Advanced Features of Qt QSS Style Sheets: Dynamic Styles, State Expressions, and Sub-Controls Styling

# 1. An Introduction to Qt QSS Style Sheets

## 1.1 What are Qt QSS Style Sheets

Qt QSS style sheets are a technique for customizing the appearance of Qt application interfaces. They adopt a syntax and selectors similar to CSS, allowing you to change the style attributes of controls such as color, font, borders, and more by setting style properties.

## 1.2 Basic Syntax and Usage of QSS Style Sheets

In Qt, QSS style sheets can be applied to controls using the `setStyleSheet()` method. QSS style sheets support selectors, properties, and values, where selectors are used to match controls, properties specify which style attributes to modify, and values set the new values for those properties.

Here is a simple example of a QSS style sheet:

QPushButton {
     background-color: red;
     color: white;
}

QLabel {
     font-size: 20px;
     font-weight: bold;
}

In the example above, `QPushButton` and `QLabel` are selectors indicating the type of control to which the style should be applied. `background-color` and `color` are properties that specify the button's background color and text color, respectively. `font-size` and `font-weight` are properties specific to `QLabel` used to set the font size and boldness.

To use the above style sheet, it can be saved as a .qss file and loaded using the `setStyleSheet()` method:

QApplication app(argc, argv);
app.setStyleSheet("style sheet content");

With the introduction above, we can understand the basic concepts and usage of Qt QSS style sheets. The next chapter will introduce how to use dynamic styles to change the appearance of controls.

# 2. Qt QSS Dynamic Styles

### 2.1 How to Change the Appearance of Controls with Dynamic Styles

In Qt QSS, we can use dynamic styles to achieve changes in the appearance of controls. With dynamic styles, we can dynamically change the style of a control based on changes in the control's state or attributes.

For example, we can use the `QPushButton` control as an example. First, we need to set a basic style as the default style:

QPushButton {
     background-color: blue;
     color: white;
     border-radius: 5px;
}

Then, we can use the state of the control to change the style. Suppose we want the button's background color to turn red when the mouse hovers over it:

QPushButton:hover {
     background-color: red;
}

This way, when the mouse hovers over the button, the button's background color will dynamically change to red.

### 2.2 Introducing the Usage of Dynamic Properties and Property Selectors in Qt QSS

In addition to the state of the control, we can also use dynamic properties of the control to change the style. Dynamic properties can be set using the `setProperty()` function.

For example, we want to change the appearance of the button by setting a dynamic property. First, we need to set a basic style as the default style:

QPushButton {
     background-color: blue;
     color: white;
     border-radius: 5px;
}

Then, we can change the style by setting a dynamic property of the button. Suppose we want the button's background color to turn gray when the button's `isEnabled` property is `False`:

QPushButton[isEnabled="false"] {
     background-color: gray;
}

This way, when the button's `isEnabled` property is `False`, the button's background color will dynamically change to gray.

It is important to note that when using dynamic properties, the corresponding property values need to be set in the code, for example:

button = QPushButton()
button.setProperty("isEnabled", False)

This is the basic usage of dynamic styles in Qt QSS. With dynamic styles, we can dynamically change the appearance of controls based on changes in the control's state or attributes. This flexible styling method can greatly enhance the customizability and interactivity of the user interface.

# 3. Qt QSS State Expressions

In this chapter, we will introduce how to use state expressions to implement different styles for controls in various states. We will also introduce the syntax and application of state selectors and state expressions in Qt QSS. State expressions are an essential part of Qt QSS style sheets, enabling us to achieve more flexible and intelligent style control.

#### 3.1 Using State Expressions to Implement Different Styles for Controls in Various States

In Qt QSS, we can use state expressions to control the styles of controls in different states. For example, when the mouse hover