Add-ons: add method resourcePath() to addons.Addon ghosts

This method takes a string such as "this/is/a/relative/path" and returns
"[addon=ADDON_ID]this/is/a/relative/path", substituting the add-on
identifier for ADDON_ID.

This way, add-on authors don't even have to know the special syntax
'[addon=ADDON_ID]relative/path' used for add-on-specific resource paths,
and don't need to hardcode their add-on identifier inside each such path
either.

src/Add-ons/Addon.cxx

@@ -24,6 +24,7 @@
 #include <vector>
 
 #include <simgear/misc/sg_path.hxx>
+#include <simgear/misc/strutils.hxx>
 #include <simgear/nasal/cppbind/Ghost.hxx>
 #include <simgear/nasal/cppbind/NasalHash.hxx>
 #include <simgear/nasal/naref.h>
@@ -36,8 +37,11 @@
 #include "Addon.hxx"
 #include "AddonMetadataParser.hxx"
 #include "AddonVersion.hxx"
+#include "exceptions.hxx"
 #include "pointer_traits.hxx"
 
+namespace strutils = simgear::strutils;
+
 using std::string;
 using std::vector;
 
@@ -180,6 +184,17 @@ SGPath Addon::getBasePath() const
 void Addon::setBasePath(const SGPath& addonBasePath)
 { _basePath = addonBasePath; }
 
+std::string Addon::resourcePath(const std::string& relativePath) const
+{
+  if (strutils::starts_with(relativePath, "/")) {
+    throw errors::invalid_resource_path(
+      "addon-specific resource path '" + relativePath + "' shouldn't start "
+      "with a '/'");
+  }
+
+  return "[addon=" + getId() + "]" + relativePath;
+}
+
 std::string Addon::getMinFGVersionRequired() const
 { return _minFGVersionRequired; }
 
@@ -315,6 +330,7 @@ void Addon::setupGhost(nasal::Hash& addonsModule)
     .member("licenseUrl", &Addon::getLicenseUrl)
     .member("tags", &Addon::getTags)
     .member("basePath", &Addon::getBasePath)
+    .method("resourcePath", &Addon::resourcePath)
     .member("minFGVersionRequired", &Addon::getMinFGVersionRequired)
     .member("maxFGVersionRequired", &Addon::getMaxFGVersionRequired)
     .member("homePage", &Addon::getHomePage)

src/Add-ons/Addon.hxx

@@ -127,6 +127,11 @@ public:
   SGPath getBasePath() const;
   void setBasePath(const SGPath& addonBasePath);
 
+  // Return a resource path suitable for use with the simgear::ResourceManager.
+  // 'relativePath' is relative to the add-on base path, and should not start
+  // with a '/'.
+  std::string resourcePath(const std::string& relativePath) const;
+
   // Should be valid for use with simgear::strutils::compare_versions()
   std::string getMinFGVersionRequired() const;
   void setMinFGVersionRequired(const std::string& minFGVersionRequired);

src/Add-ons/addon_fwd.hxx

@@ -59,6 +59,7 @@ class error_loading_metadata_file;
 class duplicate_registration_attempt;
 class fg_version_too_old;
 class fg_version_too_recent;
+class invalid_resource_path;
 
 } // of namespace errors
 

src/Add-ons/exceptions.hxx

@@ -59,6 +59,9 @@ class fg_version_too_old : public error
 class fg_version_too_recent : public error
 { using error::error; };
 
+class invalid_resource_path : public error
+{ using error::error; };
+
 } // of namespace errors
 
 } // of namespace addons