This is a text-only version of the following page on https://raymii.org:

This is a text-only version of the following page on https://raymii.org:
---
Title : Fade in / fade out in Qt/QML
Author : Remy van Elst
Date : 19-08-2022 21:30
URL : https://raymii.org/s/tutorials/Fade_in_Fade_out_in_QML.html
Format : Markdown/HTML
---

This guide shows you how to add a fade in / fade out effect to a control in QML. There are a lot of built in animations in Qt/QML, but no fade in/fade out. Using a state machine and a `SequentialAnimation`, we can first animate the opacity, then set the visibility, achieving a fade in / fade out effect. Other ways like a `PropertyAnimation` are also available but are less expressive or configurable.

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The `visibility` property of an item cannot be animated directly, since it's a
`bool`. We therefore have to animate the `opacity` property,
which is a number from 0.0 to 1.0. Using a `NumberAnimation` gives control
over the duration and putting those in a `SequentialAnimation` makes them
happen in order. Combining that with the built in state machine every QML
control has (to invert the order of the effects when hiding the item) we
achieve a nicely animated fade in / fade out, without resorting to writing
custom OpenGL code in C++ for our own QML control.

Here is a GIF showing the full effect and also how it looks when you're just
toggling visibility:

![animated gif][1]

Is this convoluted? Yes I think so, an entire state machine for just a fade
in/fade out effect. Is it nice that Qt/QML allows you to hack this together
using their builtin standard library? Yes, I do think so. Would I rather have
an effect I can simply apply, much like, for example, one of the built in
[blur effects][2]? Yes, that would be even better. Other animations and
effects are easy to do, so why not add a built in for this effect?

### QML Fade In / Fade Out

Add the following state machine and transitions to your QML control,
then bind the state to a property or trigger it directly. The id
of the control is `exampleControl` and the property that I use to
trigger the fade in / fade out animation is named `folded`.

id: exampleControl
property bool folded: false
state: !folded ? "Visible" : "Invisible"
states: [
State{
name: "Visible"
PropertyChanges{target: exampleControl; opacity: 1.0}
PropertyChanges{target: exampleControl; visible: true}
},
State{
name:"Invisible"
PropertyChanges{target: exampleControl; opacity: 0.0}
PropertyChanges{target: exampleControl; visible: false}
}
]

transitions: [
Transition {
from: "Visible"
to: "Invisible"

SequentialAnimation{
NumberAnimation {
target: exampleControl
property: "opacity"
duration: 500
easing.type: Easing.InOutQuad
}
NumberAnimation {
target: exampleControl
property: "visible"
duration: 0
}
}
},

Transition {
from: "Invisible"
to: "Visible"
SequentialAnimation{
NumberAnimation {
target: exampleControl
property: "visible"
duration: 0
}
NumberAnimation {
target: exampleControl
property: "opacity"
duration: 500
easing.type: Easing.InOutQuad
}
}
}
]

### Full example source code

This is the code that creates the recorded GIF in the article. It shows
the animation code and how to bind it to a property which can be triggered.
I found the state machine example on stackoverflow, but I cannot find the
specific topic in my browser history anymore, so I cannot link to the source
example. If you do happen to know, please send me an email so I can update
this article.

import QtQuick 2.15
import QtQuick.Controls 1.4
import QtQuick.Window 2.15

Window {
width: 640
height: 480
visible: true
title: qsTr("Fade in / Fade out demo by raymii.org")

Column {
anchors.fill: parent
anchors.margins: 20
spacing: 20

Row {
spacing: 20
Button {
text: fadeRect.folded ? "Fade in" : "Fade out"
onClicked: fadeRect.folded = !fadeRect.folded
}

Button {
text: toggleRect.visible ? "Hide" : "Show"
onClicked: toggleRect.visible = !toggleRect.visible
}

}

Rectangle {
id: fadeRect
width: 410
height: 60
border.width: 3
property bool folded: true
border.color: "#cccccc"
color: "#efefef"

Row {
anchors.fill: parent
anchors.margins: 10
spacing: 5

Button {
text: "Button 1"
}
Button {
text: "Button 2"
}
Button {
text: "Button 3"
}
}

state: !folded ? "Visible" : "Invisible"
states: [
State{
name: "Visible"
PropertyChanges{target: fadeRect; opacity: 1.0}
PropertyChanges{target: fadeRect; visible: true}
},
State{
name:"Invisible"
PropertyChanges{target: fadeRect; opacity: 0.0}
PropertyChanges{target: fadeRect; visible: false}
}
]

transitions: [
Transition {
from: "Visible"
to: "Invisible"

SequentialAnimation{
NumberAnimation {
target: fadeRect
property: "opacity"
duration: 500
easing.type: Easing.InOutQuad
}
NumberAnimation {
target: fadeRect
property: "visible"
duration: 0
}
}
},

Transition {
from: "Invisible"
to: "Visible"
SequentialAnimation{
NumberAnimation {
target: fadeRect
property: "visible"
duration: 0
}
NumberAnimation {
target: fadeRect
property: "opacity"
duration: 500
easing.type: Easing.InOutQuad
}
}
}
]

}

Rectangle {
id: toggleRect
width: 410
height: 60
border.color: "#cccccc"
color: "#efefef"
border.width: 3
visible: false

Row {
anchors.fill: parent
anchors.margins: 10
spacing: 5

Button {
text: "Button 1"
}
Button {
text: "Button 2"
}
Button {
text: "Button 3"
}
}
}
}
}

[1]: /s/inc/img/qml-fade-in-fade-out.gif
[2]: https://web.archive.org/web/20220819095850/https://doc.qt.io/qt-5/qml-qtgraphicaleffects-zoomblur.html

---

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