.g. for the value field for message buttons can contain up to 2.000 characters. Keep in mind that you need to consider the length of your data after serialization. e.g. if you convert binary data into Base64 so it can be transported with HTTP you will end up with about 1.33 characters for every byte.
Contents
In general I would recommend to keep your data container as small as possible and not include the actual data, but only IDs. Here are two common approaches:
- Include IDs of your data objects and load the actual objects
from a data store when the request is later processed.
- Include the ID of server session and when processing the request you
can restore the corresponding server session which contains all data
objects.
In addition you might need to include functional values so that the functionality of the field you are using still works (e.g. value of a menu option, see below)
Implementation
Dialogs
Dialogs provide an official field for custom data called state. Up to 3.000 characters.
Message buttons
For Message buttons you can use the message action fields / value. Up to 2.000 characters. Its also possible to use the name field, but I would advise against it, because the maximum allowed length of that field is not documented.
Message menus
For Message menus you can use the value field of an option or the name field of the menu action.
Usually the value field is the better approach, since you have a documented max length of 2.000 and it gives you more flexibility. However, you will need to combine you custom data with the actual functional value for each option. Also, this will not work for dynamic select elements (like users), where you can not control the value field.
When using the name field note, keep in mind that the maximum allowed length of name is not documented, so you want to keep you data as short as possible. Also, if you want to use more than one menu per attachment you need to include the actual name of the menu into your data container.
Normal message attachments
Normal message attachments do not contain any suitable field to be used as container for custom data, since all fields are linked to Slack functionality.
Technically you could use the fallback field, but only if you are 100% sure that your app is never used on a client that can not display attachments. Otherwise your data will be displayed to the user.
