Interpreter Pattern

In this article I would like to demonstrate the usage of Interpreter pattern. This is not a widely used pattern but reveals a good way of programming. It says about creating a language along with an interpreter for the language. The language can be used to represent states, actions, loops etc. depending on the programming requirement. Here we are using the pattern to resolve a persistence problem.

Challenge

You are working on a Drawing application which allows creating lines and circles on a canvas. The classes Line and Circle do the job for you. You need to communicate the drawing with multiple users who have their own copy of the application. The current persistence of drawing using serialization of the above classes is creating large sized files and does not allow sending the message over a chat window.

How to provide a better solution for persistence with smaller size and easier communication?

Definition

“Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language”

Implementation

We can use the Interpreter pattern to approach the problem. We need to construct a language to include the commands and an interpreter to execute the actions.

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Following are the classes involved for our drawing application:

  • A Line Class
  • A Circle Class
  • An Interpreter Class

The Line and Circle class includes the drawing functionality. The Interpreter class takes care of converting the commands to invoking of appropriate line or circle classes.

The Language

The language part could be constituted by a series of sentences. The sentence could be as following:

line,10,10,10,10

Line command stating x, y, width, height

circle,10,10,50

Circle command stating x, y, radius

Please note that the arguments are separated using comma (,).

The sentence could be separated using semicolon (;) as shown below.

line,60,60,50,0;line,110,110,50,0;

Executing the Application

We can enter the series of commands separated by semicolon (;). The commands are entered in the application.

clip_image004

Clicking Execute you can see the above drawings of lines and circles.

The body of the Interpreter Execute() method is given below:

public void Execute(string commands, Graphics graphics)

{

graphics.Clear(Color.FromArgb(237, 253, 200));

int i = 0;

foreach (string rawCommand in commands.Split(‘;’))

{

string command = rawCommand.Replace(Environment.NewLine, string.Empty).Trim();

if (command.StartsWith(“line”))

{

i = 0;

int x = 0, y = 0, width = 0, height = 0;

foreach (string argument in command.Split(‘,’))

{

if (i == 1)

x = int.Parse(argument);

else if (i == 2)

y = int.Parse(argument);

else if (i == 3)

width = int.Parse(argument);

else if (i == 4)

height = int.Parse(argument);

i++;

}

new Line(graphics).Draw(x, y, width, height);

}

else if (command.StartsWith(“circle”))

{

i = 0;

int x = 0, y = 0, radius = 0;

foreach (string argument in command.Split(‘,’))

{

if (i == 1)

x = int.Parse(argument);

else if (i == 2)

y = int.Parse(argument);

else if (i == 3)

radius = int.Parse(argument);

i++;

}

new Circle(graphics).Draw(x, y, radius);

}

}

}

Following is the class diagram:

clip_image006

This concludes our application using the Interpreter Pattern. Now the actions could be converted to a language for interpreting later.

Comparison of Command Pattern and Interpreter Pattern

You can find some similarities between the Command and Interpreter patterns. The command pattern says about converting the actions into command for logging or undo purposes. Here the commands are objects but for the Interpreter pattern the commands are sentences.

The Command pattern can be used for persisting objects by serializing the command list and results in large sized files compared with Interpreter pattern.

The Interpreter pattern provides an easier approach in the runtime but comes at the cost of developing and interpreter.

Applications of Interpreter Pattern

You are creating your own Photoshop application.  The objects drawn on the layer can be converted to a series of commands.  Later the entire drawing could be represented using the command file which will be small in size compared to the entire canvas serialized.  You will get the advantage of sending the file to another person where he/she can:

· Regenerate the Drawing

· View all the Draw Actions

· Modify the Draw Action Items

· Undo the last draw actions

References

http://www.dofactory.com/Patterns/PatternInterpreter.aspx#_self1

Summary

In this article we have seen about the Interpreter pattern with a small application using it. The above application shows only a simple usage for learning purpose but in the real world scenarios the language may include loops, conditions etc. The attachment contains the source code of application we have discussed.

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