In this section, you will learn:
1. How to update the UI using state variables
To create interactive and dynamic UIs, you need to connect widget properties that you want to change dynamically with variables. In the Creating Variables section, we discussed the difference between read-only (non-state) and non-read-only (state) variables. State variables can be changed after the widget has been loaded and displayed on the screen, allowing you to dynamically update the UI.
In this guide, we'll demonstrate how to update the UI by changing a state variable connected to a widget property using Circuit.
Updating State Variables with Circuit
- Create a state variable to one of the widget properties that you want to change
- Open Circuit to create logic to be executed for a specific action, such as On pressed event for Button widget or Gesture Detector wrapper
- When clicking on the state variable name inside the Locals section in Circuit, you will see Get and Set options. Get allows you to read the variable value and use it as input to another node, while Set lets you change the variable to a new value. To update the variable, choose set then connect the input pin for the Set node for that variable to the value that you need to change it to, you can enter the value manually in the node itself or use the output of other nodes as the input for that Set node.
- After changing a state variable, add the Refresh node to the flow to update the UI. The Refresh node won't be visible if all variables in the Screen/Component are read-only.
Non-state variables don't have the get and set options, as they can only be read as inputs to other nodes but not set to new values. Therefore you will see only the name of the variable listed in the section Locals which is used as Get only
Example: Changing an Image on Button Press
In this example, we'll build an app that changes an image when a button is pressed, the image will be displayed online images using the url property.
- Create a state variable called imageURL for the url property of the Image widget, allowing you to control the property dynamically with a variable. Set the variable to be a state variable (uncheck the read-only option).
- Add logic to the button by selecting it and clicking on the On Pressed field. This logic will execute when the user presses the button.
- Inside Circuit, click the plus button next to the "OnPressed" node to add the next node to the logic flow.
- Select the imageURL variable in the locals section. Choose set and enter the new URL for the imageURL variable after the button is clicked.
- Add a Refresh node at the end of the flow. The Refresh node is also found within the locals section.
Now, when you play the app and click the button, the image URL will change from the initial value to the new one, updating the image displayed in the UI.
Making a variable private is a common practice for state variables since you want the state variable to be updated within the screen itself so you want to hide it from being accessed from outside.
Usually, the variables that are non-state are public, since their default value is usually set from outside the screen, like a previous screen or an API source, so they should be accessed from outside the screen/component, but for state variables, it's usually that they change according to the user interactions inside that screen or component, and they don't need an initial value to be passed from outside.
In case of the initial value of the state variable to be received from outside, a common practice is to create two independent variables, one is non-state and public for receiving the value from outside, and another one is state and private variable for updating the UI state inside the screen or component. The initial value for that state variable is then set up to be the value of the other non-state variable that receives its value from outside. This concept is known in software development as encapsulation.