Qt for HarmonyOS/user development guide/embedded subwindow guide

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1. Concepts and Scope

1.1 What is an Embedded Subwindow

In-Process Embedding

• It is not a system-level "window", but a child node under the root node of the current window.
• An XComponent can be mounted inside the child node for custom rendering.
• Layout, input events, and lifecycle are controlled by the Qt top-level window.

Cross-Process Embedding (EmbeddedComponent / EmbeddedUIExtensionAbility)

• The host and the content run in separate processes; Qt can act as the Provider.
• Qt as the Provider: The Qt side provides window content, while the Host side handles embedding and display.
• A binding key can be used to map Host instances to Qt windows on the Provider side one-to-one (for multi-instance scenarios).

1.2 Differences from HarmonyOS System Subwindows

Type	Key Differences
In-Process Embedding	Not a system subwindow: Not managed individually by the window manager, and has no system-level Z-Order. No independent lifecycle: Created and destroyed along with the Qt top-level window. Position and clipping constrained by the parent node: No system-level cross-screen or multi-window positioning capability.
Cross-Process Embedding	Not a system subwindow: Still not managed individually by the system window manager. Lifecycle driven by the Host: Created and destroyed along with the EmbeddedComponent / EmbeddedUIExtensionAbility session. Position and clipping controlled by the Host: The embedded area is constrained by the Host's layout and clipping.

2. Type Classification and Selection

Type	Method	Applicable Scenarios	Key Qt Interfaces
In-Process Embedding	winId() / Qt::WA_NativeWindow	Embedding third-party renderers requiring native window handles, such as CEF, SDL	QWidget::winId(), Qt::WA_NativeWindow
Cross-Process Embedding (Qt as Provider)	EmbeddedComponent / EmbeddedUIExtensionAbility	Embedding Qt content into other host applications	QEmbeddedUiExtensionAbility / QtOhosExtras::setBundledAbilityAndQWindowBindingKeyForQWindow()

3. In-Process Embedding

3.1 Triggering a Native Node via winId()

Suitable for third-party frameworks that require a native window handle (e.g., CEF, SDL). Core point: Force the creation of a native window handle.

class NativeNodeHost : public QWidget {
    Q_OBJECT
public:
    explicit NativeNodeHost(QWidget *parent = nullptr)
        : QWidget(parent, Qt::Widget) {
        this->setAttribute(Qt::WA_NativeWindow); // Optional; not required if winId() is called
    }

    void bindThirdPartyRenderer() {
        WId wid = winId();
        QWindow *qw = windowHandle();
        Q_UNUSED(qw);
        // third_party_bind_native_handle(wid);
    }
};

Notes:

• setAttribute(Qt::WA_NativeWindow) and winId() are two separate approaches; usually either one is sufficient.
• winId() forces Qt to create a native window handle and implicitly sets Qt::WA_NativeWindow.
• setAttribute(Qt::WA_NativeWindow) only marks the widget as native; actual creation typically occurs during show() / create().
• When triggering an embedded native node, the widget is generally required to have a non-null parent (as the embedding container).
• If the parent is a top-level window, the native node is attached directly to the root node level of that window.
• winId() is a per-widget forced creation method, while setting the environment variable QT_USE_NATIVE_WINDOWS=1 is a global method.

4. Cross-Process Embedding (EmbeddedComponent / EmbeddedUIExtensionAbility)

Qt supports cross-process embedding. When Qt acts as the Provider, it can be divided into single-instance and multi-instance modes.

Two ExtensionAbility types are pre-configured in the Qt HarmonyOS project template:

• QEmbeddedUiExtensionAbility: extensionProcessMode: "instance", single-instance mode.
• QBundledEmbeddedUiExtensionAbility: extensionProcessMode: "bundle", multi-instance mode.

4.1 Qt as the Provider (Other Frameworks as Host)

4.1.1 Single Process, Single Instance (QEmbeddedUiExtensionAbility)

• Applicable to: Only one instance is needed in the same Provider process.
• Method: The Host creates an EmbeddedComponent, and the Provider creates and shows a top-level Qt window.

Minimal Provider-side (Qt) example:

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    auto *window = new QWidget;
    window->setWindowTitle("Single Instance UiExtension");
    window->resize(800, 600);
    window->show();

    return app.exec();
}

4.1.2 Single Process, Multiple Instances (QBundledEmbeddedUiExtensionAbility)

• Applicable to: Multiple concurrent instances within the same Provider process.
• Method: Use a binding key to map each Host instance to a corresponding Qt window on the Provider side.
• Standard parameter name:

  io.qt.public.bundledAbilityAndQWindowBindingKey
  

  .

Host-side (ArkUI) example key points: Set a unique binding key in want.parameters for each EmbeddedComponent.

private want1: Want = {
  bundleName: 'com.ohos.ohosqttemplate',
  abilityName: 'QBundledEmbeddedUiExtensionAbility',
  moduleName: 'entry',
  parameters: {
    'io.qt.public.bundledAbilityAndQWindowBindingKey': 'A',
  }
}

Provider-side (Qt) example key points: Use the interface QtOhosExtras::setBundledAbilityAndQWindowBindingKeyForQWindow().

Item	Details
Declaration	qtohosextras/src/ohosextras/qohoswindowutils.h
Parameters	window: The QWindow to bind; key: Unique binding key
Function	Establishes a mapping for multi-instance correspondence

QApplication app(argc, argv);

auto getOrCreateUnderlyingQWindow = [](QWidget *widget) {
    widget->winId();
    return widget->windowHandle();
};

auto windowA = makeBundledUiExtensionWindow("A");
QtOhosExtras::setBundledAbilityAndQWindowBindingKeyForQWindow(
    getOrCreateUnderlyingQWindow(windowA.get()), "A");

return app.exec();

5. Frequently Asked Questions

• Q: What is the difference between winId() and windowHandle()?
  • A: winId() forces the generation of a native window handle; windowHandle() may return null before the native handle is created.
• Q: Why is a binding key required for multi-instance EmbeddedUIExtensionAbility?
  • A: It maps the Host's EmbeddedComponent to the corresponding Qt window on the Provider side, preventing incorrect bindings.

6. Notes

• Embedded subwindows remain inside the same Qt top-level window; their Z-Order and clipping are affected by the parent node.
• In multi-instance scenarios, binding must be completed before the window is shown.

7. References

• Official Introduction to EmbeddedUIExtensionAbility
• HarmonyOS Application Template