QML fade out changed item - binding

Question

I'm new to Qt and QML - this seems like a rather simple issue but I can't find a simple solution to it.
Say I have the following example QML
Item {
... lots of other stuff
Item {
id: obj_container
property var obj
Text {
text: obj.name
}
Image {
source: obj.source
}
}
}
Now when the obj property becomes null, I would like to fade out the obj_container item, while still displaying the values it had before it was set to null.
Alternatively, if the obj item changes to a different obj, I would like to fade out the obj_container item (still displaying its previous values) and then fade it in again with the new values.
How would I go about this?
UPDATE
the obj in the example is a Q_PROPERTY of an object set using setContextProperty from C++, as in
engine.rootContext()->setContextProperty("obj_holder", &obj_holder);
the obj property in the example above would then be set like
obj_container.obj = obj_holder.obj
though I think for the purposes above it doesn't make a difference where the obj property is coming from or how it was set/changed. When the obj changes, the above should happen (fade out obj_container with old values, fade in obj_container with new values)

Answer 1

Fading out can be easy done with one of Animation.
For example:
import QtQuick 2.4
import QtQuick.Window 2.2
Window {
width: 600
height: 400
visible: true
Text {
id: txt
anchors.centerIn: parent
property int count: 0
text: count
opacity: 1
font.pixelSize: 100
SequentialAnimation {
id: anim
PropertyAnimation {
target: txt
property: "opacity"
to: 0
duration: 1000
easing.type: Easing.OutQuart
}
PropertyAction {
target: txt
property: "count"
value: txt.count + 1
}
PropertyAnimation {
target: txt
property: "opacity"
to: 1
duration: 1000
easing.type: Easing.InOutCubic
}
}
}
Timer {
interval: 2000
repeat: true
running: true
onTriggered: anim.running = true
}
}