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83 lines
2.5 KiB
Markdown
83 lines
2.5 KiB
Markdown
---
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title: "Binding QML with Python: PyViewer"
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date: 2021-08-29T12:53:19+02:00
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draft: false
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tags:
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- python
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- qml
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- gui
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- code
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---
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[PyViewer](https://git.leene.dev/lieuwe/pyviewer) is a example project which
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implements a simple image browser / viewer in a scrollable grid array. This main
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objective here was using QML to define a graphical layout and bind it to a
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python code-base. Note that this code base is compatible with both Pyside2 and
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Pyside6. This is because while Pyside6 is preferred it is not readily available
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on all platforms. Running Pyside6 instead only recommend the qml library version
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requirements to omitted.
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Please take a look at the git repository for exact implementation details. A
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brief summary of this interaction is presented below.
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## Emitting QML Calls
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Creating a `QObject` and adding `PySide2.QtCore.Slot` decorators to its methods
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will allow a python object to be added to the qml context as a referenceable
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object. For example here we add "viewer" to the qml context which is a
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"PyViewer" python object.
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```Python
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pyviewer = PyViewer()
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engine.rootContext().setContextProperty("viewer", pyviewer)
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```
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This way we can call the object's python procedure "update_tag_filter" from
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within the QML script as follows:
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```QML
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viewer.update_tag_filter(false);
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```
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Further using the `PySide2.QtCore.Property` decorator further allows us to call
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states in our python object and manipulate them as it were a qml object.
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```QML
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viewer.path.split("::")
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```
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## Emitting Python Calls
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Once this context is working we can create a `PySide2.QtCore.Signal` object to
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call QML methods from within the python context. A python procedure could then
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"emit" this signal and thereby prompt any connected qml methods.
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```python
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self.path_changed.emit()
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```
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In the qml contect we can connect the signals from the python "viewer" object
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to a qml function call "swipe.update_paths" for example.
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```qml
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viewer.path_changed.connect(swipe.update_paths)
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```
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## Downside
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Debugging and designing QML in this environment is limited since the pyside
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python library does not support all available QML/QT6 functionality. In most
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cases you are looking at C++ Qt documentation for how the pyside data-types
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and methods are supposed to behave without good hinting.
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Also the variety in data types that can be passed from one context to the other
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is constrained although in this case I was able to manage with strings and byte
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objects.
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## Other Notes: TODO
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```python
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ImageCms.profileToProfile(img, 'USWebCoatedSWOP.icc',
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'sRGB Color Space Profile.icm', renderingIntent=0, outputMode='RGB')
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```
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