Singleton {R6P}

Singleton Pattern

Description

Ensure a class only has one instance, and provide a global point of access to it.

Details

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Singleton ensures a class only has one instance, and provide a global point of access to it.

How It Works

1. Create only one instance of the Singleton class; and

2. If an instance exists, then serve the same object again.

The main features of Singleton are:

• Ensuring that one and only one object of the class gets created;

• Providing an access point for an object that is global to the program; and

• Controlling concurrent access to resources that are shared.

When to Use It

• In situations that require exactly one instance of a class, that must be accessible to clients from a well-known access point. See the Counter example.

<div class="alert alert-danger"> **Caution:** Singletons can be a problem in multi-threaded applications, especially when they manipulate mutable data. </div> <div class="alert alert-info"> **Tip:** Singletons work well for immutable data, such as reading from some data source, since anything that can’t change isn’t going to run into thread clash problems. </div>

Methods

Public methods

• Singleton$new()

Method new()

Create or retrieve an object

Usage

Singleton$new()

See Also

Other base design patterns: NullObject(), ValueObject()

Examples

# See more examples at <https://tidylab.github.io/R6P/articles>
address <- function(x) sub('<environment: (.*)>', '\\1', capture.output(x))

# In this example we implement a `Counter` that inherits the qualities of
# Singleton
Counter <- R6::R6Class("Counter", inherit = R6P::Singleton, public = list(
    count = 0,
    add_1 = function(){self$count = self$count + 1; invisible(self)}
))

# Whenever we call the constructor on `Counter`, we always get the exact same
# instance:
counter_A <- Counter$new()
counter_B <- Counter$new()

identical(counter_A, counter_B, ignore.environment = FALSE)

# The two objects are equal and located at the same address; thus, they are
# the same object.

# When we make a change in any of the class instances, the rest of the
# instances are changed as well.

# How many times has the counter been increased?
counter_A$count

# Increase the counter by 1
counter_A$add_1()

# How many times have the counters been increased?
counter_A$count
counter_B$count