Power-Fx/docs/imperative.md

Imperative logic

Note: Microsoft Power Fx is the new name for canvas apps formula language. These articles are work in progress as we extract the language from canvas apps, integrate it with other products of the Power Platform, and make available as open source. Start with the Power Fx Overview for an introduction to the language.

Most Microsoft Power Fx formulas calculate a value. Like an Excel spreadsheet, recalculation happens automatically as values change. For example, you might want to show the value in a Label control in red if the value is less than zero or in black otherwise. So you can set the Color property of that control to this formula:

If( Value(TextBox1.Text) >= 0, Color.Black, Color.Red )

In this context, what does it mean when the user selects a Button control? No value has changed, so there is nothing new to calculate. Excel has no equivalent to a Button control.

By selecting a Button control, the user initiates a sequence of actions, or behaviors, that will change the state of the app:

• Change the screen that's displayed: Back functions.
• Control a signal (Power Apps only): Enable and Disable functions.
• Refresh, update, or remove items in a data source: Refresh, Update, UpdateIf, Patch, Remove, RemoveIf functions.
• Update a context variable (Power Apps canvas only): UpdateContext function.
• Create, update, or remove items in a collection: Collect, Clear, ClearCollect functions.

Because these functions change the state of the app, they can't be automatically recalculated. You can use them in the formulas for the OnSelect, OnVisible, OnHidden, and other On... properties, which are called behavior formulas.

More than one action

Use semicolons to create a list of actions to perform. For example, you might want to update a context variable and then return to the previous screen:

UpdateContext( { x: 1 } ); Back()

Actions are performed in the order in which they appear in the formula. The next function won't start until the current function has completed. If an error occurs, subsequent functions might not start.