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<h1><a href="http://freetype.org/index.html">FreeType</a>
Design / V</h1>
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<h2>V. Interfaces and Services</h2>
<p>We shall now go into detail about interfaces and services
in FreeType.</p>
<p><em>Interfaces</em> in FreeType are analogous to those
found in object-oriented programming. They can be thought
of as internal public APIs, and are essentially tables of
function pointers.</p>
<p>Interfaces only describe the form and functionality, but
the actual function body may be implemented elsewhere.
The module that is implementing the interface will then
pass the required function pointers to the table. This
gives modularity and easy extendability.</p>
<p>There are two main kinds of interfaces: <em>module</em>
interfaces, and <em>services</em>.</p>
<p>Module interfaces are defined for each module. For
example, every font driver provides its own set of
procedures for use in the base layer, which are registered
in an <tt>FT_Driver</tt>. This way, very different font
drivers can be used in the same way in the base layer.</p>
<p>Services are cross-module interfaces. These provide
functionality needed in several font drivers. </p>
<p>Services are created when code from one module needs to
be used in another. Rather than include files from
another module, a service is created instead. Now, the
other module just needs to include the header defining the
interface.</p>
<p>Helper modules are an extreme example of this; all their
public functionality is made for use in other font drivers
and hence are in a single service.</p>
<h3>In-depth guide: Creating a service</h3>
<p>This section will teach you how to write your own
service.</p>
<ol>
<li>
<p>Make the service interface header.</p>
<p>We will be calling our service demo for demonstration
purposes. First, create the header file, which goes
in <tt>include/freetype/internal/services</tt>, and
add the boilerplate.</p>
<pre>
#include FT_INTERNAL_SERVICE_H
FT_BEGIN_HEADER
#define FT_SERVICE_ID_DEMO "demo"
/* ... */
FT_END_HEADER</pre>
<p>This line in particular is required to register the
new service later on.</p>
<pre>
#define <em>service-id service-tag</em></pre>
</li>
<li>
<p>We will have identified some functions that are
needed in another module. Extract the function
signatures of these and place them in the header.</p>
<pre>
[typedef <em>return-type</em>
(*<em>type-name</em>)(<em>function-signature</em>);]+</pre>
<p>Example:</p>
<pre>
typedef FT_Error
(*SampleDoSomethingFunc)( int foo );
typedef void
(*SampleDoAnotherFunc)( int foo,
float bar );</pre>
</li>
<li>
<p>Define the service interface.</p>
<p>Use the <tt>FT_DEFINE_SERVICE</tt> macro to do
this.</p>
<pre>
FT_DEFINE_SERVICE( <em>service-name</em> )
{
[<em>type-name interface-entry</em>;]+
}</pre>
<p>Example:</p>
<pre>
FT_DEFINE_SERVICE( Demo )
{
SampleDoSomethingFunc doSomething;
SampleDoAnotherFunc doAnother;
};</pre>
<p>Here is the definition of the above macro
(in <tt>ftserv.h</tt>).</p>
<pre>
#define FT_DEFINE_SERVICE( name ) \
typedef struct FT_Service_ ## name ## Rec_ \
FT_Service_ ## name ## Rec ; \
typedef struct FT_Service_ ## name ## Rec_ \
const * FT_Service_ ## name ; \
struct FT_Service_ ## name ## Rec_</pre>
<p>This defines a new struct
called <tt>FT_Service_DemoRec</tt>, along with a
handle type for the
struct, <tt>FT_Service_Demo</tt>.</p>
</li>
<li>
<p>Register (and/or implement) the functions for the
newly created interface.</p>
<p>In the module implementing the interface, add a
definition like the following</p>
<pre>
static const <em>service-record table-name</em> =
{
<em>function-name</em>
[,<em>function-name2</em> [,...]]
}</pre>
<p>which corresponds to the interface defined in
step 3. Example:</p>
<pre>
static const FT_Service_DemoRec demo_service_rec =
{
function1,
function2
};</pre>
<p>This initializes the function pointers table. In
this example, <tt>function1</tt> has the function
signature of <tt>SampleDoSomethingFunc</tt> and
implements the <tt>doSomething</tt> functionality, and
so on.</p>
</li>
<li>
<p>Register the new service.</p>
<p>Next, add this code to define the service list in
this module, or append the new service to an existing
list.</p>
<pre>
static const FT_ServiceDescRec <em>service-list-name</em>[] =
{
{ <em>service-id</em>, &<em>table-name</em> },
[{ <em>service-id2</em>, &<em>table-name2</em> }, [...]]
{ NULL, NULL }
}</pre>
<p>Example:</p>
<pre>
static const FT_ServiceDescRec demo_services[] =
{
{ FT_SERVICE_ID_DEMO, &demo_service_rec },
{ NULL, NULL }
};</pre>
<p><em>service-id</em> is what we <tt>#define</tt>'d in
the service header file in step 1. Note that
the <tt>{NULL, NULL}</tt> sentinel value at the
end is required.</p>
<p>Now we need a way for other modules to find this
service. First, we need to implement
the <tt>get_interface</tt> function in
<tt>FT_Module_Class</tt>. Here is a minimal example
that does not do any validation.</p>
<pre>
FT_CALLBACK_DEF( FT_Module_Interface )
<em>get-interface-name</em>( FT_Module module,
const char* module_interface )
{
return ft_service_list_lookup( <em>service-list-name</em>,
module_interface );
}</pre>
<p>Then, pass it into the <tt>FT_DEFINE_MODULE</tt>
macro for this module.</p>
<pre>
(FT_Module_Requester) <em>get-interface-name</em></pre>
<p>The last step is optional but recommended, which is
to register the new service header
in <tt>ftserv.h</tt>.
<pre>
#define FT_SERVICE_DEMO_H <freetype/internal/services/svdemo.h></pre>
</li>
<li>
<p>Use the new service.</p>
<p>Now, in the file that wants to use the service, add
the following code to get the service.</p>
<pre>
<em>service-record-handle</em> service;
FT_FACE_FIND_GLOBAL_SERVICE( face,
service,
<em>service-id-tail</em> );</pre>
<p><tt>face</tt> should be of type <tt>FT_Face</tt>,
which is usually the current face instance being used
in the driver, and FreeType tries to find the service
in the driver of this face first, before searching all
other modules.</p>
<p><em>service-id-tail</em> is the part
of <em>service-id</em>
following <tt>FT_SERVICE_ID_</tt>.
<p>Now to call some function in the service.</p>
<pre>
service-><em>interface-entry</em>( <em>params</em> );</pre>
<p>Example:</p>
<pre>
FT_Service_Demo demo;
FT_Error error;
FT_FACE_FIND_GLOBAL_SERVICE( face, demo, DEMO );
error = demo->doSomething(0);
</pre>
</li>
</ol>
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<p>Last update: 13-Sep-2017</p>
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