# Loading Components Dynamically
The easiest way to dynamically load different parts of QML is to use the Loader
element. It serves as a placeholder to the item that is being loaded. The item to load is controlled through either the source
property or the sourceComponent
property. The former loads the item from a given URL, while the latter instantiates a Component
.
As the loader serves as a placeholder for the item being loaded, its size depends on the size of the item, and vice versa. If the Loader
element has a size, either by having set width
and height
or through anchoring, the loaded item will be given the loader’s size. If the Loader
has no size, it is resized in accordance to the size of the item being loaded.
The example described below demonstrates how two separate user interface parts can be loaded into the same space using a Loader
element. The idea is to have a speed dial that can be either digital or analog, as shown in the illustration below. The code surrounding the dial is unaffected by which item that is loaded for the moment.
The first step in the application is to declare a Loader
element. Notice that the source
property is left out. This is because the source
depends on which state the user interface is in.
Loader {
id: dialLoader
anchors.fill: parent
}
In the states
property of the parent of dialLoader
a set of PropertyChanges
elements drives the loading of different QML files depending on the state
. The source
property happens to be a relative file path in this example, but it can just as well be a full URL, fetching the item over the web.
states: [
State {
name: "analog"
PropertyChanges { target: analogButton; color: "green"; }
PropertyChanges { target: dialLoader; source: "Analog.qml"; }
},
State {
name: "digital"
PropertyChanges { target: digitalButton; color: "green"; }
PropertyChanges { target: dialLoader; source: "Digital.qml"; }
}
]
In order to make the loaded item come alive, it is speed
property must be bound to the root speed
property. This cannot be done as a direct binding as the item not always is loaded and changes over time. Instead, a Binding
element must be used. The target
property of the binding is changed every time the Loader
triggers the onLoaded
signal.
Loader {
id: dialLoader
anchors.left: parent.left
anchors.right: parent.right
anchors.top: parent.top
anchors.bottom: analogButton.top
onLoaded: {
binder.target = dialLoader.item;
}
}
Binding {
id: binder
property: "speed"
value: root.speed
}
The onLoaded
signal lets the loading QML act when the item has been loaded. In a similar fashion, the QML being loaded can rely on the Component.onCompleted
signal. This signal is actually available for all components, regardless of how they are loaded. For instance, the root component of an entire application can use it to kick-start itself when the entire user interface has been loaded.
# Connecting Indirectly
When creating QML elements dynamically, you cannot connect to signals using the onSignalName
approach used for static setup. Instead, the Connections
element must be used. It connects to any number of signals of a target
element.
Having set the target
property of a Connections
element, the signals can be connected, as usual, that is, using the onSignalName
approach. However, by altering the target
property, different elements can be monitored at different times.
In the example shown above, a user interface consisting of two clickable areas is presented to the user. When either area is clicked, it is flashed using an animation. The left area is shown in the code snippet below. In the MouseArea
, the leftClickedAnimation
is triggered, causing the area to flash.
Rectangle {
id: leftRectangle
width: 290
height: 200
color: "green"
MouseArea {
id: leftMouseArea
anchors.fill: parent
onClicked: leftClickedAnimation.start()
}
Text {
anchors.centerIn: parent
font.pixelSize: 30
color: "white"
text: "Click me!"
}
}
In addition to the two clickable areas, a Connections
element is used. This triggers the third animation when the active, i.e. the target
of the element, is clicked.
Connections {
id: connections
function onClicked() { activeClickedAnimation.start() }
}
To determine which MouseArea
to target, two states are defined. Notice that we cannot set the target
property using a PropertyChanges
element, as it already contains a target
property. Instead a StateChangeScript
is utilized.
states: [
State {
name: "left"
StateChangeScript {
script: connections.target = leftMouseArea
}
},
State {
name: "right"
StateChangeScript {
script: connections.target = rightMouseArea
}
}
]
When trying out the example, it is worth noticing that when multiple signal handlers are used, all are invoked. The execution order of these is, however, undefined.
When creating a Connections
element without setting the target
property, the property defaults to parent
. This means that it has to be explicitly set to null
to avoid catching signals from the parent
until the target
is set. This behavior does make it possible to create custom signal handler components based on a Connections
element. This way, the code reacting to the signals can be encapsulated and re-used.
In the example below, the Flasher
component can be put inside any MouseArea
. When clicked, it triggers an animation, causing the parent to flash. In the same MouseArea
the actual task being triggered can also be carried out. This separates the standardized user feedback, i.e. the flashing, from the actual action.
import QtQuick
Connections {
function onClicked() {
// Automatically targets the parent
}
}
To use the Flasher
, simply instantiate a Flasher within each MouseArea, and it all works.
import QtQuick
Item {
// A background flasher that flashes the background of any parent MouseArea
}
When using a Connections
element to monitor the signals of multiple types of target
elements, you sometimes find yourself in a situation where the available signals vary between the targets. This results in the Connections
element outputting run-time errors as signals are missed. To avoid this, the ignoreUnknownSignal
property can be set to true
. This ignores all such errors.
TIP
It is usually a bad idea to suppress error messages, and if you do, make sure to document why in a comment.
# Binding Indirectly
Just as it is not possible to connect to signals of dynamically created elements directly, nor it is possible to bind properties of a dynamically created element without working with a bridge element. To bind a property of any element, including dynamically created elements, the Binding
element is used.
The Binding
element lets you specify a target
element, a property
to bind and a value
to bind it to. Through using a Binding
element, it is, for instance, possible to bind properties of a dynamically loaded element. This was demonstrated in the introductory example in this chapter, as shown below.
Loader {
id: dialLoader
anchors.left: parent.left
anchors.right: parent.right
anchors.top: parent.top
anchors.bottom: analogButton.top
onLoaded: {
binder.target = dialLoader.item;
}
}
Binding {
id: binder
property: "speed"
value: root.speed
}
As the target
element of a Binding
not always is set, and perhaps not always has a given property, the when
property of the Binding
element can be used to limit the time when the binding is active. For instance, it can be limited to specific modes in the user interface.
The Binding
element also comes with a delayed
property. When this property is set to true
the binding is not propagated to the target
until the event queue has been emptied. In high load situations this can serve as an optimization as intermediate values are not pushed to the target
.