Build Expressions in Altinn Studio
How to build expressions with Altinn Studio.
This documentation will describe how to build expressions connected to component fields using the Expressions tool in Altinn Studio.
Expressions are a concept in Altinn apps that lets app-developers dynamically assign values to different elements in an application. Please read more in the Expressions section of Apps documentation of what expressions can be used for and how the syntax works.
In Altinn Studio the concept is called logic where an expression is addressed as a logic rule.
- Expression: A unit consisting of the boolean component field and the actual expression that will evaluate to be a boolean value when calculated in a running app. The most simple expression can consist of a single sub expression, while a more advanced expression can consist of multiple sub expressions combined with an operator.
- Sub Expression: A term used to address the most elementary unit of an expression that only consists of a function and two values, whereas a value either can be a single element or a combined element with the first part of the element defining a source where the following value can be found.
- Property or Field: The boolean property or field of the component that the expression should be connected to.
- Function: The function that should be used to compare the two values in the sub expression.
- Data Source: The source for the values in the sub expression. Can also be addressed as the type of the value.
- Data Source Value: The actual value that should be used to evaluate the sub expression. However, if the value has
data source being
instance context, it acts as a reference to a value rather than an explicit value.
- Operator: The operator will only be relevant when there are more than one sub expression and will be used to evaluate if all the sub expressions should imply (use AND) or if it is enough that only one of the sub expressions implies (use OR).
Building expressions in Altinn Studio
When building expressions in the Expressions panel in Studio, it is always possible to add one expression for each of the following states:
For group components, also these states can have attached expressions:
If all these fields have expressions added to them you will be shown an info message saying that you have reached the limit for expressions.
when going forward configuring an expression for your component in the Expression tool in Altinn Studio for the first time, follow this guide:
Choose a component field to add the expression to
Choose a function that you want your first sub expression to use
Choose a data source that you want the first element in your sub expression to be
This data source can be either an actual source which will provide you with a set of available values to choose from, or the data source can be a type for a value. The available data sources that will provide you with a set of given values are:
- Data Model: Fields from the current selected datamodel
- Component: All component ids present across layouts
- Application Settings: All custom configuration values added in the
- Instance Context: One of the following values that exist on the instance object in Altinn storage;
While the available data sources that will act as types are:
- String: Lets you assign any custom string as the value
- Number: Lets you assign any custom number as the value
- Boolean: Lets you assign the value as
- Null: Automatically assigns the value ass
WARNING: The given values provided when choosing
Application Settingsas the data source are not yet implemented.
Choose a value that you want the first element in your sub expression to have
Choose a comparable data source and value for the second element in your sub expression
Save expression or continue adding sub expressions
Saving the expression will connect the expression to the component property you chose and visualize it in a preview mode.
Or you can keep adding sub expression by expanding the rule and set the operator, that should evaluate the sub expressions together, to either
Then you repeat these steps from point 2 - adding a function to the new sub expression and so on.
Then you can save the complete expression, with the arbitrary number of sub expressions, and visualize it in preview mode.
There is also a possibility for adding expressions by writing them directly in the syntax expected by the configuration of a running Altinn app. This functionality will be enabled in Studio if the expression is manually added to the field through Gitea or an editor IDE and if the expression is written in a way that can not be interpreted by the Studio Expressions tool. This accounts for nested expressions, as well as for expressions that are written in a simplified way, e.g. not including the function, since it will be interpreted by app-frontend implicitly.
The possibility to edit the expression by freestyle writing can also be enabled as an option at any given point. However, when editing the expression freestyle you will only be able to switch back to using the Studio tool if you write an expression that can be interpreted by the tool.
See that the switch is available to enable freestyle editing:
Enable it and see your expression in the freestyle editing text area:
Making changes to an invalid (or not-interpretable) expression will disable the switch:
As mentioned there are a few limitations in the Studio tool for configuring expressions.
Available Component Fields
Firstly, there are only some component fields that Studio are able to interpret and build connected expressions for. In a later stage of development we will enable building and interpreting expressions connected to
- text resource bindings on components
Second, Studio is limited to building expressions with only one level of nesting. This means that a value in a sub expression only can be either an implicit or explicit value and not a sub expression. If the value is a sub expression you will end up with a “complex” expression as in the example above.
Existing Boolean properties are lost when adding expressions
If you have defined any of the boolean properties/fields on the component to have a boolean value,
and you connect an expression to it, Studio will not remember this value. This means that if you add an expression to a
field that originally had a boolean value and you delete the expression, the field will disappear from the component and
be evaluated to its default value.
What is a Valid Expression from a Studio Point of View
To be allowed to save an expression to the layout file Studio will only visualize the Save button when some conditions imply:
- You have selected a component property/field that the expression should be connected to
- You have selected a function for the first sub expression of your expression
When these conditions imply you can save the expression without filling in any of the values. This will add an expression looking like this to the given component field:
"[COMPONENT_PROPERTY]": [ "[FUNCTION]", null, null ]
Be aware that the expression will be valid and calculate a boolean value that will have effect on the behaviour of your
component. Depending on the chosen function the expression can be calculated to
false. If the
would not make sense with the given function the default calculated expression value will be