A blog about web development

Paul Sturgess

Using NSUserDefaults in RubyMotion

Apple describe NSUserDefaults as “a programmatic interface for interacting with the defaults system”.

It also provides a convenient store for persisting and retrieving user preferences for your application. You can think of it as a key value hash like structure, although it is a bit smarter than that.

Here’s a really simple implementation I’ve been using:

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class Cache

  class << self

    def set_default(key, data)
      return if read(key)
      write(key, data)
    end

    def write(key, data)
      NSUserDefaults.standardUserDefaults[storage_key(key)] = data
      NSUserDefaults.standardUserDefaults.synchronize
    end

    def read(key)
      NSUserDefaults.standardUserDefaults[storage_key(key)]
    end

    private

    def storage_key_prefix
      "#{cache_namespace}_"
    end

    def storage_key key
      "#{storage_key_prefix}#{key}"
    end

    def cache_namespace
      raise NotImplementedError
    end

  end

end
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class UserSearchSettings < Cache

  class << self

    private

    def cache_namespace
      "user_search"
    end

  end

end

I use it like this:

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UserSearchSettings.write("sort_by", "name")
=> true
NSUserDefaults.standardUserDefaults.dictionaryRepresentation
=> {"user_search_sort_by" => "name"} # note there will be a lot of other key values in the hash
UserSearchSettings.read("sort_by")
=> "name"

My intention is not to call the Cache class directly and to only use the subclass UserSearchSettings. This forces me to group settings with a prefix defined by the cache_namespace method. This will help avoid accidentally defining the same key name for different values. It also nicely groups related settings.

Each application has its’ own NSUserDefaults.standardUserDefaults so there is no need to prefix the keys with the application name.

This store will persist until the user removes your application from their device.