0105: D Snippets I - Observer vs. Singleton

When it comes to building a UI, it isn’t always about the Widgets. Often, you need to find ways to handle Widgets in non-Widget-y ways just to keep things manageable.

For instance, over the last year, I’ve touched on a few design patterns—sometimes, quite sloppily—while incorporating such things as OS detection, accelerator keymaps, fonts, and groups of Widgets such as Radio- or ToggleButtons.

So, we’ll start off with a couple of more strictly-built (read: adhering more to the strict definitions) design patterns and, over the next while, we’ll go over other bits of D-specific code that will help in various ways when building a UI. So, without further ado, let’s dig in…

Singleton vs. Observer

First, what’s the difference between the two?

The Singleton assures us that we have one instance of an object. Because of the way the Singleton is structured, no matter how many times we create new pointers to it, they’ll always point to the same Singleton.

The Observer, on the other hand, allows a single object to be monitored by multiple other objects.

In both cases, we’re talking about a one-to-many relationship—one Singleton with multiple pointers… one object monitored by multiple others. The difference from a number-of-objects point of view is the direction of the one-to-many relationship.

Another difference can be found by thinking about the definition of the Observer pattern—a single object is monitored by multiple others. Why do they need to monitor it? Because the observed object is dynamic. It changes over the lifetime of the application.

The Singleton, on the other hand, remains static. That’s why we see use cases such as a print spooler, a window manager, a font list, or other OS-specific stuff. In other words, things we expect to remain static during the application’s lifetime.

So in a nutshell, a Singleton is used to encapsulate a static resource and Observers encapsulate objects that keep an eye on a dynamic resource.

The rest of this post will first present the general specs for a Singleton in D, then look at a working example.

The Singleton – a Generic Example

Note: There is no UI for this example. You’ll have to run it in a terminal.

Results of this example:
Current example terminal output
Current example terminal output (click for enlarged view)

Here’s what the DSingleton preamble looks like:

class DSingleton
	// Cache instantiation flag in thread-local bool
	static bool instantiated_;

	// Thread global
	__gshared DSingleton instance_;

These two properties work together to make the Singleton thread-safe. In essence, they allow us to work with the Singleton whether we’re working with a single- or multi-threaded application. The mechanism is a bit more complex than that, but for our purposes, this is deep as we need to dig.

Next, the constructor…

this() {}

In other words, we don’t really have a constructor. It won’t always be empty, but for this demo, we don’t need it to do anything. (In our next example, it’ll be used for setting a static variable.)

Note also that the constructor definition is in the part of the class marked private which means no outsider can call it. Why? Because the work of the constructor should only be carried out once during the life of the application and we don’t want anyone calling it by accident and screwing up our nefarious plan. And why don’t we want anyone to call it?

Besides, we don’t instantiate the DSingleton with a direct call to the constructor. We pass all requests for instantiation through a getter instead:

static DSingleton get()
				instance_ = new DSingleton();
				write("Creating the singleton and");

			instantiated_ = true;
		write("Singleton already exists, so just");

	writeln(" returning an instance.");
} // get()

All those writeln() calls are there just as proof that the system works. They can be safely axed when you’re doing a production-ready Singleton.

Wherever your code needs access to the Singleton, you start by calling get(). In the guts of the get(), there’s a bit of D-language hocus pocus going on to assure that multi-threaded applications won’t end up with a new Singleton in every thread. It checks to see if an instance of the Singleton already exists either locally (in the current thread) or globally (in any possible thread) before deciding whether or not to create one.

The getInstance Function

Don’t let the name fool you. This isn’t another getter:

DSingleton* getInstance()
	writeln("the Singleton instance at ", &instance_);

} // getInstance()

It spits out the address of the pointer to the Singleton and then returns the address of the Singleton itself so we know, even though we may have multiple pointers to the DSingleton scattered throughout our code, they all point to the same instance of the DSingleton.

The Singleton at Work

Note: As with the last example, there is no UI for this. You’ll have to run it in a terminal.

Results of this example:
Current example terminal output
Current example terminal output (click for enlarged view)

Detecting the user’s OS—something that’s definitely not expected to change while the application is running—is a quintessential example of the Singleton at work.

How we get from the generic example to this specific one comes down to this:

  • add a string property in the preamble to store the OS type,
  • add code to the constructor that detects and stores the OS type, and
  • because getInstance() is really just for testing purposes, replace it with getOS().

It’s also a good idea to rename the Singleton to reflect what we’re dealing with. (Don’t forget to edit all those places in the code where we refer to it by name.)

So, all that boils down to:

  • change the name from DSingleton to S_DetectedOS (I’m here using the leading ‘S’ as a naming convention to indicate this is a Singleton class/object),
  • edit all references to DSingleton throughout the code to reflect the name change,
  • add the property string _os_type, and
  • rewrite the constructor to set this new variable:
		_os_type = "Windows";
	else version(OSX)
		_os_type = "OSX";
	else version(Posix)
		_os_type = "Posix";

} // this()

The version() function, if you hadn’t guessed, is D’s way of finding out which OS it’s running on.

Our replacement for getInstance()—because we really don’t care about the address of the Singleton as long as it does its job—looks like this:

string getOS()
} // getOS()


Next time, we’ll start by taking a closer look at the Observer.

Until then, happy coding.

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