Improve error message for source_root and add a shortcut to define it.

lib/thor/actions.rb

@@ -20,7 +20,13 @@ module ClassMethods
       # inherited paths and the source root.
       #
       def source_paths
-        @source_paths ||= []
+        @_source_paths ||= []
+      end
+
+      # Stores and return the source root for this class
+      def source_root(path=nil)
+        @_source_root = path if path
+        @_source_root
       end
 
       # Returns the source paths in the following order:
@@ -32,7 +38,7 @@ def source_paths
       def source_paths_for_search
         paths = []
         paths += self.source_paths
-        paths << self.source_root if self.respond_to?(:source_root)
+        paths << self.source_root if self.source_root
         paths += from_superclass(:source_paths, [])
         paths
       end
@@ -126,12 +132,19 @@ def find_in_source_paths(file)
         return source_file if File.exists?(source_file)
       end
 
+      message = "Could not find #{file.inspect} in any of your source paths. "
+
+      unless self.class.source_root
+        message << "Please invoke #{self.class.name}.source_root(PATH) with the PATH containing your templates. "
+      end
+
       if source_paths.empty?
-        raise Error, "You don't have any source path defined for class #{self.class.name}. To fix this, " <<
-                     "you can define a source_root in your class."
+        message << "Currently you have no source paths."
       else
-        raise Error, "Could not find #{file.inspect} in source paths: \n#{source_paths.join("\n")}"
+        message << "Your current source paths are: \n#{source_paths.join("\n")}"
       end
+
+      raise Error, message
     end
 
     # Do something in the root or on a provided subfolder. If a relative path

spec/actions/directory_spec.rb

@@ -37,7 +37,7 @@ def exists_and_identical?(source_path, destination_path)
     it "raises an error if the source does not exist" do
       lambda {
         invoke! "unknown"
-      }.must raise_error(Thor::Error, /Could not find "unknown" in source paths/)
+      }.must raise_error(Thor::Error, /Could not find "unknown" in any of your source paths/)
     end
 
     it "copies the whole directory recursively to the default destination" do

spec/actions_spec.rb

@@ -107,7 +107,7 @@ def file
       it "raises an error if source path is empty" do
         lambda {
           A.new.find_in_source_paths("foo")
-        }.must raise_error(Thor::Error, /You don't have any source path defined for class A/)
+        }.must raise_error(Thor::Error, /Currently you have no source paths/)
       end
 
       it "finds a template inside the source path" do

spec/fixtures/group.thor

@@ -6,9 +6,7 @@ class MyCounter < Thor::Group
     from_superclass(:get_from_super, 13)
   end
 
-  def self.source_root
-    File.expand_path(File.dirname(__FILE__))
-  end
+  source_root File.expand_path(File.dirname(__FILE__))
   source_paths << File.expand_path("broken", File.dirname(__FILE__))
 
   argument :first,     :type => :numeric