If you've used another programming tool, such as Visual Basic or JavaScript, you may be asking: Where are the variables? PowerApps is a little different and requires a different approach. Instead of reaching for a variable, ask yourself: What would I do in Excel?
In other tools, you may have explicitly performed a calculation and stored the result in a variable. However, PowerApps and Excel both automatically recalculate formulas as the input data changes, so you usually don't need to create and update variables. By taking this approach whenever possible, you can more easily create, understand, and maintain your app.
In some cases, you'll need to use variables in PowerApps, which extends Excel's model by adding behavior formulas. These formulas run when, for example, a user selects a button. Within a behavior formula, it's often helpful to set a variable to be used in other formulas.
In general, avoid using variables. But sometimes only a variable can enable the experience you want.
Translate Excel into PowerApps
Excel
Let's review how Excel works. A cell can contain a value, such as a number or a string, or a formula that's based on the values of other cells. After the user enters a different value into a cell, Excel automatically recalculates any formulas that depend on the new value. You don't have to do any programming to enable this behavior.
Excel doesn't have variables. The value of a cell that contains a formula changes based on its input, but there's no way to remember the result of a formula and store it in a cell or anywhere else. If you change a cell's value, the entire spreadsheet may change, and any previously calculated values are lost. An Excel user can copy and paste cells, but that's under the user's manual control and isn't possible with formulas.
PowerApps
Apps that you create in PowerApps behave very much like Excel. Instead of updating cells, you can add controls wherever you want on a screen and name them for use in formulas.
For example, you can replicate the Excel behavior in an app by adding a Label control, named TextBox1, and two Text input controls, named TextInput1 and TextInput2. If you then set the Text property of TextBox1 to TextInput1 + TextInput2, it will always shows the sum of whatever numbers are in TextInput1 and TextInput2 automatically.
Notice that the TextBox1 control is selected, showing its Text formula in the formula bar at the top of the screen. Here we find the formula TextInput1 + TextInput2. This formula creates a dependency between these controls, just as dependencies are created between cells in an Excel workbook. Let's change the value of the TextInput1:
The formula for TextBox1 has been automatically recalculated, showing the new value.
In PowerApps, you can use formulas to determine not only the primary value of a control but also properties such as formatting. In the next example, a formula for the Color property of the label will automatically show negative values in red. The If function should look very familiar from Excel:
If( Value(TextBox1.Text) < 0, Red, Black )
Now, if the result of our calculation in TextBox1.Text is negative, the number will be shown in red:
You can use formulas for a wide variety of scenarios:
- By using your device's GPS, a map control can display your current location with a formula that uses Location.Latitude and Location.Longitude. As you move, the map will automatically track your location.
- Other users can update data sources. For example, others on your team might update items in a SharePoint list. When you refresh a data source, any dependent formulas are automatically recalculated to reflect the updated data. Furthering the example, you might set a gallery's Items property to the formula Filter( SharePointList ), which will automatically display the newly filtered set of records.
Benefits
Using formulas to build apps has many advantages:
- If you know Excel, you know PowerApps. The model and formula language are the same.
- If you've used other programming tools, think about how much code would be required to accomplish these examples. In Visual Basic, you'd need to write an event handler for the change event on each text-input control. The code to perform the calculation in each of these is redundant and could get out of sync, or you'd need to write a common subroutine. In PowerApps, you accomplished all of that with a single, one-line formula.
- To understand where TextBox1's text is coming from, you know exactly where to look: the formula in the Text property. There's no other way to affect the text of this control. In a traditional programming tool, any event handler or subroutine could change the value of the label, from anywhere in the program. This can make it hard to track down when and where a variable was changed.
- If the user changes a slider control and then changes their mind, they can change the slider back to its original value. And it's as if nothing had ever changed: the app shows the same control values as it did before. There are no ramifications for experimenting and asking "what if," just as there are none in Excel.
In general, if you can achieve an effect by using a formula, you'll be better off. Let the formula engine in PowerApps work for you.
Know when to use variables
Let's change our simple adder to act like an old-fashioned adding machine, with a running total. If you select an Add button, you'll add a number to the running total. If you select a Clear button, you'll reset the running total to zero.
Our adding machine uses something that doesn't exist in Excel: a button. In this app, you can't use only formulas to calculate the running total because its value depends on a series of actions that the user takes. Instead, our running total must be recorded and updated manually. Most programming tools store this information in a variable.
You'll sometimes need a variable for your app to behave the way you want. But the approach comes with caveats:
- You must manually update the running total. Automatic recalculation won't do it for you.
- The running total can no longer be calculated based on the values of other controls. It depends on how many times the user selected the Add button and what value was in the text-input control each time. Did the user enter 77 and select Add twice, or did they specify 24 and 130 for each of the additions? You can't tell the difference after the total has reached 154.
- Changes to the total can come from different paths. In this example, both the Add and Clear buttons can update the total. If the app doesn't behave the way you expect, which button is causing the problem?
Create a global variable
To create our adding machine, we require a variable to hold the running total. The simplest variables to work with in PowerApps are global variables.
How global variables work:
- You set the value of the global variable with the Set function. Set( MyVar, 1 ) sets the global variable MyVar to a value of 1.
- You use the global variable by referencing the name used with the Set function. In this case, MyVar will return 1.
- Global variables can hold any value, including strings, numbers, records, and tables.
Let's rebuild our adding machine by using a global variable:
Add a text-input control, named TextInput1, and two buttons, named Button1 and Button2.
Set the Text property of Button1 to "Add", and set the Text property of Button2 to "Clear".
To update the running total whenever a user selects the Add button, set its OnSelect property to this formula:
Set( RunningTotal, RunningTotal + Text1 )
The first time a user selects the Add button and Set is called, RunningTotal is created with a default value of blank. In the addition, it will be treated as a zero.
To set the running total to 0 whenever the user selects the Clear button, set its OnSelect property to this formula:
Set( RunningTotal, 0 )
Add a Label control, and set its Text property to RunningTotal.
This formula will automatically be recalculated and show the user the value of RunningTotal as it changes based on the buttons that the user selects.
Preview the app, and we have our adding machine as described above. Enter a number in the text box and press the Add button a few times. When ready, return to the authoring experience using the Esc key.
To see our global variable's value, select the File menu and select Variables in the left hand pane.
To see all the places where our variable is defined and used, select it.
Types of variables
There are three types of variables in PowerApps:
| Variables type | Scope | Description | Functions |
| Global variables | App | Simplest to use. Holds a number, text string, Boolean, record, table, etc. that can be references from anywhere in the app. | Set |
| Context variables | Screen | Great for passing values to a screen, much like parameters to a procedure in other languages. Can only be referenced from one screen. | UpdateContext Navigate |
| Collections | App | Holds a table that can be references from anywhere in the app. Allows the contents of the table to be modified rather than being set as a whole. Can be saved to the local device for later use. | Collect ClearCollect Patch Update Remove SaveData LoadData etc. |
All variables are created implicitly when used in Set, UpdateContext, Navigate, or Collect functions. There is no explicit declaration of variables as is done in other programming tools. The types of the variables are also derived implicitly from the values that are placed in them.
All variables are held in memory while the app is running. After the app closes, the values held in the variables are lost. You can store the contents of a variable in a Data Source using Patch or Collect functions, or in the case of collections you can store to the local device with the SaveData function. When the app is first loaded, all variables will have the blank value.
You use the variables name to read its value. For example, once defined with Set( MyColor, Red ) you can simply use MyVar anywhere a color value can be used and it will be replaced with the Red. It is possible to have a global variable or collection with the same name as a context variable. In this case, the context variable will take precedence. You can still reference the global variable or collection using the disambiguation operator @[MyColor].
Create a context variable
Let's look at how our adding machine would be created using a context variable instead of a global variable.
How context variables work:
- You create and set context variables by using the UpdateContext function. If a context variable doesn't already exist when first updated, it will be created with a default value of blank.
- You create and update context variables with records. In other programming tools, you commonly use "=" for assignment, as in "x = 1". For context variables, use { x: 1 } instead. When you use a context variable, use its name directly.
- You can also set a context variable when a screen is displayed, by using the Navigate function. If you think of a screen as a kind of procedure or subroutine, this is similar to parameter passing in other programming tools.
- Except for Navigate, context variables are limited to the context of a single screen, which is where they get their name. You can't use or set them outside of this context.
- Context variables can hold any value, including strings, numbers, records, and tables.
Let's rebuild our adding machine by using a context variable:
Add a text-input control, named TextInput1, and two buttons, named Button1 and Button2.
Set the Text property of Button1 to "Add", and set the Text property of Button2 to "Clear".
To update the running total whenever a user selects the Add button, set its OnSelect property to this formula:
UpdateContext( { RunningTotal: RunningTotal + Text1 } )
The first time a user selects the Add button and UpdateContext is called, RunningTotal is created with a default value of blank. In the addition, it will be treated as a zero.
To set the running total to 0 whenever the user selects the Clear button, set its OnSelect property to this formula:
UpdateContext( { RunningTotal: 0 } )
Again, UpdateContext is used with the formula UpdateContext( { RunningTotal: 0 } ).
Add a Label control, and set its Text property to RunningTotal.
This formula will automatically be recalculated and show the user the value of RunningTotal as it changes based on the buttons that the user selects.
Preview the app and we have our adding machine as described above. Enter a number in the text box and press the Add button a few timnes. When ready, return to the authoring experience using the Esc key.
You can set the value of a context variable while navigating to a screen. This is useful for passing "context" or "parameters" from one screen to another. To see this, insert a new screen, and insert a button with the OnSelect property set to:
Navigate( Screen1, None, { RunningTotal: -1000 } )
Selecting this button on Screen2 (which you can do while authoring if you select the button toward the ends) will show Screen1 and also set the context variable RunningTotal to -1000.
To see our context variable's value, select the File menu and select Variables in the left hand pane.
To see where your context variable is defined and used, select it.
Create a collection
Finally, let's look at creating our adding machine with a collection. Since a collection holds a table that is easy to modify, we will make this adding machine keep a "paper tape" of each value as they are entered.
How collections work:
- Create and set collections by using the ClearCollect function. You can use the Collect function instead, but it will effectively require another variable instead of replacing the old one.
- A collection is a kind of data source and, therefore, a table. To access a single value in a collection, use the First function, and extract one field from the resulting record. If you used a single value with ClearCollect, this will be the Value field, as in this example:
First( VariableName ).Value
Let's recreate our adding machine by using a collection:
Add a Text input control, named TextInput1, and two buttons, named Button1 and Button2.
Set the Text property of Button1 to "Add", and set the Text property of Button2 to "Clear".
To update the running total whenever a user selects the Add button, set its OnSelect property to this formula:
Collect( PaperTape, TextInput1.Text )
This formula will add the new value to the end of the collection. Since we are adding a single value, Collect will automatically place it in a single column table with the column name Value which we will use later.
To clear our paper tape when the user selects the Clear button, set its OnSelect property to this formula:
Clear( PaperTape )
To display the running total, add a label, and set its Text property to this formula:
Sum( PaperTape, Value )
To run the adding machine, press F5 to open Preview, enter numbers in the text-input control, and select buttons.
To return to the default workspace, press the Esc key.
To display the paper tape, insert a Data table control, and set its Items property to this formula:
PaperTape
You will also need to select the columns to show in the right hand pane, in our case show the Value column:
To see the values in your collection, select Collections on the File menu.
To store and retrieve your collection, add two additional button controls and set their text to Load and Save. For Load, set the OnSelect property to:
Clear( PaperTape ); LoadData( PaperTape, "StoredPaperTape", true )
We need to clear the collection first as LoadData will append the stored values to the end of the collection.
For Save, set the OnSelect property to:
SaveData( PaperTape, "StoredPaperTape" )
Preview again by pressing the F5 key, enter numbers in the text-input control, and select buttons. Select the Save button. Close and reload your app, and select the Load button to reload your collection.
Note: SaveData and LoadData do not function when running in a web browser, you must be using the studio installed on Windows or one of the players for mobile device.