Log In UI - Part 3

Illustrates how to use states to create a second UI page.

Log In UI - Part 3 is the third in a series of tutorials that build on each other to illustrate how to use Qt Design Studio to create a simple UI with some basic UI components, such as pages, buttons, and entry fields. Part 3 describes how to use states to add a second page to the UI. On the first page, users can enter a username and password to log in. On the second page, they can register if they do not already have an account.

Because the second page will contain most of the same UI elements as the login page, you will use states to show and hide UI elements as necessary when a user selects the Create Account button.

These instructions build on:

The Learn Qt Quick sections provide additional information about the features of QML and Qt Quick that are relevant to the task at hand.

Adding UI Components

You will add another text field for verifying the password that users enter to create an account and a back button for returning to the login page. You are already familiar with the tasks in this section from Part 1 and Part 2 of this tutorial.

To preview the changes that you make to the UI while you make them, select the (Show Live Preview) button on the Form Editor toolbar or press Alt+P.

To add the text field and a back button needed on the registration page:

  1. Open Screen01.ui.qml in Form Editor for editing.
  2. Drag and drop a Text Field from Library > QML Types > Qt Quick Controls 2 to fieldColumn in Navigator.
  3. In Properties, change the id of the text field to verifyPasswordField.
  4. In the Geometry group, Size field, set the width of the field to 300 pixels to match the size of the existing fields.
  5. In the Placeholder field, set the placeholder text to Verify password and mark the text translatable.
  6. Drag and drop a PushButton type from Library > My QML Components to their parent rectangle in Navigator.
  7. Select the button in Navigator and change its id to backButton in Properties.
  8. In the Geometry group, Size field, set the button width to 40 pixels.
  9. In the Text field, enter <.
  10. In the Font group, Size field, set the font size to 24 pixels.
  11. Under Layout, select the (Top) and (Right) anchor buttons to anchor backButton to the top right corner of its parent with 20- and 10-pixel margins, respectively.
  12. Select File > Save or press Ctrl+S to save your changes.

Your page now should look something like this in the Design mode and live preview:

"Login page with new UI components"

Next, you will add states for the login page and the registration page, where you use the visibility property to hide the password verification field and the back button on the login page and the login button on the registration page.

Using States to Simulate Page Changes

You will now add states to the UI to show and hide UI components in the Form Editor, depending on the current page:

  1. In the States view, select Create New State.

    "States view"

  2. Enter loginState as the state name.
  3. Select verifyPasswordField in the Navigator, and deselect the Visibility check box in Properties to hide the verify password field in the login state.

  4. Repeat the above step for backButton to hide it, too.
  5. In States, select for loginState to open the Actions menu, and then select Set as Default to determine that loginState is applied when the application starts.
  6. With the base state selected, add another state and name it registerState. This state should now look identical to the base state.
  7. Select loginButton in the Navigator, and deselect the Visibility check box to hide the login button in the registration state.
  8. Select File > Save or press Ctrl+S to save your changes.

You can now see all the states in the States view. The live preview displays the default state, loginState:

"States view"

Learn Qt Quick - States

In Qt Quick, the state of a particular visual item is the set of information which describes how and where the individual component parts of the visual item are displayed within it, and all the data associated with that state. Most visual items in a UI will have a limited number of states, each with well-defined properties.

For example, an element in a list may be either selected or not, and if selected, it may either be the currently active single selection or it may be part of a selection group. Each of those states may have certain associated visual appearance (neutral, highlighted, expanded, and so on).

Qt Quick provides a State type with properties which define its semantics and can be used to trigger behavior or animations. UI components typically have a state property and a default state. The default state contains all of an item's initial property values, and is therefore useful for managing property values before state changes.

UI components can specify additional states by adding new State objects to the state property. Each state within a component has a unique name. To change the current state of an item, the state property is set to the name of the state. State changes can be bound to conditions by using the when property.

For more information, see Qt Quick States.

Next, you will create connections to specify that clicking the Create Account button on the login page triggers a transition to the registration page and that clicking the back button on the registration page triggers a transition to the login page.

Connecting Buttons to States

QML types have predefined signals that are emitted when users interact with the UI. The PushButton QML type contains a Mouse Area type that has a clicked signal. The signal is emitted whenever the mouse is clicked within the area.

You will now use the Connections view to connect the clicked signal of registerButton to registerState and that of backButton to loginState:

  1. Select View > Views > Connection View to open the Connection view.
  2. Select registerButton in the Navigator.
  3. In the Connection view, select the button to add the action that the onClicked signal handler of registerButton should apply.
  4. Double-click the value Action column and select Change state to registerState in the drop-down menu.
  5. Repeat the steps above to connect backButton to loginState by selecting Change state to loginState in the last step.
  6. Select File > Save or press Ctrl+S to save your changes.

You can now see the following connections in the Design mode:

"Connections view"

In the live preview, you can now click the Create Account button to go to the registration page and the back button to return to the login page.

Learn Qt Quick - Signal and Event Handlers

UI components need to communicate with each other. For example, a button needs to know that the user has clicked on it. In response, the button may change color to indicate its state and perform an action.

QML has a signal and handler mechanism, where the signal is the event that is responded to through a signal handler. When a signal is emitted, the corresponding signal handler is invoked. Placing logic such as a script or other operations in the handler allows the component to respond to the event.

For more information, see Signal and Handler Event System.

Next Steps

For a more complicated UI, you would typically use QML types that specify a view of items provided by a model, such as a ListView or StackView. For more information, see Using Data Models.

To learn how to use a timeline to animate the transition between the login and registration pages, see the next tutorial in the series, Log In UI - Part 4.



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