Template Method Pattern

Template Method is a widely used design pattern and it provides great flexibility in our designs. This is one of my favorite patterns.

Challenge

You are creating machines for creating Pizza and Burger. When we look closely we can see all the machines have some operations in common and in the same order.

Start
Produce
Stop

The Start operation consists of turning on the machines, do the system check for any troubles and turning on the indicators.

The Produce operation does the respective production of item.

The Stop operation shutdowns the internal workings of the machine, turns off the indicators and power offs the machine.

The challenge is to create more machines like Cheese Burger, Pan Pizza with less custom implementations. We need to take care that the duplicate codes are avoided in our design.

Definition

"Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure."

Implementation

We can see that all the operation appears in the same sequence. Often the Start and Stop operation are common to both Pizza and Burger machines.

So we can define a skeleton of algorithm (or the order of invocation) like below:

Start
Produce
Stop

We can see all the machines have this order of execution. Only the Produce operation will be different.

So creating an Execute() method to incorporate this would look like:

public class Machine
{
public void Execute()
{
Start();
Produce();
Stop();
}
}

By calling the Execute() method we can invoke all the 3 methods in the right order. Keeping the Execute() method in the base class as public, we can create Produce() method as virtual so that it can be overridden. For more customization we are making all the 3 methods as virtual.

Note

For virtual methods, a default implementation will be provided in the base class. The derived class may/may not override it.

For abstract methods, we need to override them in the derived class. There won't be any default implementation in the base class.

The modified Machine class will look like below:

public class Machine
{
public void Execute()
{
Start();
Produce();
Stop();
}

protected virtual void Start()
{
Trace.WriteLine("Machine.Starting..");
}

protected virtual void Produce()
{
Trace.WriteLine("Machine.Producing..");
}

protected virtual void Stop()
{
Trace.WriteLine("Machine.Stopping..");
}
}

Please note the class uses virtual keyword so that the derived classes can override.

The derived class PizzaMachine will look like:

public class PizzaMachine : Machine
{
protected override void Produce()
{
Trace.WriteLine("PizzaMachine.Producing..");
}
}

The derived class BurgerMachine will look like:

public class BurgerMachine : Machine
{
protected override void Produce()
{
Trace.WriteLine("BurgerMachine.Producing..");
}
}

Please note that both of them derives from Machine class and use the keyword override to custom implement the Produce() method.

Execution

Now we can execute the PizzaMachine using the Execute() method of base class.

new PizzaMachine().Execute();

Let us examine how the order of execution works.

Machine.Execute() is invoked
Machine.Start() is invoked
PizzaMachine.Produce() is invoked
Machine.Stop() is invoked

Please note the step 3 above, in which the execution shifts from Machine class to PizzaMachine class. This is the power of Template Method pattern. Here the order of execution is maintained and a custom production is done.

Note

The underlying detail of how the above code works goes to Virtual Method Table (VMT) where the base class method address is kept in a table. The derived classes will be having a new address for the derived method. This will be replacing the original base class method address.

The control flow is depicted in the image below.

Test Application

A test application is created with a GUI using WinForms.

Summary

In this article we have seen the advantage of Template Method design pattern and the problem it tries to address. The source code contains the classes and application explained.