QML property on change ?

[SPlatten]

Is there a way to create an event handler for when a property changes ?

I have a property define in the QML:

property double waterDepth: vehicle.data.totalWaterDepth

vehicle.data.totalWaterDepth is somewhere in the C++, I haven't been able to locate it. What I would like to do is create a handler to mange when the value changes.

Something like:

onWaterDepthChanges: {
...
}

[Pradeep P N]

Hi @SPlatten ,

You should also have signals with totalWaterDepth in C++ which can help for value change updates

[SPlatten]

@Pradeep-P-N , thank you, thats exactly the way I would do it, the trouble is I didn't code the C++ and I have no idea where the change is made, I've tried to locate it and failed.

[JoeCFD]

@SPlatten Can you try?

    Connections { 
        target: vehicle.data
        function onTotalWaterDepthChanged() {
            -----
        }
    }

You can set a break point at ------

[SPlatten]

@JoeCFD, thank you, I will at some point soon, I have other build issues now.

[freedbrt]

@SPlatten said in QML property on change ?:

onWaterDepthChanges: {
...
}

It won't work, if totalWaterDepth doesn't have signal or signal not emitted. Only way to do this without signals, is checking property changes by the timer, but it is a bad way.

[SPlatten]

@JoeCFD , the property:

property double waterDepth : vehicle.data.totalWaterDepth ? vehicle.data.totalWaterDepth : 0

I tried the following:

 Connections {
      target: vehicle.data
      function onTotalWaterDepthChanged() {
          console.log("onTotalWaterDepthChanged: ", root.waterDepth)
      }
  }

  onWaterDepthChanged: {
      console.log("New water depth: ", root.waterDepth)
  }

I rebuilt and run up and when I send a new water depth, I can see it being received by another process but nothing is displayed in the console. I would have thought that the QML property would have had events such as Changed. vehicle.data.totalWaterDepth comes from the C++ application.

[SPlatten]

@freedbrt , doesn't the property type itself have any events that can be implemented ?

[Drooke]

@SPlatten the (assumed) Qproperty is defined in the (vehicle.data) class generally with a type, getter, setter, and Signal. If you can find in your codebase where the Qproperty is defined you can piggyback off of that signal.

[SPlatten]

@Drooke , this is the biggest problem I'm having the source is huge and I've searched many times for something that could explain how it is accessed, I can see nothing, "vehicle" exists in various places, "totalWaterDepth" also exists in several places, but there nothing obvious that explains the access of vehicle.data.totalWaterDepth.

[JoeCFD]

@SPlatten totalWaterDepthChanged() is a signal and has to be defined by the coder. If there is no reaction for your change, that means totalWaterDepthChanged() signal does not exist. You can search for it with grep.

[fcarney]

Look for Q_PROPERTY macros:

Q_PROPERTY(int ted READ ted WRITE setted NOTIFY tedChanged)

Note: the NOTIFY signal can be named anything.

[JoeCFD]

@fcarney is right. The signal name can be anything. I normally use tedChanged in my app.

[lemons]

Couldn't vehicle.data also be a struct with a Q_GADGET macro?
→ in this case there might only be a dataChanged signal, instead of a signal for each member of the struct

[SPlatten]

@lemons , its not my code and I have no say on how it is or was coded.

[Drooke]

@SPlatten try console.log(vehicle) and console.log(vehicle.data) in a component.oncompleted block. which may print out the classname / QObject type. This may point to the location of your code you're looking for.

[GrecKo]

To answer the original question :

Is there a way to create an event handler for when a property changes ?

Something like:

property double waterDepth: vehicle.data.totalWaterDepth
onWaterDepthChanges: {
   ...
}

The corresponding change signal would be waterDepthChanged, with its handler being onWaterDepthChanged: ....

Scrolling through your past exchange would make me guess that vehicle has a property data which is a QQmlPropertyMap where the totalWaterDepth is eventually set. Doesn't a global search followed by some code unravelling give you any clues?

But why do you need a signal handler in the first place? It's generally a code smell and should be available when possible (moreso when it's not a boolean property).

[SPlatten]

@GrecKo , I tried that and it doesn't work, I think because the primitive type double is not an object.

[SPlatten]

@GrecKo said in QML property on change ?:

QQmlPropertyMap

Thank you, you have given me a helpful hint in that I have now located:

Q_PROPERTY (QQmlPropertyMap *data READ data CONSTANT)

[GrecKo]

@SPlatten said in QML property on change ?:

@GrecKo , I tried that and it doesn't work, I think because the primitive type double is not an object.

That doesn't prevent it from triggering change signals.

Maybe your vehicle or data is changed in C++ without emitting any NOTIFY signal? if totalWaterDepth is in a QQmlPropertyMap it shouldn't be able to be changed without emitting a signal so that leaves the other two.

How do you know the final value changes?