adding a new topic overview about writing custom widgets; currently there's no official guide...

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@57057 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/doxygen/overviews/customwidgets.h

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+/////////////////////////////////////////////////////////////////////////////
+// Name:        customwidgets.h
+// Purpose:     topic overview
+// Author:      wxWidgets team
+// RCS-ID:      $Id$
+// Licence:     wxWindows license
+/////////////////////////////////////////////////////////////////////////////
+
+/**
+
+@page overview_customwidgets Creating a custom widget
+
+@li @ref overview_customwidgets_whenwhy
+@li @ref overview_customwidgets_how
+
+<hr>
+
+
+@section overview_customwidgets_whenwhy When and why you should write your custom widget
+
+Typically combining the existing @ref group_class_ctrl controls in wxDialogs and
+wxFrames is sufficient to fullfill any GUI design.
+Using the wxWidgets standard controls makes your GUI looks native on all ports
+and is obviously easier and faster.
+
+However there are situations where you need to show some particular kind of data
+which is not suited to any existing control.
+In these cases rather than hacking an existing control for something it has not
+been coinceived for, it's better to write a new widget.
+
+
+@section overview_customwidgets_how How to write the custom widget
+
+There are at least two very different ways to implement a new widget.
+
+The first is to build it upon wxWidgets existing classes, thus deriving it from
+wxControl or wxWindow. In this way you'll get a @b generic widget.
+This method has the advantage that writing a single
+implementation works on all ports; the disadvantage is that it the widget will
+look the same on all platforms, and thus it may not integrate well with the
+native look and feel.
+
+The second method is to build it directly upon the native toolkits of the platforms you
+want to support (e.g. GTK+, Carbon and GDI). In this way you'll get a @b native widget.
+This method in fact has the advantage of a native look and feel but requires different
+implementations and thus more work.
+
+In both cases you'll want to better explore some hot topics like:
+- @ref overview_windowsizing
+- @ref overview_eventhandling_custom to implement your custom widget's events.
+You will probably need also to gain some familiarity with the wxWidgets sources,
+since you'll need to interface with some undocumented wxWidgets internal mechanisms.
+
+
+@subsection overview_customwidgets_how_generic Writing a generic widget
+
+Generic widgets are typically derived from wxControl or wxWindow.
+They are easy to write. The typical "template" is as follows:
+
+@code
+
+enum MySpecialWidgetStyles
+{
+    SWS_LOOK_CRAZY = 1,
+    SWS_LOOK_SERIOUS = 2,
+    SWS_SHOW_BUTTON = 4,
+
+    SWS_DEFAULT_STYLE = (SWS_SHOW_BUTTON|SWS_LOOK_SERIOUS)
+};
+
+class MySpecialWidget : public wxControl
+{
+public:
+    MySpecialWidget() { Init(); }
+
+    MySpecialWidget(wxWindow *parent,
+                    wxWindowID winid,
+                    const wxString& label,
+                    const wxPoint& pos = wxDefaultPosition,
+                    const wxSize& size = wxDefaultSize,
+                    long style = SWS_DEFAULT_STYLE,
+                    const wxValidator& val = wxDefaultValidator,
+                    const wxString& name = "MySpecialWidget")
+    {
+        Init();
+
+        Create(parent, winid, label, pos, size, style, val, name);
+    }
+
+    bool Create(wxWindow *parent,
+                wxWindowID winid,
+                const wxString& label,
+                const wxPoint& pos = wxDefaultPosition,
+                const wxSize& size = wxDefaultSize,
+                long style = SWS_DEFAULT_STYLE,
+                const wxValidator& val = wxDefaultValidator,
+                const wxString& name = wxCollapsiblePaneNameStr);
+
+    // accessors...
+
+protected:
+
+    void Init() {
+        // init widget's internals...
+    }
+
+    virtual wxSize DoGetBestSize() const {
+        // we need to calculate and return the best size of the widget...
+    }
+
+    void OnPaint(wxPaintEvent&) {
+        // draw the widget on a wxDC...
+    }
+
+private:
+    DECLARE_DYNAMIC_CLASS(MySpecialWidget)
+    DECLARE_EVENT_TABLE()
+};
+@endcode
+
+
+
+@subsection overview_customwidgets_how_native Writing a native widget
+
+Writing a native widget is typically more difficult as it requires you to
+know the APIs of the platforms you want to support.
+See @ref page_port_nativedocs for links to the documentation manuals of the
+various toolkits.
+
+The organization used by wxWidgets consists in:
+- declaring the common interface of the control in a generic header, using
+  the 'Base' postfix; e.g. MySpecialWidgetBase.
+  See for example the wxWidgets' @c "wx/button.h" file.
+
+- declaring the real widget class inheriting from the Base version in
+  platform-specific headers; see for example the wxWidgets' @c "wx/gtk/button.h" file.
+
+- separing the different implementations in different source files, putting
+  all common stuff in a separate source.
+  See for example the wxWidgets' @c "src/common/btncmn.cpp", @c "src/gtk/button.cpp"
+  and @c "src/msw/button.cpp" files.
+
+*/
+