Using EventFilters

[emad-elsaid]

I noticed that some functions expect to inherit qt.QObject for example InstallEventFilter(filterObj *qt6.QObject) expect filterObj to inherit QObject.
in Go how would I implement a new EventFilter in this case? I thought of creating a struct and embeding qt6.QObject in it. then pass the pointer to this object.

type Filter struct {
	qt6.QObject
}

func (f *Filter) EventFilter(obj *qt6.QObject, event *qt6.QEvent) bool {
	slog.Info("EventFilter", "event", event.Type(), "object", obj.ObjectName())
	return false
}

then

application.InstallEventFilter(&new(Filter).QObject)

tried to add the filter to the window itself I also didn't get any logs. tried to add it to the widget I'm trying to intercept its events (WebEngineView). also didn't print logs.

So either I'm missing something related to Qt in general or to how the binding works. any pointers will be apprechiated

[rcalixte]

I'm working on a precise example that should be of help (see below) but the long and short of it is that my understanding is that you would create a new QObject type in addition to your other object (let's say a QLabel in this instance), no need for a struct:

o := qt.NewQObject()

You would then use o.OnEventFilter() to set the filter for the newly-constructed object and then on your original QObject-derived instance, call InstallEventFilter, so the steps would be effectively:

l := qt.NewQLabel2()
o := qt.NewQObject()

o.OnEventFilter(func(super func(watched *qt.QObject, event *qt.QEvent) bool, watched *qt.QObject, event *qt.QEvent) bool {
  // do your things here
}

l.InstallEventFilter(o)

I'm basing this on my reading of the documentation for QObject::installEventFilter and how it would apply for the bindings for Go. I hope this helps!

EDIT: It's a little sloppy but this works for me:

package main

import (
	"fmt"
	"os"
	"strconv"

	qt "github.com/mappu/miqt/qt6"
)

func main() {

	qt.NewQApplication(os.Args)

	w := qt.NewQWidget2()
	w.SetFixedWidth(400)
	w.SetFixedHeight(100)

	l := qt.NewQLabel5("Press any of the keys on your keyboard!", w)
	l.SetFocusPolicy(qt.StrongFocus)

	o := qt.NewQObject()
	o.OnEventFilter(func(super func(watched *qt.QObject, event *qt.QEvent) bool, watched *qt.QObject, event *qt.QEvent) bool {
		if event.Type() == qt.QEvent__KeyPress {
			k := qt.UnsafeNewQKeyEvent(event.UnsafePointer())
			if k.Key() == int(qt.Key_Return) {
				fmt.Println("Ate your Enter key")
				return true
			}
		}
		return super(watched, event)
	})

	l.InstallEventFilter(o)

	l.OnKeyPressEvent(func(super func(ev *qt.QKeyEvent), ev *qt.QKeyEvent) {
		key := ev.Key()
		text := fmt.Sprintf("You pressed key code: " + strconv.Itoa(key))
		l.SetText(text)
	})

	w.Show()

	qt.QApplication_Exec()

	fmt.Println("OK!")
}

This works with either order of operations between InstallEventFilter and OnKeyPressEvent so you should be able to have some flexibility in your code. Feel free to play with this code and adapt it to your needs.

[emad-elsaid]

Thank you, this worked for me too. I tried at first to use the QApplication object to be the filter instead of creating another QObject instance. it didn't work.

Then I followed exactly this example. and it worked as I expected.

I also tried to cast the event pointer kevent, ok := any(event).(*qt.QKeyEvent) it didn't work. I used the same function you used UnsafeNewQKeyEvent it worked. although the Unsafe in the name is giving me the creeps. I'm not sure what's unsafe about it and what dangers I'm stepping into when using it.

[rcalixte]

although the Unsafe in the name is giving me the creeps. I'm not sure what's unsafe about it and what dangers I'm stepping into when using it.

It's purely technical and a sign of caution/certainty because we are downcasting the pointer object. Imagine the case where it was a QMouseEvent or QWheelEvent instead of a QKeyEvent and we still tried to cast the object's pointer to a QKeyEvent. That is technically an unsafe operation and trying to access the data at the pointer will either crash or lead to undefined behavior. To mitigate this downcasting unsafety, we check for the type of the QEvent first before executing the operation.

The inverse (upcasting) is considered safe in most operations which is why we can pass QObject-derived classes as QObject type parameters without having to invoke any unsafe methods.