This document specifies an API that allows web applications to request a wake lock. A wake lock prevents some aspect of the device from entering a power-saving state (e.g., preventing the system from turning off the screen).
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//request to keep the display awake when the recipe is showing:
screen.keepAwake = true;
//stop keeping the display awake:
screen.keepAwake = false;
2.
Wake Locks
A wake lock prevents some aspect of the device or operating system from entering a power-saving state.
This specification deals specifically with the screen wake lock which prevents the screen(s) of the device from entering a power-saving state.
3. Extension to Screen interface
This specification extends Screen interface defined by [
CSSOM-VIEW] to allow document scripts to inform the browser that they prefer to keep the device screen awake:
Each browsing context has a request flag for the
wake lock, represented by its screen.keepAwake attribute. The request flag is set to false when the
browsing context is created or its active document is changed. Request flags are used by the user agent to track if a particular document is requesting the lock (for the purpose of
acquiring or releasing the corresponding operating system lock).
A document requests wake lock by setting
browsing context's lock request flag. This means that the document indicates to the user agent a preference that the underlying operating system apply wake lock, i.e. keep the screen on.
A document can successfully request wake lock regardless of whether the actual lock can be applied by the user agent and/or the underlying operating system at the time when the request is made.
Conversely, a document cancels wake lock
request by resetting its browsing context's lock request
flag. This means that the document indicates to the user
agent that it no longer prefers that the underlying system apply the wake lock.
Note
Setting or resetting a lock request flag merely indicates a preference of the document to have the wake lock applied. Requesting the lock or cancelling wake lock request does not entail synchronous acquisition or release of the corresponding operating system lock, and can only result in an error when the document is explicitly prohibited from executing those operations by some security mechanism.
No document "owns" the operating system lock and the state of operating system lock does not affect the state of the lock request flag for any document's browsing context.
Instead, the user agent is responsible for mapping the state of request flags for all browsing contexts to the state the operating system lock.
5. Managing Wake Locks
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 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 same-origin requirement prevents unauthorized nested third-party content from requesting a wake lock on behalf of the top-level page.
An alternative approach could be requiring a permission as defined in [
PERMISSIONS] which would effectively delegate decisions about wake lock in nested browsing contexts to the user agent. The downside is that permission inheritance model is currently non-portable between implementations and sometimes inconsistent even within the same implementation. For more details see e.g. this
Chromium design document.
The wake lock is applicable if the state of the operating system permits application of the lock (e.g. there is sufficient battery charge and wake locking is not disabled by a user setting).
The user agentMUSTacquire the wake lock when all of the following conditions become true:
The user agentMUSTrelease the wake lock when any of the conditions above become false.
Note
From the user's perspective, a document which has requested the wake lock will no longer cause the lock to be applied if one of the following happens:
Browser tab or window is closed.
Switching from one tab to another.
Browser is minimized.
Navigating to a different page or reloading a page.
Should the tab and/or browser window once again come to foreground, the lock will be applied again if it is still requested and the state of the operating system permits its application.
6. Additional lock types
The lock request and management mechanism presented in this document can be extended to support additional types of locks by adding new lock request flags to appropriate interface objects. E.g. a "system" wake lock could be introduced which would prevent the device's cpu, radio(s), and other system services from entering a power-saving state while allowing the screen to switch off.
7.
Security and privacy considerations
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.
8.
Dependencies
The following concepts and interfaces are defined in [HTML]:
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key word MUST is to be interpreted as described in [RFC2119].
This specification defines conformance criteria for a single product: a
user agent that implements the interfaces that it contains.
The user agentMUST implement the APIs defined in this specification in a manner that conforms to the ECMAScript
Bindings defined in [WEBIDL].
A.
Acknowledgments
This section is non-normative.
We would like to offer our sincere thanks to Mounir Lamouri, Sergey Konstantinov, Matvey Larionov, Dominique Hazael-Massieux (via the HTML5Apps project) for their contributions to this work.