This document specifies an API that allows web applications to request a screen wake lock. Under the right conditions, and if allowed, the screen wake lock prevents the system from turning off a device's screen.
Implementors need to be aware that this specification is extremely unstable. Implementors who are not taking part in the discussions will find the specification changing out from under them in incompatible ways. Vendors interested in implementing this specification before it eventually reaches the Candidate Recommendation phase should subscribe to the repository on GitHub and take part in the discussions.
Modern operating systems achieve longer battery life by implementing aggressive power management, meaning that shortly after the lack of user activity, a host device may lower the screen brightness, turn the screen off and even let the CPU go into a deep power state, limiting power usage as much as possible.
Though this is great for prolonged battery life, it can sometime hinder some use cases such as scanning a barcode, reading an ebook, following a recipe, presenting to an audience, and so on. See also [[[wake-lock-use-cases]]].
A wake lock will generally prevent something from happening, but UAs (and the underlying OS) may time limit a wake lock given the battery status (wall power connected, discharging, low battery level), or even disallow wake locks in the case a power saving mode is activated.
This specification defines the following wake lock type:
In the API, the [=wake lock types=] are represented by the {{WakeLockType}} enum values.
Other specifications might define different wake lock types.
The Screen Wake Lock API defines a policy-controlled feature identified by the string `"screen-wake-lock"`. Its default allowlist is `["self"]`.
The [[PERMISSIONS]] API provides a uniform way for websites to request permissions from users and query which permissions they have.
A user agent can deny a wake lock of a particular wake lock type for a particular {{Document}} by any implementation-specific reason, such as platform setting or user preference.
It is RECOMMENDED that a user agent show some form of unobtrusive notification that informs the user when a wake lock is active, as well as provides the user with the means to block the ongoing operation, or simply dismiss the notification.
The `"screen-wake-lock"` powerful feature enables the capability defined by this specification.
To block a permission, run these steps:
To obtain permission for wake lock type |name|, run these steps in parallel. This async algorithm returns either {{PermissionState/"granted"}} or {{PermissionState/"denied"}}.
The term platform wake lock refers to platform interfaces with which the user agent interacts to query state and acquire and release a wake lock.
A platform wake lock can be defined by the underlying platform (e.g. in a native wake lock framework) or by the user agent, if it has direct hardware control.
Each platform wake lock (one per wake lock type) has an associated state record per [=environment settings object / responsible document=] with the following internal slots:
| Internal slot | Initial value | Description (non-normative) |
|---|---|---|
| [[\ActiveLocks]] | The empty list. | A list of {{WakeLockSentinel}} objects, representing active wake locks associated with the [=environment settings object / responsible document=]. |
[SecureContext]
partial interface Navigator {
[SameObject] readonly attribute WakeLock wakeLock;
};
The {{WakeLock}} interface allows a document to acquire a [=screen wake lock=].
[SecureContext, Exposed=(Window)]
interface WakeLock {
Promise<WakeLockSentinel> request(WakeLockType type);
};
The {{WakeLock/request()}} method, when invoked, MUST run the following steps. The method takes one argument, the {{WakeLockType}} |type:WakeLockType|:
[SecureContext, Exposed=(Window)]
interface WakeLockSentinel : EventTarget {
readonly attribute WakeLockType type;
Promise<void> release();
attribute EventHandler onrelease;
};
A {{WakeLockSentinel}} object provides a handle to a platform wake lock, and it holds on to it until it is either manually released or until the underlying platform wake lock is released. Its existence keeps a platform wake lock for a given wake lock type active, and releasing all {{WakeLockSentinel}} instances of a given wake lock type will cause the underlying platform wake lock to be released.
The type attribute corresponds to the {{WakeLockSentinel}}'s wake lock type.
The {{WakeLockSentinel/release()}} method, when invoked, MUST run the following steps:
The {{WakeLockSentinel/onrelease}} attribute is an event
handler whose corresponding event handler event type is
release.
It is used to notify scripts that a given {{WakeLockSentinel}} object's handle has been released, either due to the {{WakeLockSentinel/release()}} method being called or because the wake lock was released by the user agent.
For the purpose of wake lock type description, this specification defines the following enumeration to represent [=wake lock types=]:
enum WakeLockType { "screen" };
This section applies to each wake lock type equally and independently, unless a particular wake lock type is explicitly mentioned.
The user agent acquires the wake lock by requesting the underlying operating system to apply the lock. The lock is considered acquired only when the request to the operating system succeeds.
Conversely, the user agent releases the wake lock by requesting the underlying operating system to no longer apply the wake lock. The lock is considered released only when the request to the operating system succeeds.
The wake lock is applicable if the state of the operating system permits application of the lock (e.g. there is sufficient battery charge).
The screen wake lock MUST NOT be applicable after the screen is manually switched off by the user until it is switched on again.
A user agent may release a wake lock at any time it:
When the user agent determines that the visibility state of the [=environment settings object / responsible document=] of the current settings object changes, it must run these steps:
To acquire a wake lock for a given |lock:WakeLockSentinel| and |type:WakeLockType|, run these steps in parallel:
To release a wake lock for a given |lock:WakeLockSentinel| and |type:WakeLockType|, run these steps in parallel:
Application of a wake lock causes various device components such as display or CPU to operate at higher power levels than they otherwise would. This can lead to undesirable and potentially dangerous effects such as excessive heating and faster than normal battery charge depletion. The latter is particularly relevant to mobile devices which may not have a stationary power source readily available. Complete battery depletion at an unexpected time can lead to inability of the user to make or receive calls and use network services, including the emergency call service. Implementations should consider preventing wake lock application if they determine that the remaining battery capacity is low.
When the user agent does not acquire wake lock even though a browsing context has requested it, this can be observed by the browsing context and can possibly disclose sensitive information about the state of the device such as that battery level is low.
function tryKeepScreenAlive(minutes) {
navigator.wakeLock.request("screen").then(lock => {
setTimeout(() => lock.release(), minutes * 60 * 1000);
});
}
tryKeepScreenAlive(10);
This example allows the user to request a screen wake lock by clicking on a checkbox, but updates the checkbox checked state in case the wake lock state changes:
const checkbox = document.createElement("input");
checkbox.setAttribute("type", "checkbox");
document.body.appendChild(checkbox);
const sentinel = await navigator.wakeLock.request("screen");
checkbox.checked = true;
sentinel.onrelease = () => checkbox.checked = false;
In this example, two different wake lock requests are created and released independently:
let lock1 = await navigator.wakeLock.request("screen");
let lock2 = await navigator.wakeLock.request("screen");
lock1.release();
lock2.release();
Document's hidden attribute,
and visibility
state are defined in [[PAGE-VISIBILITY]].
This specification defines conformance criteria for a single product: a user agent that implements the interfaces that it contains.
We would like to offer our sincere thanks to Mounir Lamouri, Sergey Konstantinov, Matvey Larionov, Dominique Hazael-Massieux, Domenic Denicola, Thomas Steiner, Raphael Kubo da Costa for their contributions to this work.