The Qt 6 Book

# Serving UI via HTTP

To load a simple user interface via HTTP we need to have a web-server, which serves the UI documents. We start off with our own simple web-server using a python one-liner. But first, we need to have our demo user interface. For this, we create a small RemoteComponent.qml file in our project folder and create a red rectangle inside.

// RemoteComponent.qml
import QtQuick

Rectangle {
    width: 320
    height: 320
    color: '#ff0000'
}

To serve this file we can start a small python script:

cd <PROJECT>
python -m http.server 8080

Now our file should be reachable via http://localhost:8080/RemoteComponent.qml. You can test it with:

curl http://localhost:8080/RemoteComponent.qml

Or just point your browser to the location. Unfortunately, your browser does not understand QML and will not be able to render the document. Fortunately, a QML web browser does exist. It's called Canonic (opens new window). Canonic is itself built with QML and runs in your web browser via WebAssembly. However, if you are using the WebAssembly version of Canonic, you won't be able to view files served from localhost; in a bit, you'll see how to make your QML files available to use with the WebAssembly version of Canonic. If you want, you can download the code to run Canonic as an app on your desktop, but there are security concerns related to doing so (see here (opens new window) and here (opens new window) for more details).

Furthermore, Qt 6 provides the qml binary, which can be used like a web browser. You can directly load a remote QML document by using the following command:

qml http://localhost:8080/RemoteComponent.qml

Sweet and simple.

TIP

If the qml program is not in your path, you can find it in the Qt binaries: <qt-install-path>/<qt-version>/<your-os>/bin/qml.

Another way of importing a remote QML document is to dynamically load it using QML ! For this, we use a Loader element to retrieve for us the remote document.

// LocalHostExample.qml
import QtQuick

Loader {
    id: root
    source: 'http://localhost:8080/RemoteComponent.qml'
    onLoaded: {
        root.width = root.item.width  // qmllint disable
        root.height = root.item.height  // qmllint disable
    }
}

Now we can ask the qml executable to load the local LocalHostExample.qml loader document.

qml LocalHostExample.qml

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If you do not want to run a local server you can also use the gist service from GitHub. The gist is a clipboard like online services like Pastebin and others. It is available under https://gist.github.com (opens new window). For this example, I created a small gist under the URL https://gist.github.com/jryannel/7983492 (opens new window). This will reveal a green rectangle. As the gist URL will provide the website as HTML code we need to attach a /raw to the URL to retrieve the raw file and not the HTML code.

Since this content is hosted on a web server with a public web address, you can now use the web-based version of Canonic to view it. To do so, simply point your web browser to https://app.canonic.com/#http://gist.github.com/jryannel/7983492 (opens new window). Of course, you'll need to change the part after the # to view your own files.

// GistExample.qml
import QtQuick

Loader {
    id: root
    source: 'https://gist.github.com/jryannel/7983492/raw'
    onLoaded: {
        root.width = root.item.width  // qmllint disable
        root.height = root.item.height  // qmllint disable
    }
}

To load another file over the network from RemoteComponent.qml, you will need to create a dedicated qmldir file in the same directory on the server. Once done, you will be able to reference the component by its name.

# Networked Components

Let us create a small experiment. We add to our remote side a small button as a reusable component.

Here's the directory structure that we will use:

./src/SimpleExample.qml
./src/remote/qmldir
./src/remote/Button.qml
./src/remote/RemoteComponent.qml

Our SimpleExample.qml is the same as our previous main.qml example:

import QtQuick

Loader {
    id: root
    anchors.fill: parent
    source: 'http://localhost:8080/RemoteComponent.qml'
    onLoaded: {
        root.width = root.item.width  // qmllint disable
        root.height = root.item.height  // qmllint disable
    }
}

In the remote directory, we will update the RemoteComponent.qml file so that it uses a custom Button component:

// remote/RemoteComponent.qml
import QtQuick

Rectangle {
    width: 320
    height: 320
    color: '#ff0000'

    Button {
        anchors.centerIn: parent
        text: qsTr('Click Me')
        onClicked: Qt.quit()
    }
}

As our components are hosted remotely, the QML engine needs to know what other components are available remotely. To do so, we define the content of our remote directory within a qmldir file:

# qmldir
Button 1.0 Button.qml

And finally we will create our dummy Button.qml file:

// remote/Button.qml
import QtQuick.Controls

Button {
    
}

We can now launch our web-server (keep in mind that we now have a remote subdirectory):

cd src/serve-qml-networked-components/
python -m http.server --directory ./remote 8080

And remote QML loader:

qml SimpleExample.qml

# Importing a QML components directory

By defining a qmldir file, it's also possible to directly import a library of components from a remote repository. To do so, a classical import works:

import QtQuick
import "http://localhost:8080" as Remote

Rectangle {
    width: 320
    height: 320
    color: 'blue'

    Remote.Button {
        anchors.centerIn: parent
        text: qsTr('Quit')
        onClicked: Qt.quit()
    }
}

TIP

When using components from a local file system, they are created immediately without a latency. When components are loaded via the network they are created asynchronously. This has the effect that the time of creation is unknown and an element may not yet be fully loaded when others are already completed. Take this into account when working with components loaded over the network.

WARNING

Be very cautious about loading QML components from the Internet. By doing so, you introduce the risk of accidentally downloading malicious components that will do evil things to your computer. These security risks have been documented (opens new window) by Qt. The Qt page was already linked to on this page, but the warning is worth repeating.

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