In Cloudformation YAML, use a Ref in a multiline string (? use Fn:Sub) - string

Question

Imagine you have a aws resource such as
Resources:
IdentityPool:
Type: "AWS::Cognito::IdentityPool"
Properties:
IdentityPoolName: ${self:custom.appName}_${self:provider.stage}_identity
CognitoIdentityProviders:
- ClientId:
Ref: UserPoolClient
The Ref for "AWS::Cognito::IdentityPool" returns the id of this resource. Now lets say I want to reference that id in a multiline string. I've tried
Outputs:
AmplifyConfig:
Description: key/values to be passed to Amplify.configure(config);
Value: |
{
'aws_cognito_identity_pool_id': ${Ref: IdentityPool}, ##<------ Error
'aws_sign_in_enabled': 'enable',
'aws_user_pools_mfa_type': 'OFF',
}
I've also tried to use Fn:Sub but without luck.
AmplifyConfig:
Description: key/values to be passed to Amplify.configure(config);
Value:
Fn::Sub
- |
{
'aws_cognito_identity_pool_id': '${Var1Name}',
'aws_sign_in_enabled': 'enable',
}
- Var1Name:
Ref: IdentityPool
Any way to do this?

Answer 1

Using a pipe symbol | in YAML turns all of the following indented lines into a multi-line string.
A pipe, combined with !Sub will let you use:
your resources Ref return value easily like ${YourResource}
their Fn::GetAtt return values with just a period ${YourResource.TheAttribute}
any Pseudo Parameter just as is like ${AWS:region}
As easy as !Sub |, jumping to the next line and adding proper indentation. Example:
Resources:
YourUserPool:
Type: AWS::Cognito::UserPool
Properties:
UserPoolName: blabla
Outputs:
AmplifyConfig:
Description: key/values to be passed to Amplify.configure(config);
Value: !Sub |
{
'aws_cognito_identity_pool_id': '${YourUserPool}',
'aws_sign_in_enabled': 'enable',
'aws_user_pools_mfa_type': 'OFF',
}
AdvancedUsage:
Description: use Pseudo Parameters and/or resources attributes
Value: !Sub |
{
'aws_region': '${AWS::Region}',
'user_pool_arn': '${YourUserPool.Arn}',
}

Answer 2

I found out how to do this using Join
AmplifyConfig:
Description: key/values to be passed to Amplify.configure(config);
Value:
Fn::Join:
- ''
- - "{"
- "\n 'aws_cognito_identity_pool_id':"
- Ref : IdentityPool
- "\n 'aws_user_pools_id':"
- Ref : UserPool
- "\n 'aws_user_pools_web_client_id':"
- Ref : UserPoolClient
- ",\n 'aws_cognito_region': '${self:provider.region}'"
- ",\n 'aws_sign_in_enabled': 'enable'"
- ",\n 'aws_user_pools': 'enable'"
- ",\n 'aws_user_pools_mfa_type': 'OFF'"
- "\n}"
This works but it's kinda ugly. I'm going to leave this answer unaccepted for a while to see if anyone can show how to do this with Fn::Sub.

Answer 3

Using YAML you could compose this simply:
Outputs:
AmplifyConfig:
Description: key/values to be passed to Amplify.configure(config);
Value: !Sub '
{
"aws_cognito_identity_pool_id": "${IdentityPool}",
"aws_sign_in_enabled": "enable",
"aws_user_pools_mfa_type": "OFF",
}'

Answer 4

Leaving this here as I encountered a Base64 encoding error when doing something similar and this question came up when searching for
a solution.
In my case I was a using multi line string + !Sub to populate UserData and receiving the following error in AWS Cloudformation.
Error:
Invalid BASE64 encoding of user data. (Service: AmazonEC2; Status
Code: 400; Error Code: InvalidUserData.Malformed; Request ID: *;
Proxy: null)
Solution:
Can be solved by combining two built in Cloudformation functions; Fn::Base64 and !Sub:
UserData:
Fn::Base64: !Sub |
#!/bin/bash
echo ${SomeVar}