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To determine the characteristics of available video devices, your application can use the
DMGetFirstScreenDevice
function to obtain a handle to the
GDevice
structure for the first video device in the device list. The
DMGetFirstScreenDevice
function is similar to the QuickDraw function
GetDeviceList
, except that when returning
GDevice
structures,
GetDeviceList
does not distinguish between the
GDevice
structures for video devices and the
GDevice
structures associated with no video devices. (For example, if system software uses the function
DMDisableDisplay
to disable the last remaining device in the device list, then
DMDisableDisplay
inserts into the device list a
GDevice
structure that is not associated with any video device. The
DMGetFirstScreenDevice
function will not return this
GDevice
structure, but
GetDeviceList
might.)
After using the
DMGetFirstScreenDevice
function to obtain a handle to the first
GDevice
structure for a display in the device list, your application can use the
DMGetNextScreenDevice
function to loop through all of the video devices in the device list. The
DMGetNextScreenDevice
function is similar to the QuickDraw function
GetNextDevice
, except that when returning
GDevice
structures,
GetNextDevice
does not distinguish between the
GDevice
structures for video devices and the
GDevice
structures associated with no video devices.
Another important difference between these two Display Manager functions (
DMGetFirstScreenDevice
and
DMGetNextScreenDevice
) and their related QuickDraw functions (
GetDeviceList
and
GetNextDevice
) is that with both Display Manager functions, your application can specify that the Display Manager return handles only to active video devices. (An active device is a video device whose display area is included in the user's desktop; the display area of an inactive device does not appear on the user's desktop.)
To get a handle to the
GDevice
structure for a video device that mirrors another, your application can use the
DMGetNextMirroredDevice
function.
Your application can pass the
GDevice
handle returned for any of these video devices to a QuickDraw function like
TestDeviceAttribute
or
HasDepth
to determine various characteristics of the video device, or your application can examine the
gdRect
field of the
GDevice
structure to determine the dimensions of the screen it represents.
Macintosh system software uses the
DMCheckDisplayMode
function to determine whether a video device supports a particular display mode and pixel depth. Typically, your application does not need to know whether a display mode is supported, but only whether a specific pixel depth is supported, in which case your application can use the Color QuickDraw function
HasDepth
.
To determine whether QuickDraw supports video mirroring on the user's computer system, your application can use the
DMQDIsMirroringCapable
function. Your application can use the
DMCanMirrorNow
function to determine whether video mirroring can activate. And to determine whether the user's computer system currently uses video mirroring, your application can use the
DMIsMirroringOn
function.
Finally, your application can use the
DMGetDisplayIDByGDevice
function to determine the display ID for a video device. A display ID is a long integer used by the Display Manager to uniquely identify a video device. Associating a display by its display ID is helpful when using functions such as
DMRemoveDisplay
that could change the
GDevice
structure associated with a video device. You can first determine the display ID for a device by using the
DMGetDisplayIDByGDevice
function. To later retrieve that device's
GDevice
structure after calling various Display Manager functions, your application can use the
DMGetGDeviceByDisplayID
function. Display IDs are not guaranteed to be persistent across reboots or sleep.
