iokit/IOKit/IOService.h

1372     The interested driver should override @link powerStateWillChangeTo powerStateWillChangeTo@/link and @link powerStateDidChangeTo powerStateDidChangeTo@/link to receive these power change messages.
1373     Interested drivers must acknowledge power changes in <code>powerStateWillChangeTo</code> or <code>powerStateDidChangeTo</code>, either via return value or later calls to @link acknowledgePowerChange acknowledgePowerChange@/link.
1382     @discussion The retain from <code>registerInterestedDriver</code> is released. This method is synchronous against any <code>powerStateWillChangeTo</code> or <code>powerStateDidChangeTo</code> call targeting the interested driver, so when this method returns it is guaranteed those interest handlers will not be entered.
1392     @discussion When power management informs an interested object (via @link powerStateWillChangeTo powerStateWillChangeTo@/link or @link powerStateDidChangeTo powerStateDidChangeTo@/link), the object can return an immediate acknowledgement via a return code, or it may return an indication that it will acknowledge later by calling <code>acknowledgePowerChange</code>.
1393     Interested objects are those that have registered as interested drivers, as well as power plane children of the power changing driver. A driver that calls @link registerInterestedDriver registerInterestedDriver@/link must call <code>acknowledgePowerChange</code>, or use an immediate acknowledgement return from <code>powerStateWillChangeTo</code> or <code>powerStateDidChangeTo</code>.
1643 /*! @function powerStateWillChangeTo
1645     @discussion Power management informs interested parties that a device is about to change to a different power state. Interested parties are those that have registered for this notification via @link registerInterestedDriver registerInterestedDriver@/link. If you have called <code>registerInterestedDriver</code> on a power managed driver, you must implement <code>powerStateWillChangeTo</code> and @link powerStateDidChangeTo powerStateDidChangeTo@/link to receive the notifications.
1646     <code>powerStateWillChangeTo</code> is called in a clean and separate thread context. <code>powerStateWillChangeTo</code> is called before a power state transition takes place; <code>powerStateDidChangeTo</code> is called after the transition has completed.
1652     virtual IOReturn powerStateWillChangeTo(
1659     @discussion Power management informs interested parties that a device has changed to a different power state. Interested parties are those that have registered for this notification via @link registerInterestedDriver registerInterestedDriver@/link. If you have called <code>registerInterestedDriver</code> on a power managed driver, you must implemnt @link powerStateWillChangeTo powerStateWillChangeTo@/link and <code>powerStateDidChangeTo</code> to receive the notifications.
1660     <code>powerStateDidChangeTo</code> is called in a clean and separate thread context. <code>powerStateWillChangeTo</code> is called before a power state transition takes place; <code>powerStateDidChangeTo</code> is called after the transition has completed.