Handling Touch Events in Qt5 on Linux: A Guide to Avoiding Duplicates

Описание: Discover how to manage touch events in your Qt5 application on Linux without rewriting your driver. We explore a solution to avoid duplicated touch input from the OS.
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Handling Touch Events in Qt5 on Linux: A Guide to Avoiding Duplicates

Touch screen functionality has become increasingly common in applications, but integrating it can pose unique challenges. This is especially true in environments where native touch support is either absent or conflicts with existing custom implementations. In this guide, we will explore a real-world scenario faced by developers working with Qt5 on Linux and propose a solution to a frustrating issue: duplicate touch events.

The Problem

A developer faced the challenge of integrating touch support into an application initially built for Scientific Linux 6.4, which lacked native multi-touch support. The application relied on a custom driver to interpret touch events by reading from the /dev/input/event file. After upgrading to RedHat 8, which included native touch event handling, the developer noticed that touch events were being registered twice:

Once from the operating system as part of its native support

Once from the custom driver required for specific application behavior

This overlap led to issues such as double-typing on the on-screen keyboard, causing user frustration and hindering the application's usability.

The Solution

To solve the problems associated with duplicate events, the developer implemented a straightforward yet effective solution using xinput. The following sections will break down this solution and provide context on why it is beneficial.

Understanding xinput

xinput is a utility available on X server systems that allows users to configure and test input devices. By leveraging this tool, developers can enable or disable specific input devices easily. In this context, it would be used to disable the native input from the touchscreen, ensuring that only the custom driver's input is processed.

Steps to Disable Native Touch Input

Identify Your Touchscreen Device: Start by listing all input devices connected to the system. You can use the following command:

[[See Video to Reveal this Text or Code Snippet]]

This will give you a list of all connected input devices along with their IDs.

Disable the Touchscreen Device: Once you have identified the touchscreen device ID, you can disable it. For example, if the ID of your touchscreen is 12, the command will be:

[[See Video to Reveal this Text or Code Snippet]]

This action prevents the operating system from receiving touch events from the native touchscreen, allowing your driver to handle all inputs without interference.

Benefits of This Approach

No Need for Code Rewrite: By disabling the native touch input, the developer avoids the need to refactor a large codebase. This saves significant time and resources while maintaining application functionality and performance.

Custom Gesture Support: The custom driver continues to function as intended, handling all touch actions, including multi-touch and custom gestures. These gestures are essential for the specific use cases of the application.

Immediate Effect: The changes made with xinput affect the input handling immediately upon execution, allowing for quick testing and adjustments without needing to restart the application.

Conclusion

Managing touch input in a complex application can be challenging, especially when transitioning from legacy systems to ones with modern support. By leveraging tools like xinput to disable native touch event processing, developers can effectively manage input without diving into significant code alterations. This solution ensures that touch functionalities work smoothly, maintaining the integrity of custom gestures and user experiences.

Implementing this strategy can save developers from the headaches associated with duplicate touch events a