main.qml

    Loader {
        id: loader
        anchors.fill: parent
        asynchronous: true
        opacity: 0
        focus: true;

        sourceComponent: PageStack {

            initialPage: YourPage {

                onPresented: {
                    window.visible = true;
                    loader.opacity = 1;
                }
            }
        }

        Behavior on opacity {
            NumberAnimation {
                duration: 200
                easing.type: Easing.OutQuad;
            }
        }
    }