Allow manifest to be lazily set

lib/rake/sprocketstask.rb

@@ -47,7 +47,11 @@ def index
     # Will be created by default if an environment and output
     # directory are given
     def manifest
-      @manifest ||= Sprockets::Manifest.new(index, output)
+      if !@manifest.is_a?(Sprockets::Manifest) && @manifest.respond_to?(:call)
+        @manifest = @manifest.call
+      else
+        @manifest
+      end
     end
     attr_writer :manifest
 
@@ -93,6 +97,7 @@ def log_level=(level)
     def initialize(name = :assets)
       @name         = name
       @environment  = lambda { Sprockets::Environment.new(Dir.pwd) }
+      @manifest     = lambda { Sprockets::Manifest.new(index, output) }
       @logger       = Logger.new($stderr)
       @logger.level = Logger::INFO
       @keep         = 2

test/test_rake_task.rb

@@ -69,4 +69,21 @@ def teardown
     assert Dir["#{@dir}/manifest-*.json"].first
     assert File.exist?("#{@dir}/#{digest_path}")
   end
+
+  test "lazy custom manifest" do
+    Rake::SprocketsTask.new do |t|
+      t.environment = nil
+      t.manifest    = lambda { @manifest }
+      t.assets      = ['application.js']
+      t.log_level   = :fatal
+    end
+
+    digest_path = @env['application.js'].digest_path
+    assert !File.exist?("#{@dir}/#{digest_path}")
+
+    @rake[:assets].invoke
+
+    assert Dir["#{@dir}/manifest-*.json"].first
+    assert File.exist?("#{@dir}/#{digest_path}")
+  end
 end