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Procedure ComPortTimeOut(Sender: TObject) Procedure ComPortLineEvent(Sender: TObject Error: TLineError) Procedure ComPortDataReceived(Sender: TObject Trigger: TTrigger) Now have a look at the same abstract for class TController with the mediator applied to this structure: Property ComPort: TComPort read FComPort Property OnTransmitted: TNotifyEvent read FOnTransmitted write FOnTransmitted Property OnTimeOut: TNotifyEvent read FOnTimeOut write FOnTimeOut Property OnLineEvent: TLineEvent read FOnLineEvent write FOnLineEvent Property OnDataReceived: TTriggerEvent read FOnDataReceived write FOnDataReceived TTriggerEvent = procedure (Sender: TObject Trigger: TTrigger) of object TLineEvent = procedure (Sender: TObject Error: TLineError) of object Here's an abstract of the interface and implementation of TComport and TController without the mediator:

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This TController class would typically wire and handle the Comport events to update it's internal state.

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A TController class could 'use a' TComPort class to transmit data through a serial port. This class defines a series of events to asynchronously inform clients of state changes: data triggers, line errors etc. Suppose we've created a class TComPort which encapsulates a serial port device driver. The mediator pattern relies on these event type definitions to construct and update event handlers. In order to construct a correct event handler method it needs the event type definition.

Make sure you unwire the event again in appropriate cases to avoid wired objects from calling destroyed or otherwise illegal objects.

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Wire the event handler to the event with code like: FSample.OnChange := SampleChange Create a event handler method with the correct parameter list and possibly the correct function result.If you ever manually created event handlers you know that you have to: So what's the need for a Mediator pattern?Дelphi's Object Inspector uses RTTI and form designers to create event handlers on forms and Delphi's component streaming mechanism takes care of actually wiring these events at run time. The mediator is the form which handles all events and takes corresponding actions, coupling all related components together with snippets of code. Being able to delegate behaviour to another class using events, saves you, for example, from subclassing TButton when you need a button that interacts with an TEdit control. They even delivered a great tool with it: the Object Inspector. The class TForm is their standard mediator class which handles (wires) events from components put on the form. The use of events makes de-coupling of objects possible. Their solution to this problem is essentially: events (or method pointers).

As a result, you may be forced to define many subclasses to customise the system's behaviour.Дevelopers at Borland probably were aware of the above mentioned paragraphs when designing the Delphi Object Pascal language and the VCL components.

Moreover it can be difficult to change the systems' behaviour in any significant way, since behaviour is distributed among many objects. Lots of interconnections make it less likely that an object can work without the support of others - the system acts as though it were monolithic. Though partitioning a system into many objects generally enhances reusability, proliferating interconnections tend to reduce it again.

Delphi object inspector