c# barcode code 39 Client xIFlyweight FlyweightFactory Flyweight intrinsicState extrinsicState unSharedState in Visual C#.NET

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Our implementation of the Flyweight pattern makes use of two interesting features from C# 1.0 and three from C# 3.0. They are: Structs Indexers Implicit typing for local variables and arrays Object and collection initializers Anonymous types
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In addition, it uses generic collections from C# 2.0, as discussed in the section on the Composite pattern. I will introduce these features in the text that follows. This pattern encompasses three types: IFlyweight, Flyweight, and FlyweightFactory. As in the Composite pattern, we shall put them in a namespace and use the namespace as the theory example for the pattern.
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Because the types are in an interface, they and their constructors must be declared as public.
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We ll start with the interface:
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public interface IFlyweight { void Load (string filename); void Display (PaintEventArgs e, int row, int col); }
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The Load method will load into memory a thumbnail version of a given image using its filename. The Display method will display that thumbnail. Later, we will need to add further Display options for displaying the full version of the image (its extrinsic state). Now, let s consider the first of our four features: structs.
C# Feature Structs
C# has two typing constructs that define attributes and operations: the well-known class and the lesser-known struct. Structs are similar to classes in that they represent types that can contain data members and function members. However, a variable of a struct type directly contains the data of the struct, whereas a variable of a class type contains a reference to the data. Instances of both class and structs are objects. However, Structs have value semantics that is, the value is passed on assignment, rather than the reference, as is the case with objects instantiated from classes. Structs are lightweight and are implemented without the overhead. They are particularly useful for small data structures. The restriction on structs is that they cannot inherit from other types; they can, however, implement interfaces. cf. C# Language Specification Version 3.0, September 2007, Section 11
A Flyweight is an ideal candidate for a struct; Flyweights are small, and they do not inherit from anything (although the type does implement an interface). Here is our Flyweight struct:
public struct Flyweight : IFlyweight { // Intrinsic state Image pThumbnail;
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3: Structural Patterns: Composite and Flyweight
public void Load (string filename) { pThumbnail = new Bitmap("images/"+filename). GetThumbnailImage(100, 100, null, new IntPtr( )); } public void Display(PaintEventArgs e, int row, int col) { e.Graphics.DrawImage(pThumbnail,col*100+10, row*130+40, pThumbnail.Width,pThumbnail.Height); } }
As desired, Load uses the image at the given filename, but it doesn t retain a copy of it in its intrinsicState; it keeps only the thumbnail. The computations done to position the thumbnails across the screen are shown in Figure 3-3. They could obviously be improved using constants. Notice, however, that the row and col information is derived from the unSharedState of the group, which is kept by the Client. Next, let s consider the factory. We ll look further at factories in 6, but for now, I ll just say that in their simplest form, they generate objects according to specific conditions. Here, we are interested in checking whether an object exists already before we add it to a collection. Before we dive into the code, let s take a quick look at indexers. Now, we can give the FlyweightFactory class:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 public class FlyweightFactory { // Keeps an indexed list of IFlyweight objects in existence Dictionary <string,IFlyweight> flyweights = new Dictionary <string,IFlyweight> ( ); public FlyweightFactory ( ) { flyweights.Clear( ); } public IFlyweight this[string index] { get { if (!flyweights.ContainsKey(index)) flyweights[index]=new Flyweight( ); return flyweights[index]; } } }
In lines 3 4, a Dictionary is declared, mapping strings to flyweights. On lines 13 14, the Dictionary is accessed using the indexer that is part of the Dictionary definition. Line 13 is an example of the use of the set accessor, and line 14 calls a get accessor. This is all part of the member declared in lines 10 16, where we define our own indexer for the FlyweightFactory class. Line 10 specifies the return type and the key type, and lines 12 14 give the body of the get accessor. It starts by checking whether there is already a Flyweight with this key. If not, it allocates one. In either case, it returns the element in the dictionary with the correct index.
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