UIAbility Component Lifecycle

Overview

When a user opens, switches, and returns to an application, the UIAbility instances in the application transit in their different states. The UIAbility class provides a series of callbacks. Through these callbacks, you can know the state changes of the UIAbility instance, for example, being created or destroyed, or running in the foreground or background.

The lifecycle of UIAbility has four states: Create, Foreground, Background, and Destroy, as shown in the figure below.

Figure 1 UIAbility lifecycle states

Description of Lifecycle States

Create

The Create state is triggered when the UIAbility instance is created during application loading. The system invokes the onCreate() callback. In this callback, you can perform application initialization operations, for example, defining variables or loading resources.

import UIAbility from '@ohos.app.ability.UIAbility';
import window from '@ohos.window';

export default class EntryAbility extends UIAbility {
    onCreate(want, launchParam) {
        // Initialize the application.
    }
    // ...
}

WindowStageCreate and WindowStageDestory

After the UIAbility instance is created but before it enters the Foreground state, the system creates a WindowStage instance and triggers the onWindowStageCreate() callback. You can set UI loading and WindowStage event subscription in the callback.

Figure 2 WindowStageCreate and WindowStageDestory

In the onWindowStageCreate() callback, use loadContent() to set the page to be loaded, and call on('windowStageEvent') to subscribe to WindowStage events, for example, having or losing focus, or becoming visible or invisible.

import UIAbility from '@ohos.app.ability.UIAbility';
import window from '@ohos.window';

export default class EntryAbility extends UIAbility {
    // ...

    onWindowStageCreate(windowStage: Window.WindowStage) {
        // Subscribe to the WindowStage events (having or losing focus, or becoming visible or invisible).
        try {
            windowStage.on('windowStageEvent', (data) => {
                console.info('Succeeded in enabling the listener for window stage event changes. Data: ' +
                    JSON.stringify(data));
            });
        } catch (exception) {
            console.error('Failed to enable the listener for window stage event changes. Cause:' +
                JSON.stringify(exception));
        };

        // Set the UI loading.
        windowStage.loadContent('pages/Index', (err, data) => {
            // ...
        });
    }
}

NOTE

For details about how to use WindowStage, see Window Development.

Before the UIAbility instance is destroyed, the onWindowStageDestroy() callback is invoked to release UI resources. In this callback, you can unsubscribe from the WindowStage events.

import UIAbility from '@ohos.app.ability.UIAbility';
import window from '@ohos.window';

export default class EntryAbility extends UIAbility {
    // ...

    onWindowStageDestroy() {
        // Release UI resources.
        // Unsubscribe from the WindowStage events such as having or losing focus in the onWindowStageDestroy() callback.
        try {
            windowStage.off('windowStageEvent');
        } catch (exception) {
            console.error('Failed to disable the listener for window stage event changes. Cause:' +
                JSON.stringify(exception));
        };
    }
}

Foreground and Background

The Foreground and Background states are triggered when the UIAbility instance is switched to the foreground and background respectively. They correspond to the onForeground() and onBackground() callbacks.

The onForeground() callback is triggered before the UI of the UIAbility instance becomes visible, for example, when the UIAbility instance is switched to the foreground. In this callback, you can apply for resources required by the system or re-apply for resources that have been released in the onBackground() callback.

The onBackground() callback is triggered after the UI of the UIAbility component is completely invisible, for example, when the UIAbility instance is switched to the background. In this callback, you can release useless resources or perform time-consuming operations such as saving the status.

For example, an application needs to use positioning, and the application has requested the positioning permission from the user. Before the UI is displayed, you can enable positioning in the onForeground() callback to obtain the location information.

When the application is switched to the background, you can disable positioning in the onBackground() callback to reduce system resource consumption.

import UIAbility from '@ohos.app.ability.UIAbility';

export default class EntryAbility extends UIAbility {
    // ...

    onForeground() {
        // Apply for the resources required by the system or re-apply for the resources released in onBackground().
    }

    onBackground() {
        // Release useless resources when the UI is invisible, or perform time-consuming operations in this callback,
        // for example, saving the status.
    }
}

Destroy

The Destroy state is triggered when the UIAbility instance is destroyed. You can perform operations such as releasing system resources and saving data in the onDestroy() callback.

The UIAbility instance is destroyed when terminateSelf() is called or the user closes the instance in Recents.

import UIAbility from '@ohos.app.ability.UIAbility';
import window from '@ohos.window';

export default class EntryAbility extends UIAbility {
    // ...

    onDestroy() {
        // Release system resources and save data.
    }
}