6. Numeric Widgets

Qt offers several widgets designed for numeric input:

• QSpinBox - integer values
• QDoubleSpinBox - floating-point (decimal) values
• QSlider - linear selection with a sliding handle
• QDial - selection with a circular dial

These widgets form two distinct inheritance trees. QDoubleSpinBox and QSpinBox are inherited from QAbstractSpinBox, while QSlider and QDial are inherited from QAbstractSlider:

All of them are bounded (they have configurable minimum and maximum values) and emit the valueChanged() when the user modifies the value.

A QSpinBox allows the user select an integer either by clicking the up/down arrows, pressing the keyboard arrows or typing directly.

To use a spin box in your application:

1. Create the QSpinBox instance and configure it. The default spinbox range is 0-99, but in this example we restrict it to (arbitrary) budget limit sizes ($0-$100,000), set the step size $1,000, the initial value to $1,000, and prefix the unit with setPrefix('$ '). Note that we set the initial value after setting the range so that it doesn’t get clamped to 99, the default maximum value.

2. Define a slot that receives the new value and decorate it with @Slot(int). The slot name is set_limit() and it sets a label’s text to the spinbox value.

3. Connect the valueChanged signal to the slot. The slot receives the spinbox value as its argument.

The running application should look like this:

QDoubleSpinBox lets the user select a floating-point (float) value. In this example we use it to drive a calculation where a small change in the decimal value produces a genuinely different result.

To use a QDoubleSpinBox in your application:

1. Create the spinbox and configure its properties. We set the range to 0.00-15.00, choose a single step of 0.05, append the unit with setSuffix(' %'), and set an initial value of 4.50.

2. Create a slot to react to the valueChanged() signal. The slot calculate_projected_earnings() receives the new rate as a float, applies it to a fixed principal balance (e.g. $10,000), and updates a label with the projected first-year interest, formatted as currency.

3. Connect the signal to the slot, then call `calculate_projected_earnings() once directly with the spinbox’s initial value so the label shows a projection before the user makes any changes..

When you run the application, it should show a warning banner with adjustable opacity:

QSlider provides a vertical or horizontal slider that lets the user select an integer value by dragging a handle. Its main signals are:

To use a slider in your application:

1. Create and configure the slider. Set the range with setRange(min, max) In this example we use 0-255 because we want to control the alpha channel of the overdue-payment banner’s background color in a stylesheet (rgba(0, 128, 0, alpha)). We start at 255 (fully opaque - maximum urgency) and make the slider horizontal.

2. Create a slot to react to value changes. The slot receives an int. Here we dynamically update the alert banner’s stylesheet, inserting the slider value as the alpha component of the rgba() color.

3. Connect the signal to the slot.

The running application should look like this:

Unlike the low-balance warning from the previous section, the alert text itself stays fully legible here - only the background’s urgency color fades.

The QDial class provides a rounded range control. Unlike the QSlider, which is linear, QDial lets the user “turn” a value by dragging around a circular widget.

To use a dial in your application:

1. Create and configure the dial.

2. Create a slot to respond to the dial value changes.

3. Connect the signal to the slot.

In the example, turning the QDial updates a QProgressBar in real time:

Which of the four numeric widgets you choose depends on the data type and precision you need, and their visual appearance:

QSpinBox and QDoubleSpinBox values are meant to be exact. QSlider and QDial value changes are imprecise, and QDial is also somewhat awkward to handle with a mouse, and is a good fit mostly for specialized applications like audio mixers and studio equipment where users expect that appearance.