feat(session): add flash messages spanning redirects

lib/prism/flash.rb

@@ -0,0 +1,50 @@
+module Prism
+  class Flash
+    SessionKey = '_flash'.freeze
+
+    def initialize(session)
+      @session = session
+      loaded = session[SessionKey]
+      @current = loaded ? Params.from(loaded) : Params.new
+      @pending = Params.new
+      session.delete(SessionKey)
+    end
+
+    def [](key)
+      @current[key]
+    end
+
+    def []=(key, value)
+      @pending[key] = value
+    end
+
+    def now
+      @current
+    end
+
+    def keep(key = nil)
+      if key
+        @pending[key] = @current[key] if @current.key?(key)
+      else
+        @current.each { |name, value| @pending[name] = value }
+      end
+    end
+
+    def discard(key = nil)
+      key ? @current.delete(key) : @current.clear
+    end
+
+    def empty?
+      @current.empty?
+    end
+
+    def to_h
+      @current.to_h
+    end
+
+    def sweep
+      @session.delete(SessionKey)
+      @session[SessionKey] = @pending.to_h unless @pending.empty?
+    end
+  end
+end