GrabbaMRZ_Listener Class Reference

Description

Base MRZ-event listener class, for receiving events from the Grabba device's MRZ reader.

The following events are supported by this class:

• Data - machine-readable zone data was read
• ValidatedData - machine-readable zone data was read and it passed validation

Each event may invoke callbacks in two ways:

• Subclassing - the relevant event methods (e.g. dataEvent) are called directly
  • This approach is recommended wherever feasible, due to its simplicity.
  • To use it, create an object of a suitable subclass, which overrides the desired event method(s).
• Delegation - event methods trigger method calls for a delegate object
  • This approach is only recommended when the callback class must inherit from a different base class
  • To use it, create an object of this class, create an object of a class which implements GrabbaMRZ_Protocol, then set the delegate property of the former to point to the latter.

Note

Delegation will be disabled for an event if that event's method is subclassed, unless the subclass calls the superclass' equivalent method from within its override thereof.

By default, each listener object will register itself for callbacks upon construction, and deregister itself at destruction; the enable and disable methods may be used if an object requires manual control over when its callbacks are triggered.

The default behaviour for each event, if not overridden, on an enabled listener object is as follows:

• If delegate has not been set, or has been set to null, then take no action
• If delegate has been set to a non-null pointer, then delegate to the pointed-to object

Overrides need not call the superclass' equivalent method unless it is necessary to preserve the delegation (i.e. to support both subclassing and delegation from a single object).

Thread safety: This interface is intended to be thread-safe; any classes deriving or delegating from it should ensure that the relevant methods are callable from any thread.

See also

GrabbaMRZ_API for related API functions

Inheritance diagram for GrabbaMRZ_Listener:

Instance Methods
(void)	- dataEvent:
	Callback which is invoked when a machine-readable zone (MRZ) is read. More...
(void)	- disable
	Disable listening for events from this object, if not already disabled. More...
(void)	- enable
	Enable listening for events from this object, if not already enabled. More...
(instancetype _Nullable)	- init
	Default initialiser - builds an object then enables event callbacks to it. More...
(instancetype _Nullable)	- initWithEnabled:
	Initialiser which allows control over whether event callbacks should be enabled. More...
(void)	- validatedDataEvent:
	Callback which is invoked when a machine-readable zone (MRZ) is read and successfully validated. More...
(void)	- validationFailedEvent:reason:
	Callback which is invoked when a machine-readable zone (MRZ) is read, but fails validation. More...

Properties
ProtocolType _Nullable	delegate
	Delegate for receiving events from non-subclassed listener objects; defaults to nil. More...

Method Documentation

dataEvent:()

- (void) dataEvent:

(GrabbaMRZ_Data *_Nonnull)

data

Callback which is invoked when a machine-readable zone (MRZ) is read.

This event is triggered when the optical character recognition (OCR) module encounters an MRZ (swiped through the Grabba device's reading slot) and is able to read data from it.

This event will be triggered regardless of data validity - i.e. even if some characters are not read successfully, or if the data which was read does not conform to any of the formats defined in ICAO 9303.

Most applications can safely ignore this event unless they need the raw MRZ data even from validated MRZs - consider using validatedDataEvent and/or validationFailedEvent instead. Each trigger of this event will be coupled with exactly one of those two (with the choice depending on the validation outcome); no guarantees are made as to the order in which the two events are triggered.

Override this method to receive callbacks when the object is enabled (via init or enable) and the event is triggered.

Note

Overrides of this method are required to be thread-safe; no guarantees are made about which thread(s) they will be called from.

Parameters

data	Passport, visa or identity card data which was read

disable()

- (void) disable

Disable listening for events from this object, if not already disabled.

This will remove the listener object from the relevant event handler, if it had previously been added. Callbacks will no longer be triggered on this object, although they may be re-enabled by calling enable.

Note

This method should not be overridden.

enable()

- (void) enable

Enable listening for events from this object, if not already enabled.

This will add the listener object to the relevant event handler, if it has not already been added. Events will then trigger callbacks on this object until it is destroyed or disable is called.

Note

This method should not be overridden.

init()

- (instancetype _Nullable) init

Default initialiser - builds an object then enables event callbacks to it.

Returns

Pointer to initialised object if initialisation succeeded; nil if it failed

initWithEnabled:()

- (instancetype _Nullable) initWithEnabled:

(BOOL)

startEnabled

Initialiser which allows control over whether event callbacks should be enabled.

Parameters

startEnabled

If YES, the newly-allocated listener object is added to the relevant event handler, enabling receipt of event callbacks. If NO, no callbacks are received until enable is called.

Returns

Pointer to initialised object if initialisation succeeded; nil if it failed

validatedDataEvent:()

- (void) validatedDataEvent:

(GrabbaMRZ_Validated *_Nonnull)

data

Callback which is invoked when a machine-readable zone (MRZ) is read and successfully validated.

This event is triggered when the optical character recognition (OCR) module encounters an MRZ (swiped through the Grabba device's reading slot), it is able to successfully read data in one of the formats defined in ICAO 9303, and the read data passes all validation checks for that format.

This event is mutually exclusive with validationFailedEvent; applications which handle this event should typically ignore dataEvent and pick up validationFailedEvent if they need to process scan failures. If dataEvent is handled, however, it should be noted that no guarantees are made as to the order in which this event and that event will be triggered after a scan.

Override this method to receive callbacks when the object is enabled (via init or enable) and the event is triggered.

Note

Overrides of this method are required to be thread-safe; no guarantees are made about which thread(s) they will be called from.

Parameters

data	Passport, visa or identity card data which was read and validated

validationFailedEvent:reason:()

- (void) validationFailedEvent:		(GrabbaMRZ_Data *_Nonnull)	data
reason:		(NSString *_Nonnull)	reason

Callback which is invoked when a machine-readable zone (MRZ) is read, but fails validation.

This event is triggered when the optical character recognition (OCR) module encounters an MRZ (swiped through the Grabba device's reading slot), but the data which was read does not pass validation checks. This could be due to any of the following causes:

• The scan was incomplete (e.g. one end of the MRZ did not pass the read head)
• One or more characters could not be successfully identified
• The MRZ was in a format not consistent with those defined by ICAO 9303
• The MRZ was incorrectly written (e.g. check digit incorrectly calculated)

This event is mutually exclusive with validatedDataEvent; applications which handle this event should typically ignore dataEvent and pick up validatedDataEvent if they need to process scan failures. If dataEvent is handled, however, it should be noted that no guarantees are made as to the order in which this event and that event will be triggered after a scan.

Override this method to receive callbacks when the object is enabled (via init or enable) and the event is triggered.

Note

Overrides of this method are required to be thread-safe; no guarantees are made about which thread(s) they will be called from.

Parameters

data	Passport, visa or identity card data which was read, but failed to pass validation
reason	Brief description of the validation failure reason

Property Documentation

delegate

- (ProtocolType _Nullable) delegate

readwritenonatomicweakinherited

Delegate for receiving events from non-subclassed listener objects; defaults to nil.

Delegation may be used as an alternative to subclassing for receiving event callbacks. Any Grabba-defined listener class object (as opposed to a subclass thereof) will pass events on to a delegate, if one has been set and it has implemented the relevant protocol method.

If the delegate is to be written to whilst a callback to it is in progress, the write operation will block until that operation has completed. This ensures thread-safety.

Note

Delegates must provide thread-safe implementations of all protocol methods; no guarantees are made about which thread(s) they will be called from.