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AET65 API
Version 1.0
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Page 51 of 84
Page 51 of 84
Description
A type of the callback functions that an application can supply to the
BSAPI to enable itself to display GUI state information to user.
The callback is passed into the BSAPI function of interactive
operations
through
ABS_OPERATION
structure.
Interactive
operations call the callback repeatedly while the operation is in
process. Thus the application can react accordingly to the process
stage of the operation and update user interface.
Note that exact way when the callback is called can be further
determined by member Flags of ABS_OPERATION.
Most applications will probably implement a callback in a way that it
will create a dialog when first message of a biometric operation
appears and then update a text and/or image in the dialog,
according to the messages received. For this reason, BSAPI.DLL
delays sending of some messages if there would be danger that the
letter would replace the former too quickly so that end user would
have no chance to catch the message. For example when user
swipes incorrectly, and bad quality of the swipe or of resulted image
is detected, the callback is called with appropriate feedback
message. Then there is some delay before the callback is called
again, with a prompt for new swipe so user has time to see the bad
quality feedback.
However this default behavior might not be desired in some other
scenarios. For example if the callback implementation writes every
message to a new line, so user can review complete history of the
messages, or when the callback implementation provides no
feedback to the user. In these cases the delays between some
subsequent messages only protract time of the biometric operation.
To
disable
all
those
delays,
set
the
flag
ABS_OPERATION_FLAG_LL_CALLBACK
in
ABS_OPERATION::Flags.
The second supported flag of ABS_OPERATION related to
ABS_CALLBACK is ABS_OPERATION_FLAG_USE_IDLE. When
set, BSAPI.DLL guarantees the callback is called quite often (about
100 milliseconds). If there is nothing it would report, it uses
message ABS_MSG_IDLE. The only purpose of this is to allow the
canceling of the operation from the callback in a reasonable way
(see ABSCancelOperation for more info). However this comes at
some cost: it eats more CPU cycles, so when this is not needed
(e.g. you know that you don't cancel the operation or when you can
cancel it from other thread), you should avoid use of this flag.
When the flag ABS_OPERATION_FLAG_USE_IDLE is not set, the
message ABS_MSG_IDLE is never used and it there might be quite
a long time between two subsequent calls of the callback, e.g. when
the biometric operation has been started but user does not touch
the sensor for long time.
Parameters
pOperation
Pointer to ABS_OPERATION structure used when calling the
interactive operation.
The caller of the interactive operation can use member Context of
the structure to pass data into the callback.
