Implementation of event bus pattern for Delphi XE
EventBus is designed to provide interaction between different components, without increasing connectivity.
- Development
- The type of event is determined by the class.
- Events are inherited.
- The base class for any event is TbtkEventObject.
- Filtering
- Events can contain filters.
- Filter values are case sensitive.
- To declare filters, annotations of event class methods are used.
- As a filter, functions without parameters are used that should return the filter value as a string.
- Filters are identified by name.
- The filter name is specified in the filter annotation.
- Filter names are not case sensitive.
- Filters use two modes:
- Simple mode.
- The event filter value corresponds to empty and exactly matching handler filter values.
- This mode is used by default.
- Hashable mode.
- The event filter value only matches exactly the same handler filter value.
- Hashing accelerates the formation of lists of handlers to be called.
- Simple mode.
- Filter mode is specified in the filter annotation.
- The base class contains one hash filter named "Topic"
- Handlers
- Adding event handlers is done by registering a listener on the bus.
- Removal of event handlers is performed by unregistration of the listener in the bus.
- The filter values of listener event handlers are set after registration.
- Filter values are bound to the event type, and are equal for different handlers of the same event.
- To declare handlers, annotations of listener methods are used.
- Handlers must contain one input parameter with the class type of the event being processed.
- The type of the handler parameter determines the events that it will process.
- The handler is invoked for all heirs of the event processed by it.
- Two types of event handlers are used:
- Simple handlers.
- When calling, the filtering conditions are taken into account.
- The order of the call is not guaranteed.
- Hooks.
- Called before calling simple handlers.
- Ignore filtering conditions.
- The order of the call corresponds to the reverse order of registration.
- Simple handlers.
- The type of handler will be determined by annotation.
//event class declaration Type TFooEventObject = class(TbtkEventObject) //.......................... public const sFooFilterName = 'FooFilter'; constructor Create(ATopic: string; AFooFliter: string); [EventFilter(sFooFilterName)] //filter parameter declaration function FooFilter: string; //.......................... end; //preparation of the listener TFooEventListener = class //.......................... public [EventHandler] //handler declaration procedure FooHandler(AFooEvent: TFooEventObject); [EventHook] //hook declaration procedure FooHook(AEventObject: TFooEventObject); //.......................... end; EventBus := TbtkEventBus.GetEventBus('FooEventBus'); ListenerInfo := EventBus.Register(FooEventListener); //setting filter parameters ListenerInfo.HandlerFilters[TFooEventObject][TFooEventObject.sEventFilterTopicName].Value := 'TopicValue'; ListenerInfo.HandlerFilters[TFooEventObject][TFooEventObject.sFooFilterName].Value := 'FooFilterValue'; //creating and sending events EventBus.Send(TFooEventObject.Create('TopicValue', 'FooFilterValue')); //listener unregistration EventBus.Unregister(FooListener);program EventHookExample; {$APPTYPE CONSOLE} uses System.SysUtils, btkEventBus; type TFooEventListener = class public [EventHook] //hook declaration procedure FooHook(EventObject: TbtkEventObject); end; { TFooEventListener } //hook implementation procedure TFooEventListener.FooHook(EventObject: TbtkEventObject); begin Writeln(Format('======'#13#10'Event with topic "%s" sended', [EventObject.Topic])); end; const ebFoo = 'FooEventBus'; var FooEventListener: TFooEventListener; FooTopicName: string; begin //register class for eventbus with name 'FooEventBus' RegisterEventBusClass(TbtkEventBus, ebFoo); FooEventListener := TFooEventListener.Create; try //register listener EventBus(ebFoo).Register(FooEventListener); Write('Write topic: '); ReadLn(FooTopicName); //create and send event EventBus(ebFoo).Send(TbtkEventObject.Create(FooTopicName)); finally FooEventListener.Free; end; Readln; end.program EventHookExample; {$APPTYPE CONSOLE} uses System.SysUtils, btkEventBus; type TFooEventListener = class public [EventHandler] //handler declaration procedure FooHandler(EventObject: TbtkEventObject); end; { TFooEventListener } //handler implementation procedure TFooEventListener.FooHandler(EventObject: TbtkEventObject); begin Writeln(Format('Event with topic "%s" sended', [EventObject.Topic])); end; const FooTopicName = 'FooTopic'; var FooEventListener: TFooEventListener; FooListenerInfo: TbtkListenerInfo; begin //register class for eventbus with empty name RegisterEventBusClass(TbtkEventBus); FooEventListener := TFooEventListener.Create; try //register listener and get listner info FooListenerInfo := EventBus.Register(FooEventListener); //set topicfilter for handler FooListenerInfo.HandlerFilters.Items[TbtkEventObject].Filters[TbtkEventObject.sEventFilterTopicName].Value := FooTopicName; //create and send event EventBus.Send(TbtkEventObject.Create(FooTopicName)); finally FooEventListener.Free; end; Readln; end.