Power Apps Variables


By:   |   Updated: 2021-07-28   |   Comments (1)   |   Related: > Power Apps


Problem

Power Apps is an advanced citizen development platform, with its own formula-style language allowing you to create complex expressions for logic and behavior. Variables are integral part of this environment. This tip examines and different scenarios where variables are needed and how to set and manage them.

Solution

There are two formulas that allow the maker to create a variable. These are Set() and UpdateContext(). Set() is used for creating a global variable (available throughout the app, on every screen), while UpdateContext() creates a local variable (available only on the screen where it was created). Let us first examine where and how you create variables.

Where to set variables

Naturally, you set a variable using a behavioral property of an object. That is a property that "acts" or "does" something upon an event.

App

The App object is available for every canvas app and has one property allowing you to set a variable: OnStart.

app object variables

This is a good place to use for global variable setting, for example configuration variables and collection initialization. If you are, however, performing a call to an outside data source and assigning the result back to a variable, keep in mind this operation may negatively affect user experience if the data source responds slowly or there are too many records to retrieve. The App OnStart property executes before anything else on the app.

Also, we must note that here we are concerned with a generic canvas app. If you are building an app on top of a SharePoint list or library then you get the SharePoint Integration control which has different behavioral properties, e.g. OnNew, OnSave and others. You can refer to this tip for more details on that.

Screen

Every screen you insert in your app has two properties which you can use to set variables: OnHidden and OnVisible.

Screen OnVisible

In my practice, I have not used OnHidden much, but it can be handy. For example, if you want to reset inputs in a multi-screen app where the user flips through a number of screens back and forth. OnVisible, on the other hand, is frequently used for both local and global variables. You can use it to initialize logic pertinent to the specific screen. Additionally, you can use the very first screen of the app to initialize some variables instead of using App OnStart.

Button

The button object also has an OnSelect property which is by default set to false. Every interaction with the button (click) will invoke OnSelect and the logic it contains. A frequent use is disabling a submit button in the current session by modulating the DisplayMode property of the button with a variable that sets to false. So, for example, you can initiate a variable on the screen OnVisible property:

creating a variable for a button

Then change the variable to false, once the button is clicked:

set button variable to false

And finally, make DisplayMode dynamic:

use a variable to flip between display modes of a button

Toggle and Checkbox controls

These are quite powerful controls allowing the maker to design complex menus, custom forms and imbue business rules in them. The toggle control has an OnChange, OnCheck, OnSelect and OnUncheck properties:

power apps toggle control

This allows you create complex logic every time the toggle changes, the true value is selected, the whole control is selected, or the false value is selected.

On the other hand, the Checkbox has the same minus OnChange.

power apps checkbox control

The OnSelect may seem redundant but it will ensure you can exploit the check’s behavior every time it is used, regardless checked or unchecked.

Gallery

The gallery is a ubiquitous control and hardly any app goes live without at least one gallery in it. There is an OnSelect property which enables the maker to access the currently selected item inside the gallery. Keep in mind, all objects inside the gallery template also have an OnSelect property. In a default gallery, this OnSelect property refers to the Parent – in this case the gallery itself.

using the gallery onselect property

This setting means when you select an item, the gallery’s OnSelect will be invoked. Change this setting if you want custom behavior for every different row in the gallery.

Forms

The form control possesses three behavioral properties: OnFailure, OnReset and OnSuccess.

Form behavioral properties

Here is how they work:

  • OnFailure fires if the form fails to submit. Typically you submit a form by using the SubmitForm() formula on the OnSelect of a button.
  • OnReset fires when you reset the form with ResetForm(). That formula is typically used on a cancel or reset button to give the user an option to clear the form and start filling in anew.
  • OnSucess fires when SubmitForm() executes successfully and the data gets stored in the underlying data source.

Camera

The Camera control has an OnSelect and OnStream property:

camera properties

The OnSelect property is frequently used to capture the image coming from the camera. If we assume you initiated a new collection in the OnVisible property of the current screen, you can add images to it by assigning

Collect(Images, CameraControl.Photo) to the camera’s OnSelect. On the other hand, OnStream lets you configure how the app responds when the Stream property is updated.

Barcode Scanner

This control has two properties we are interested in here: OnCancel and OnScan:

barcode scanner behavioral properties

OnScan will fire every time a barcode was successfully recognized by the app, while OnCancel will execute in case the user cancels the scanning process. These are handy properties for providing meaningful feedback to the user, for example with the Notify() function.

This is not an exhaustive list of all objects and controls, however, it does lay out all behavioral properties you will encounter when developing a power app.

Variable types

To fully understand how useful and powerful variables in Power Apps are, let us also investigate variable types. The variable type is automatically decided by the Power Apps formula language at run-time so we can compare it to a dynamic language. Variables are also strongly typed, meaning you can’t intermix different types unless you assign an integer value to a decimal-type variable or vice-versa. The type is determined as soon as the value is assigned to the variable name.

Binary/text/number

These three should be self-explanatory, here is an example of a binary, text, and numerical variable:

Set(
    binaryVar,
    true
);
Set(
    textVar,
    "name"
);
Set(
    numericalVar,
    10.5
);

Working with a binary variable allows you to be very flexible and easily reassign the same variable for the purpose of alternating automatically between true/false. In turn, this alteration allows you use the variable for setting the ascending/descending order or use it anywhere else where a binary switch is required. For example:

Set(
    sortDirection,
    true
);
SortByColumns(
    MyCollection,
    "column1",
    If(
        sortDirection,
        Ascending,
        Descending
    )
)

On a relevant icon’s OnSelect property you can then just hook up the following expression:

Set(
    sortDirection,
    !sortDirection
);

Thus, you will alternate between the sort directions depending on the value of the variable.

Other types

Power Apps supports also other, more complex types. If you add a Barcode Scanner control with no extra modifications, you can for example set the following two variables:

Set(
    colorVar,
    BarcodeScanner1.Color
);
Set(
    displayModeVar,
    BarcodeScanner1.DisplayMode
)

If you then go to File > Variables you will see a summary of existing variables in the app, their types (as mentioned above) and their locations (global or local, i.e. on a specific control):

variables overview in powerapps

The displayModeVar is an Enum type. In this case it can have a value of either Disabled, Edit or View which are the typical values for this property on any control. On the other hand, the colorVar is of type Color signifying a color value in the RGBA format.

Conclusion

With this tip we hope to provide an overview of the abilities of variables in Power Apps. For almost any app variables are needed to provide enhanced functionality and user experience.

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About the author
MSSQLTips author Hristo Hristov Hristo Hristov is a Microsoft certified data professional, specializing in Power Apps and Power BI.

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Article Last Updated: 2021-07-28

Comments For This Article




Monday, August 2, 2021 - 8:01:40 AM - Hristo Hristov Back To Top (89080)
Small clarification about the App OnStart property. If you go to File > Setting > Upcoming features, there is a setting called "Use non-blocking OnStart rule". By default it is on, but if you switch it off, App OnStart will execute in parallel with other rules.


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