Design Patterns
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The document discusses the lazy loading design pattern, which defers the initialization of objects until they are needed to improve efficiency. It outlines various implementations like lazy initialization, virtual proxy, ghost, and value holder, explaining their functionalities and scenarios of use. Additionally, it compares the lazy loading pattern with other design patterns such as adapter, proxy, bridge, factory, and builder patterns.
Design Patterns
- 3. DEFINITION "Lazy loading is a design pattern commonly used in computer programming to defer initialization of an object until the point at which it is needed. It can contribute to efficiency in the program's operation if properly and appropriately used. The opposite of lazy loading is eager loading.“ -- Wikipedia 3
- 4. WHAT DOES THAT MEAN? You want to be as lazy as possible, and hope that someone else comes along and does all the work for you. 4
- 5. WAIT WHAT? Lazy loading really means waiting to get data or process an object until you need it; not before. If you get data before and it's never used, the time it took was wasted. Also, getting all your data up front can make the user think the application is slow or unresponsive at first. Deferring certain data/processing to lazy load lets your application focus on more immediate/important code. 5
- 7. IMPLEMENTATIONS There are four common implementations: • Lazy initialization • Virtual proxy • Ghost • Value holder 7
- 8. LAZY INITIALIZATION Constructors are used to construct the member variables of a class. As soon as the instance of the class is created the member variables of the class are initialized. This process seems fine if we have member variables which don't take huge memory. But suppose if we have a memory consuming resource and if we are initializing it as member variable, it can be prove to be memory hungry process. To prevent from initializing the member variable as soon as the instance of class is constructed we can define member variable as property which can be initialized on demand(Lazy initialization). Let's check it through code snippet. 8
- 10. LAZY INITIALIZATION Is the code at the previous slide is thread safe? Definitely not. What if two threads enter if block at the same time, we will end up getting different instance of Hungry for the ResourceConsumer class which is not a good thing. To make it thread safe we need to have a lock outside if block as shown in the following code snippet. 10
- 12. LAZY INITIALIZATION Lazy<T> Starting framework 4.0, Lazy<T> class can be used to help with lazy initialization. If instantiated with an argument of true, it implements a thread- safe initialization pattern just described. To use the Lazy<T>, we need to instantiate the class with the delegate that tells it how to instantiate a new value, and the argument true. We need to access its value via the Value property 12
- 14. VIRTUAL PROXY A virtual proxy object shares an interface with the "real" object. The first time a property of the virtual proxy is accessed, the real object gets initialized. From that point on, accessing properties of the virtual object returns the corresponding property of the real object. This basically combines the Lazy initialization and Proxy patterns 14
- 16. VIRTUAL PROXY 16
- 17. VIRTUAL PROXY 17
- 18. GHOST The object is partially loaded, usually only with its identifier. The rest of the data is loaded the first time a property is accessed. 18
- 20. GHOST 20
- 21. VALUE HOLDER A value holder is a generic object that handles the lazy loading behavior and appears in place of the object’s data fields. When the user needs to access it, they simply ask the value holder for its value by calling the GetValue method. At that time (and only then), the value gets loaded from a database or from a service.(this is not always needed). 21
- 22. VALUE HOLDER 22
- 23. DIFFERENCE BETWEEN ADAPTER PATTERN, PROXY PATTERN AND BRIDGE PATTERN 23
- 24. ADAPTER PATTERN • The primary purpose of the adapter pattern is to change the interface of class/library A to the expectations of client B. • The typical implementation is a wrapper class or set of classes. • The purpose is not to facilitate future interface changes, but current interface incompatibilities. 24
- 25. STRUCTURE 25
- 26. PROXY PATTERN • The purpose of the proxy pattern is to create a stand-in for a real resource. There are common situations in which the Proxy pattern is applicable: • A virtual proxy is a placeholder for "expensive to create" objects. The real object is only created when a client first requests/accesses the object. • A remote proxy provides a local representative for an object that resides in a different address space. • A protective proxy controls access to a sensitive master object. 26
- 27. STRUCTURE 27
- 28. KEY DIFFERENCES • Adapter provides a different interface to its subject. Proxy provides the same interface. • Adapter is meant to change the interface of an existing object. 28
- 29. BRIDGE PATTERN • The Bridge pattern is something you implement up front - if you know you have two orthogonal hierarchies, it provides a way to decouple the interface and the implementation in such a way that you don't get an insane number of classes. 29
- 30. BRIDGE PATTERN • Let's say you have: 30
- 31. STRUCTURE 31
- 32. KEY DIFFERENCES • Adapter makes things work after they're designed; Bridge makes them work before they are. • Bridge is designed up-front to let the abstraction and the implementation vary independently. Adapter is retrofitted to make unrelated classes work together. 32
- 33. DIFFERENCE BETWEEN FACTORY PATTERN AND BUILDER PATTERN 33
- 34. FACTORY PATTERN • The factory is in charge of creating various subtypes of an object depending on the needs. • The user of a factory method doesn't need to know the exact subtype of that object. An example of a factory method createCar might return a Ford or a Honda typed object. 34
- 35. STRUCTURE 35
- 36. BUILDER PATTERN • In the Builder pattern, different subtypes are also created by a builder method, but the composition of the objects might differ within the same subclass. • To continue the car example you might have a createCar builder method which creates a Honda typed object with a 4 cylinder engine, or a Honda typed object with 6 cylinders. The builder pattern allows for this finer granularity. 36
- 37. STRUCTURE 37