How to derive from COpenFontFile

Write a class derived from COpenFontFile to manage a file of the font format supported by the DLL. An object of this type is created by COpenFontRasterizer::NewFontFile().

To derive from COpenFontFile:

• Provide implementations for COpenFontFile's two pure virtual functions: GetNearestFontInPixelsL() and HasUnicodeCharacterL(). These are discussed further in the following sections.

• During construction call the base class constructor, passing it the arguments aUid and aFileName supplied to COpenFontRasterizer::NewFontFileL().

• During construction, extract the attributes of each font in the font file into a TOpenFontFaceAttrib. Then add the attributes for each typeface to the typeface array — using COpenFontFile::AddFaceL().

This function is called by the Font and Bitmap server to retrieve the font (from the font file) which most closely matches a given font specification. The freetype implementation is given below:

void CFreeTypeFontFile::GetNearestFontInPixelsL(RHeap* aHeap,
          COpenFontSessionCacheList* aSessionCacheList,
          const TOpenFontSpec& aFontSpec,
          TInt aPixelWidth,
          TInt aPixelHeight, COpenFont*& aFont,
          TOpenFontSpec& aActualFontSpec)
    {
    // Use the standard matcher.
    TInt face_index = 0;
    if (GetNearestFontHelper(aFontSpec,aPixelWidth,aPixelHeight,face_index,aActualFontSpec))
        {
        aFont = CFreeTypeFont::NewL(aHeap,aSessionCacheList,this,face_index,
                                    aActualFontSpec.Height(),
                                    aActualFontSpec.WidthFactor(),
                                    aActualFontSpec.SlantFactor());
        }
    }

To implement this function:

• Call the helper function COpenFontFile::GetNearestFontHelper(), passing it aDesiredFontSpec, aPixelWidth, aPixelHeight, aActualFontSpec, and the index of the typeface to search for a match. Note that a font file may contain serveral typefaces, which are accessed by this index.

  This helper function searches the typeface array for the closest matching font and returns ETrue if a match is found. It also retrieves the matching font's specification into aActualFontSpec.

  Note that the typeface array must be populated from the font file during construction of the COpenFontFile derived object.

• If a valid match was obtained from the helper function (i.e. if it returned ETrue) create a new COpenFont object using the specification aActualFontSpec. Pass the two arguments, aHeap and aSessionCacheList to the COpenFont constructor.

  The new object should be returned in aFont.

• If no match is found, aFont should be set to NULL.

HasUnicodeCharacterL() is called by the Font and Bitmap Server to test whether a font contains a particular Unicode character. The prototype is:

virtual TBool HasUnicodeCharacterL(TInt aFaceIndex,TInt aCode) const = 0;

Implementations must return ETrue if the character aCode, which is always a Unicode value, exists in the typeface. There are no other constraints on the way this function is implemented.

How to derive from COpenFontRasterizer

How to derive from COpenFont

How to derive from COpenFontRasterizerContext