Support questionnaire-constraint Extension

[delcroip]

Is your feature request related to a problem? Please describe.
I need to set validation a bit more complex than max/min on question there is an extension available
http://hl7.org/fhir/StructureDefinition/questionnaire-constraint

Describe the solution you'd like
Support of the extension and run add it to the form validation
Must support reference to the item value through:

• getValue()
• %qitem

Neither are supported today (GetValue() return always null, %qitem throw a Fhirpath exception) might be related to #1246

Describe alternatives you've considered
Min /Max

but the issue it can only do numeric validation

also the message cannot be customised

Additional context
here a simple example (like a max)

{
                        "extension": [
                            {
                                "extension": [
                                    {
                                        "url": "key",
                                        "valueId": "EmCare.A.DE13.a"
                                    },
                                    {
                                        "url": "expression",
                                        "valueExpression": {
                                            "language": "text/fhirpath",
                                            "expression": "getValue().empty() or getValue()>=0 and getValue()<4"
                                        }
                                    },
                                    {
                                        "url": "severity",
                                        "valueCode": "error"
                                    },
                                    {
                                        "url": "human",
                                        "valueString": "the number of year cannot be above 4"
                                    }
                                ],
                                "url": "http://hl7.org/fhir/StructureDefinition/questionnaire-constraint"
                            },
                            {
                                "url": "http://hl7.org/fhir/uv/sdc/StructureDefinition/sdc-questionnaire-enableWhenExpression",
                                "valueExpression": {
                                    "language": "text/fhirpath",
                                    "expression": "%resource.repeat(item).where(linkId='EmCare.A.DE12').answer.first().value.code = 'EmCare.A.DE13'"
                                }
                            }
                        ],
                        "linkId": "EmCare.A.DE13.a",
                        "text": "Age in years",
                        "type": "quantity",
                        "required": false,
                        "repeats": false
                    },

Would you like to work on the issue?
no, I don't have the skills

[delcroip]

Update on other alternatives: Min / Max is not supportted for Quantity type

[delcroip]

I think qitem should be part of that list

android-fhir/datacapture/src/main/java/com/google/android/fhir/datacapture/fhirpath/ExpressionEvaluator.kt

Line 46 in e0c92b2

private val reservedVariables =

[jingtang10]

Please see in #2003 for additional comments.

Assigned to @FikriMilano. Thanks for taking this Fikri.

[ndegwamartin]

I think qitem should be part of that list

android-fhir/datacapture/src/main/java/com/google/android/fhir/datacapture/fhirpath/ExpressionEvaluator.kt

Line 46 in e0c92b2

private val reservedVariables =

+1 on this. Additionally questionnaire because of %questionnaire?

[FikriMilano]

android-fhir/datacapture/src/main/java/com/google/android/fhir/datacapture/fhirpath/ExpressionEvaluator.kt

Line 46 in e0c92b2

private val reservedVariables =

+1 on this. Additionally questionnaire because of %questionnaire?

I'm happy to have this in a separate PR, it will be super small change, that way it can be merged quick.

[FikriMilano]

@ndegwamartin @delcroip here's the ticket #2477