- Qt Creator Manual
- Adding States
- Qt Creator Manual 5.0.3

Managing C++ Backend Objects ◦ Exporting 3D Assets

# Adding States

You can define states for components and component instances in the States view by selecting Create New State.

Click the new state to switch to it in Form Editor, and then modify the values of the properties of components or component instances in Properties.

For example, to change the appearance of a button, you can define states in the button component to hide the button image and show another image in its place or to change the button background or text color. When you use instances of the button in other components, you can define states to create different screens by hiding or showing button component instances. The preset Button control in Library > Components > Qt Quick Controls > Controls has predefined normal and down states.

To add motion to a screen, you can change the position of a component instance in Form Editor and then add animation to the change between the states.

The properties that you change in a state are highlighted with blue color. In Text Editor, you can see the changes recorded as changes to the base state.

Note: If you have locked a component in Navigator, and you attempt to remove states where you change the values of its properties, you are prompted to confirm the removal.

For more information, watch the following video:

## Setting the Default State

To determine the startup state of the application, select to open the Actions menu, and then select Set as Default.

To reset the state later, select Actions > Reset Default.

## Applying States

To determine when a state should be applied, select Actions > Set when Condition. In Binding Editor, specify a when property for the state. Set the value of the property to a boolean expression that evaluates to `true` when you want the state to be applied.

This enables you to evaluate the truthfulness of several components' properties and move the UI to the state in which these conditions apply. You can evaluate whether something is true or false, greater than or equal to something else, and so on. You can also use operators, such as AND or OR, to evaluate the truthfulness of several components.

The when conditions are evaluated from left to right and in order of appearance in the code. Therefore, if you have two conditions for two different states that both evaluate to `true`, the first state is applied.

In Binding Editor, select the component and property to create the expression. For example, to change the state when a button is pressed, you could select a button component and its pressed property.

When you compose the expressions in Binding Editor, the code completion feature lists the components and their properties you can use in the expressions.

### Summary of Logical Operators

You can use the following logical operators in the expressions to combine several conditions in one expression:

Operator | Meaning | Evaluates to `true` if
--- | --- | ---
! | NOT | The condition is not met.
&& | AND | Both conditions are met.
|| | OR | Either of the conditions is met.
< | Less than | The left operand is less than the right operand.
> | Greater than | The left operand is greater than the right operand.
>= | Greater than or equal | The left operand is greater than or equal to the right operand.
<= | Less than or equal | The left operand is less than or equal to the right operand.
== | Equal | The operands are equal.
=== | Strict equal | The operands are equal and of the same type.
!= | Not equal | The operands are not equal.
!== | Strict not equal | The operands are of the same type but not equal, or are of different type.

In addition, you can use arithmetic operators to compare numbers before checks. However, we recommend that you create separate properties for this purpose whenever possible.

### Examples of when Conditions

To apply a state to a button when the button is pressed, you could simply write:

     when: control.pressed

To apply a state when the button is not pressed, select the NOT check box.

To apply a state when the button is not pressed, selected, nor hovered on, you could combine conditions, as follows:

     when: !control.pressed && !control.checked && !control.hovered

To apply a state when the button is pressed or selected, but not hovered on, you could write:

     when: control.pressed || control.checked && !control.hovered

## Using States

To keep the code clean, you should create a base state that contains all the components you will need in the application. You can then create states, in which you hide and show a set of components and modify their properties. This allows you to:

- Align components on different views with each other.
- Avoid excessive property changes. If a component is invisible in the base state, you must define all changes to its child components as property changes, which leads to complicated code.
- Minimize the differences between the base state and the other states to keep the code short and readable and to improve performance.
- Avoid problems when using transitions and animation when changing states.

To create views for an application by using states:

1. In the base state, add all components you will need in the application (1). While you work on one view, you can click the icon in Navigator to hide components on the canvas that are not part of a view.
2. In States, select Create New State to create a new state and give it a name. For example, `Normal`.
3. In Properties (2), deselect the Visibility check box or set Opacity to 0 for each component that is not needed in this view. If you specify the setting for the parent component, all child components inherit it and are also hidden.
4. Create additional states for each view and set the visibility or opacity of the components in the view.
5. To determine which state is applied when the application starts, select Actions > Set as Default.

## Using SCXML State Machines

To use QML together with an SCXML state machine, add states and bind them to the state machine in Connection View > Backends, as described in Managing C++ Backend Objects.

In the States view, you can select Actions > Set when Condition to edit the `when` condition of states to map QML states to the states of the SCXML state machine. For an example, see Qt SCXML Traffic Light QML Example (Dynamic).

If you add animation to the states, you can preview or run the application to test the animation.

Managing C++ Backend Objects ◦ Exporting 3D Assets