Question

I have an item with value param. I wonder how to catch its change event?

Say having a RectComp.qml:

Item{
    property alias currentX: rect.x
       Rectangle {
           id: rect
           x: 617
           y: 450
       }
}

handle its currentX change from application that creates its instance say how to

       Rectangle {
           id: host
           x: 617
           y: 450
           RectComp{ id: MyRC}
           OnMyRCcurrentXChange(int){log("hello!")}
       }

Answer 1

As per Qt documentation “Adding a property to an item automatically adds a value changed signal handler to the item.”

So your code will changes to following

 Rectangle {
       id: host
       x: 617
       y: 450
       RectComp{ id: MyRC
         onCurrentXChanged:{
             console.log("Hello");
         }
       }
   }

Or you can use Connection element

 Connections {
     target: MyRC
     onCurrentXChanged: {
        console.log("Hello");
     }
 }