Power-Aware Mobilized Windows Applications

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March 9, 2009 1:00 AM PDT


Challenge

Handle the WM_POWERBROADCAST message provided by the Windows operating system and scaling features accordingly. WM_POWERBROADCAST is a Windows message sent to applications to indicate system-wide power state changes.

Hardware advances have made mobile systems lighter and more energy efficient. In addition to these advances, software developers can make applications power-aware such that they enable users to work for longer periods of time before having to reconnect to recharge the system battery. For example, when on AC power, an anti-virus scanner would run in a full-featured fashion, providing file scanning on all files opened and also running periodic system-wide scans. When on battery power, the scanner could defer the system-wide scans until a later time and continue processing safely, analyzing just the open files.

Under Windows, applications are notified of system power changes through the WM_POWERBROADCAST message. Note that the WM_POWER message is now obsolete and should be replaced with WM_POWERBROADCAST.

Solution

Handle messages through the standard message loop using GetMessage(). The WM_POWERBROADCAST message comes with two parameters: wParam and lParam. The wParam parameter can be a number of items that specify the event received. The documentation for each of these is as follows:

Event

Meaning

PBT_APMBATTERYLOW

Battery power is low.

PBT_APMOEMEVENT

OEM-defined event occurred.

PBT_APMPOWERSTATUSCHANGE

Power status has changed.

PBT_APMQUERYSUSPEND

Request for permission to suspend.

PBT_APMQUERYSUSPENDFAILED

Suspension request denied.

PBT_APMRESUMEAUTOMATIC

Operation resuming automatically after event.

PBT_APMRESUMECRITICAL

Operation resuming after critical suspension.

PBT_APMRESUMESUSPEND

Operation resuming after suspension.

PBT_APMSUSPEND

System is suspending operation.

 

lParam is function-specific and is not generally used.

The source for this item, cited below, provides further detail and use cases. Additional introductory material is available from the MSDN documentation at http://msdn.microsoft.com/*.

Source

Windows* WM_POW ERBROADCAST Messages in a Mobilized Environment