Universals
Everyone knows that global variables are bad. Sometimes though there are values that need to visible to all aspects of your code. Universals is a library that provides a class that allows for the aggregation of application or library wide data values. The Universe class becomes a centralized location for the storage or retrieval of such values. This avoids the need to use global variables or to pass such values down through the stack to the places they are actually used while avoiding the namespace pollution of global variables.
Installing The Library
The Universals library is provided as a Ruby gem and thus can be installed with a line such as…
$> gem install universals
Using The Library
The library contains a single class called Universe. This class acts very much like a Hash (with a few restrictions). Universe has mixed in Singleton and so there should only ever be one instance of the class that can be accessed using code such as the following…
Universals::Universe.instance
The value returned from this call can be treated like a Hash in that values can be set under keys using the array assignment operator like so…
Universals::Universe.instance["my_value"] = "Blah-de-blah"
Note that key names must be a String object. In addition, as the class makes the stored values available as properties on the Universe instance (see later for more details), the key must conform to many of the rules for method names. It can only start with a letter (upper or lower case) or an underscore and the remainder of the name must be made up of letters, underscores or numbers. The key may end with either ‘!’ or ‘?’. Names ending with ‘=’ or containing ‘[’ or ‘]’ are explicitly excluded as they would be confusing. If you try to set a value on a Universe instance that does not conform to these rules then you will receive an exception for your trouble.
As was mention previously, the Universe class makes values stored within it available as properties on the object itself. So, the value set in the previous example could be accessed using code such as…
Universals::Universe.instance.my_value
Likewise, values can be set as if they were properties on the object. So, the following are valid assignments…
Universals::Universe.instance.my_value = 12345
Universals::Universe.instance.other_value = "Something else."
As the Universe class is a singleton, any values assigned into it can be retrieved anywhere. Note, that no effort has been made to make the Universe class thread safe. It is recommended that you assign all of the values needed to the object in a single place and thereafter treat the object as read only if you want to use it in multi-threaded code.
Deferred Evaluation
If you want to defer the evaluation of a value you can, instead of directly storing the value under it’s key, store a Proc that evaluates to the correct value under the key. The next time this key is explicitly fetched from the Universe instance the Proc will be called (with no parameters) and the value it returns will be set under the key instead. Using this technique you can defer the creation of items until they are actually used, allowing them to be created at more appropriate points in the code and avoiding the possible overhead associated with the values creation.