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<concept id="GUID-1CF78E25-8C83-5812-AE6F-8612B3F9CDA9" xml:lang="en"><title>Deriving

from COpenFontRasterizer</title><prolog><metadata><keywords/></metadata></prolog><conbody>

<section><title>Overview</title> <p>Classes derived from <codeph>COpenFontRasterizer</codeph> are

used to recognize font files and create objects to manage them. To derive

from <codeph>COpenFontRasterizer</codeph> you need only provide an implementation

of its pure virtual function <codeph>NewFontFileL()</codeph>. The implementation

of this function is discussed in the following section. </p> <p>The derived

class <i>should</i> also define the DLL's single exported function. This function

is discussed further in the previous topic: <xref href="GUID-F477E82D-2929-5C69-BF6D-A69A61AC9EA5.dita#GUID-F477E82D-2929-5C69-BF6D-A69A61AC9EA5/GUID-ADA409CE-7326-5586-8DC4-B1E3FEBCE070">The

exported function</xref>. </p> <p>In addition, font rasterizer classes often

need to store an 'engine context'. For example, FreeType relies on a <codeph>TT_Engine</codeph> object;

this is kept in the <codeph>CFreeTypeRasterizer</codeph>, in a <codeph>CFreeTypeRasterizerContext</codeph>.

The context is derived from <codeph>COpenFontRasterizerContext</codeph>, a

utility base class that provides functions that make it easier to write the

glyph bitmap during rasterization. </p> </section>

<section><title>Implementing NewFontFileL()</title> <p>The implementation

must examine a specified file and, if it is of the correct type, attempt to

create a <codeph>COpenFontFile</codeph> derived object to open it. The function

must return NULL if it cannot recognize the file. It may also leave if there

is an error. The caller is responsible for deleting the <codeph>COpenFontFile</codeph> object. </p> <p>The

function prototype is given below: </p> <codeblock id="GUID-25C6CABB-E2C6-52E6-BEB8-36AD98AAAD0D" xml:space="preserve">virtual COpenFontFile* NewFontFileL(TInt aUid,

        const TDesC&amp; aFileName, RFs&amp; aFileSession) = 0;</codeblock> <p> <codeph>aUid</codeph> is

an ID to be used for the font file — the application framework requires that

each typeface have a UID, so one is dynamically allocated for font files that

are not Symbian-native. The ID must be passed to the <codeph>COpenFontFile</codeph> derived

class's constructor. </p> <p> <codeph>aFileName</codeph> is the full path

and filename of the file that the font rasterizer is attempting to open. This

filename must also be passed to the <codeph>COpenFontFile</codeph> derived

class's constructor. </p> <p> <codeph>aFileSession</codeph> is the file session

owned by the Font and Bitmap server. This reference should be used for any

file access. If <codeph>COpenFontFile</codeph> derived objects need to keep

files open, <codeph>aFileSession</codeph> should be stored in a place that

is accessible to them. </p> </section>

<example><p>The FreeType implementation of <codeph>NewFontFileL()</codeph> is

given below. It recognizes the file only by its extension, relying on the

construction of the font file object to perform stricter checking.</p><codeblock id="GUID-7AA7FB45-1399-5A22-892D-2B365F8F0C60" xml:space="preserve">COpenFontFile* CFreeTypeRasterizer::NewFontFileL(TInt aUid,

    const TDesC& aFileName,RFs& aFileSession)

    {

    TBool is_ttf = EFalse;

    //Tests if the filename extension is ".ttf" or ".ttc".

    if (aFileName.Length() > 4)

        {

        _LIT(KExtensionTTF,".ttf"); // Extension for truetype font file.

        _LIT(KExtensionTTC,".ttc"); // Extension for truetype collection file.

        TPtrC ext = aFileName.Right(4);

        if ((ext.CompareF(KExtensionTTF) == 0) | (ext.CompareF(KExtensionTTC)==0))

          is_ttf = ETrue;

        }

    // If the extension is not .ttf or .ttc then we can't read the file.

    if (!is_ttf)  

      return NULL; 

    //If extension is .ttf then attempt to create the font file reader.

    //return it to the caller.

    return CFreeTypeFontFile::NewL(iContext,aUid,

    aFileName,aFileSession);

    }

</codeblock></example>

</conbody><related-links>

<link href="GUID-F477E82D-2929-5C69-BF6D-A69A61AC9EA5.dita"><linktext>Creating

a Font Rasterizer Plug-in</linktext></link>

<link href="GUID-BCF6FEF0-1792-5DEA-A2D7-896E47163547.dita"><linktext>Deriving

from COpenFontFile</linktext></link>

<link href="GUID-5A89E3C4-DEE7-5823-A109-92ED0D87B58C.dita"><linktext>Deriving

from COpenFont</linktext></link>

<link href="GUID-F44D974F-AADE-5E6C-8D69-8D3EEE4BD395.dita"><linktext>Deriving

from COpenFontRasterizerContext</linktext></link>

</related-links></concept>