Writing CPicture derived classes

Each CPicture-derived class must provide:

• a constructor which creates a ‘real-world’ picture and sets a data member to the picture’s original size in device-independent units (for example, iOriginalSizeInTwips could be used for twips);

• a GetOriginalSizeInTwips() function which returns the original size of the picture in twips;

• a Draw() function which draws the picture to a particular graphics map on a graphics context — at this point, and not before, the picture’s co-ordinates should be converted from device-independent units to pixels;

• an ExternalizeL() function which externalises the picture to a stream.

Normally, a CPicture-derived class will also provide:

• an InternalizeL() function to internalise the picture from a stream;

• StoreL() and RestoreL() functions to store and restore pictures from a stream store;

• one or more data members to access and manipulate the original picture — the picture’s scaling may be altered (this changes the ‘real-world’ size of the picture in twips);

• if necessary, behaviour that performs appropriate clean-up.

CApaIconPicture is an example of a CPicture-derived class.