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If saved QML edits do not appear, first confirm you are running Qt Creator’s QML Preview for the file or project you are editing. Then check QML debugging settings for your project type, rebuild where required, and account for the fact that structural edits can rebuild components or restart the app instead of updating the visible UI in place.
1. Make sure you launched QML Preview
Open the relevant QML file and start the preview using the Live Preview control in Design/Edit mode, or choose Build > QML Preview. Confirm the preview is for the same file or project you are changing. Qt documents QML Preview as the workflow for live-reloading QML while an application is running; a separately launched executable is not necessarily the preview client. See Qt Creator: Viewing QML applications and Qt 6.12: Prototyping, Debugging and Profiling QML Applications.
2. Enable QML debugging for the project type
Qt Creator’s setup differs depending on whether the project is a Qt Quick application or a Qt Quick UI project using a .qmlproject file. Follow the matching branch in the Qt Creator QML debugging guide.
Qt Quick application
- Open Projects > Build Settings and enable QML debugging and profiling.
- Open Run Settings and set the QML debugger to Automatic or Enabled.
- Select Build > Rebuild Project before testing the preview again.
Qt Quick UI project (.qmlproject)
- Open the project’s Run Settings.
- Under Debugger Settings > QML debugger, select Automatic or Enabled.
- Run QML Preview again for the project.
QML debugging opens a TCP port. Use it only in an environment where that port is protected by an appropriate firewall.
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3. Check whether the edit requires a rebuild or restart
Live reload does not guarantee that every change is patched into the running UI without disruption. Qt’s Qt 6.12 documentation and technical explanation describe different outcomes: a simple edit may be patched, a more structural change may rebuild part of the component tree, and a change that cannot be applied at an enclosing or root level may restart the application. A reset of runtime state after an edit therefore does not, by itself, show that file watching failed. See Qt 6.12’s QML Preview overview.
Version matters: Qt Project development mailing-list contributor Ulf Hermann wrote on June 2, 2026 that QML Preview’s hot-reload support was in Qt 6.12. Do not assume an unidentified earlier Qt version has the same behavior; compare the Qt runtime and Qt Creator versions in use with the documentation for those versions. The Qt 6.12 technical explanation also notes a limitation involving multiple QML engines on multiple threads. That caveat is relevant to such architectures, not a general explanation for ordinary single-engine projects. See the June 2, 2026 Qt Project development mailing-list message.
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4. For device preview, verify the target connection
Android
Enable USB debugging on the Android device, connect it over USB, and select the kit for that device. Then choose Build > QML Preview or press Alt+P. Qt Creator’s device-preview documentation says the first preview copies a utility to the device; subsequent previews copy UI files. See Qt Creator: Previewing on devices.
Boot to Qt
Use a supported and configured connection to the device, and select the kit associated with that Boot to Qt target before starting QML Preview. The desktop toolbar workflow alone does not establish that the device connection or kit is ready.
5. Gather the details needed to isolate a persistent problem
If those checks do not explain the symptom, a useful report needs the exact environment and a reproducible edit. Record:
- Qt Creator version and Qt version.
- Project type:
.qmlprojectUI project or Qt Quick application. - Build system or project arrangement, such as CMake or qmake.
- Target platform and selected kit, including whether the preview is on desktop, Android, or Boot to Qt.
- Whether QML debugging is enabled and, for a Qt Quick application, whether you rebuilt after changing the setting.
- A minimal before-and-after QML change that fails to appear, plus whether the app remains open, rebuilds, or restarts.
Without those details, there is no evidence for one universal cause: project configuration, target path, Qt version, and the shape of the edit can lead to different behavior.
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