fpdfsdk/pwl is a Widget Library for AcroForms. CPWL_Wnd is the base class that widget classes extend. The derived widget classes are controllers for each widget. The hierarchy is: * CPWL_Wnd * CPWL_Button * CPWL_CheckBox * CPWL_PushButton * CPWL_RadioButton * CPWL_Caret * CPWL_EditCtrl * CPWL_Edit * CPWL_Icon * CPWL_ListBox * CPWL_CBListBox (combo box) * CPWL_ScrollBar Widgets are rendered to Appearance Streams, with the case all centralized in CPDFSDK_AppStream.