Add a function to find viewscreens (or parents) of a given type

This makes it possible for some plugins to detect the game state more
reliably - for example, prospector would previously fail when
embarking if the viewscreen_choose_start_sitest instance had any
children.

NEWS

@@ -114,6 +114,7 @@ DFHack Future
         plug:
           - lists all plugins
           - shows state and number of commands in plugins
+        prospect: works from within command-prompt
         quicksave: Restricted to fortress mode
         remotefortressreader: Exposes more information
         search:

library/include/modules/Gui.h

@@ -156,6 +156,15 @@ namespace DFHack
         /// Get the current top-level view-screen
         DFHACK_EXPORT df::viewscreen *getCurViewscreen(bool skip_dismissed = false);
 
+        DFHACK_EXPORT df::viewscreen *getViewscreenByIdentity(virtual_identity id, int n = 1);
+
+        /// Get the top-most viewscreen of the given type from the top `n` viewscreens (or all viewscreens if n < 1)
+        /// returns NULL if none match
+        template <typename T>
+        inline T *getViewscreenByType (int n = 1) {
+            return strict_virtual_cast<T>(getViewscreenByIdentity(T::_identity, n));
+        }
+
         inline std::string getCurFocus(bool skip_dismissed = false) {
             return getFocusString(getCurViewscreen(skip_dismissed));
         }

library/modules/Gui.cpp

@@ -1326,6 +1326,21 @@ df::viewscreen *Gui::getCurViewscreen(bool skip_dismissed)
     return ws;
 }
 
+df::viewscreen *Gui::getViewscreenByIdentity (virtual_identity id, int n)
+{
+    bool limit = (n > 0);
+    df::viewscreen *screen = Gui::getCurViewscreen();
+    while (screen)
+    {
+        if (limit && n-- <= 0)
+            break;
+        if (id.is_instance(screen))
+            return screen;
+        screen = screen->parent;
+    }
+    return NULL;
+}
+
 df::coord Gui::getViewportPos()
 {
     if (!df::global::window_x || !df::global::window_y || !df::global::window_z)

plugins/prospector.cpp

@@ -18,6 +18,7 @@ using namespace std;
 #include "Console.h"
 #include "Export.h"
 #include "PluginManager.h"
+#include "modules/Gui.h"
 #include "modules/MapCache.h"
 
 #include "MiscUtils.h"
@@ -575,7 +576,8 @@ command_result prospector (color_ostream &con, vector <string> & parameters)
     CoreSuspender suspend;
 
     // Embark screen active: estimate using world geology data
-    if (VIRTUAL_CAST_VAR(screen, df::viewscreen_choose_start_sitest, Core::getTopViewscreen()))
+    auto screen = Gui::getViewscreenByType<df::viewscreen_choose_start_sitest>(0);
+    if (screen)
         return embark_prospector(con, screen, showHidden, showValue);
 
     if (!Maps::IsValid())