Dynamic Design Patterns

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This process could be automated based on the number of items being added to the list. MyArrayList never knows what sorting algorithm is being used, because it simply doesn't care. It isn't responsible for that any more, it just needs to know that there is a strategy or behaviour defined for sorting the data. It doesn't matter how the strategy attacks the problem in the end.

It knows how to communicate with the strategy because of the interface, which is enough. By using the strategy pattern, adding new algorithms to the family of algorithms is easy. You simply add a class that implements the interface that all algorithms in the family implement. If you look at the example above again, often you don't even want sorting, so you could make a NoSort strategy which simply does nothing at all in that case or omit setting a strategy.

If you look at the class diagram, you see simply adding a class that implements ISortBehaviour will suffice in order to extend the SortBehaviour family.

No if statements, no conditions, just a new class and a call to SetSortingAlgorithm. Here's what Wikipedia has to say on the topic:. In the strategy pattern, that means that the behaviours of a class should not be inherited, but encapsulated elsewhere. An interface is then required to enable other entities in the package to communicate with those behaviours. To prevent compatibility issues and broken packages, one should only add new modules intead of modifying existing code. Here's a simple example in TypeScript that animates two little boxes. It is based on the example from the Design Patterns: Introduction with Singletons post.

It now includes the strategy pattern for the movement of the squares. The idea is that it should not matter what movement strategy is used and a new strategy can be easily added. Even though these examples show TypeScript code, this pattern and all patterns can easily be implemented in native JavaScript. You only need a little bit of inheritance and object oriented code is required.

I once wrote a little bit on class inheritance in JavaScript , which might make this a little bit easier. Unfortunately, there isn't much type safety in JavaScript. You can still apply the patterns and make your front-end code open for extension and closed for modification! I hope this was a good read, but hoping doesn't get me far. I love feedback, so let me know if I need to change anything or adjust the format. I want this to be as educational and understandable as it can be!

I'll write about the Observer pattern next, which is a little stalker pattern! Thanks for pointing out that odd sentence, I completely missed that one. We're all for progressive enhancement, but CodePen is a bit unique in that it's all about writing and showing front end code, including JavaScript. It's required to use most of the features of CodePen.

Need to know how to enable it? CodePen probably won't work great in this browser. We generally only support the major desktop browsers like Chrome, Firefox, Safari, and Edge. Use this one at your own risk! Dynamic behavior with the Strategy pattern Introduction This is the second instalment of my series on design patterns in software. Strategy Pattern The strategy pattern is one of those patterns that help you describe behaviour, but keep it loosely coupled to the class that requires that behaviour.

Defines a family of algorithms. Encapsulates each algorithm in the family. For example, Textfield s are often subclassed within an application, giving them the ability to validate user input. Standard object-oriented design principles suggest that secondary roles be implemented via the Strategy pattern. Since frameworks are usually developed separately and implemented as shared libraries, using a strongly typed language results in Figure 1.

However, Figure 1 can cause problems over the application life cycle. NextStep's Application object, for example, acquired nine new delegate methods between versions 2. These were gradual modifications brought about by changes in how people use computers, as well as changes made to remedy oversights in the original design. In Figure 1 , each of these changes would have required a recompilation of the application and a new release. Moreover, existing copies of the application installed on users' machines would need to be updated. One solution to this problem is to implement a strict versioning system.

At best, strict versioning is a stop-gap solution. DLL into memory, defeating the point of shared libraries. Additionally, older applications won't automatically inherit new functionality or have bugs repaired -- they still use the older version of the framework. Moreover, minor updates to an application can become complex; if you want to use a feature found in the new version of the framework, you must update every object that depends on the framework including all the strategy objects.

The Objective-C approach removes the abstract base class, giving the framework object a more complex setDelegate method. The framework object defines a set of strategic methods it will call if they are defined. Then, inside the setDelegate method, run-time type information RTTI determines which methods the delegate has implemented. If the delegate doesn't implement the strategic method, the framework object doesn't call it. This is illustrated in Listing One where the new delegate object is queried about each function.

If the new delegate implements the function, the method pointer is obtained. This approach eliminates the versioning problem -- older applications work with new frameworks. For example, most applications compiled under NextStep 2. If the application needs to be updated to take advantage of a new feature in the framework, the necessary code change is minimal because only the object getting updated needs to be modified.

The essence of this idiom is the principle that the framework and application should evolve independently. It is not easy to see how to do this in a statically typed language. A typical application can display a large number of images, but rarely displays them all, or even a significant percentage of them. Because images are large, application developers frequently adopt a policy of lazy fetching, getting images from the disk only when they are about to be displayed.

In the era of sprawling programs, there are benefits to being able to do this with object code as well. None of these features were used in writing this article. When I launch Word, however, it loads the object code for these functions -- and Word subsequently takes longer to launch and consumes more resources when it is running. A better approach would be to define a core program consisting of the tasks that are performed by most users each time they execute the application. When users try to use functions outside the core program, it would load a module containing code to perform the requested task.

Dynamic run times affect how programs are designed and built

Doing this properly requires that the loaded module and already-running code be dynamically linked, so that there exists only one object namespace between them. This way, the newly loaded code will be able to automatically locate Singleton objects, tie into existing event loops, and access existing objects with a minimum of programmer effort objects can be used as arguments without explicitly passing function pointers.

Fortunately, the Objective-C run time allows this functionality. You can message the run time to find out whether or not the required classes exist; if they don't, tell the run time to load them from a file and link them into the application. From then on, the loaded classes are first-class objects exactly as if they had been part of the initial executable. The Visitor Pattern The Visitor pattern is one of the most commonly used behavioral patterns.

The idea is that an object represents a thing. It encapsulates attributes facts about the thing and methods what the thing can do. However, objects frequently acquire extra methods. In particular, an object's methods often start to include "what can be done to the thing.

This lets you expand program functionality, while keeping the basic object model intact. A typical line of action for implementing the Visitor pattern is to create objects for your elements, container objects for your objects, and objects that encapsulate common complex operations on the data see Figures 2 and 3. One thing that developers often overlook is that visitors can store cross-element states; for example, a visitor can create a report about the elements it has visited.

Design Patterns: Dynamic behavior with the Strategy pattern

This sort of operation must be external to the elements and is easily implemented as a visitor. Visitor has the potential of being a dynamic pattern that supports piecemeal code growth. As you discover operations that need to be supported, you add visitors that embody the operation and "fire them" at the appropriate data objects. But the standard implementation of the Visitor pattern contains a flaw.

Over the lifetime of a project, new visitors and elements the objects that visitors act on will be needed.

Separate hierarchies for visitors and elements should enable painless modification of the software. Consider Figures 2 and 3. Unfortunately, separate hierarchies are not easy to set up in the standard strongly typed version of Visitor. Figure 2 implies that every concrete element must know about every visitor to know whether to accept a particular visitor.

And, of course, the visitor base class must know about every type of element the visitConcreteElement methods must encompass every possible type of element. When there are many visitors and many elements, implementing and modifying the Visitor pattern becomes tedious and error prone. The overall effect is poorly understood and brittle software that has bugs. This problem is partially caused by double dispatch: Elements accept or reject visitors, then call a method in the visitor and the ultimate function called is dependent on both the element and visitor.

But this also means that elements are coupled to the visitors they accept.

Introduction

Eliminating double dispatch simplifies the implementation. If, however, double dispatch is necessary, you can still eliminate most of the compile-time dependencies in the code by having both the element's accept method and the visitor's operation use RTTI. In Listing Three , for example, the argument of the accept method is a pointer to a generic Visitor object. Inside the method, this argument is more carefully type checked at run time and certain types of visitors are accepted.

The implementation of Visit is similar. Consider how this code needs to change when seven new elements are added. Obviously, seven new element classes need to be written and some visitors might have to be modified. But we have isolated the code changes to those visitors that are directly affected. Similarly, when a new visitor is added, only those elements that accept the visitor need to be modified.

Using RTTI localizes the code changes to inside specific methods , eliminates compile-time dependencies, minimizes the number of objects that need to be changed, and enables black-box reuse of most of the visitors and elements. Object Serialization As good object-oriented designers and developers, we actively practice information hiding encapsulation. This means that at run time, a program is a huge graph of interconnected objects, each with their own private state. In addition, the connections between the objects are an important part of the application state.

Object serialization is simply the idea that each object ought to be responsible for saving and restoring its own state and the connections it has through instance variables. In other words, saving and restoring is best accomplished by traversing the object graph and invoking object-level save and restore operations. This is a prime example of a simple implementation of the Visitor pattern. It also illustrates how loosely coupled the Visitor pattern can be in Objective-C, where there are three distinct types of objects:

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