Store value as a serialized Json string

Property definitions

android $ SharedPreferences :: []=
	# Store `value` as a serialized Json string
	fun []=(key: String, value: nullable Serializable)
	do
		var serialized_string = new StringWriter
		var serializer = new JsonSerializer(serialized_string)
		serializer.serialize(value)

		add_string(key, serialized_string.to_s)
		commit_if_auto
	end
lib/android/shared_preferences/shared_preferences_api10.nit:377,2--386,4