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35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 Console.WriteLine(start.HandleRequest(i)); } } /* Output Handler 5 deals up to a limit of 5000 Handler 4 deals up to a limit of 4000 Handler 3 deals up to a limit of 3000 Handler 2 deals up to a limit of 2000 Handler 1 deals up to a limit of 1000 Request for 50 handled at level 1 Request for 2000 handled at level 3 Request for 1500 handled at level 2 Request for 10000 handled BY DEFAULT at level 5 Request for 175 handled at level 1 Request for 4500 handled at level 5 */
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1000 Figure 8-3. Chain of handlers
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We ll start by exploring the implementation from the client s point of view. The client wants to call a handler with some data. For example:
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handler(2000);
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The crux of the Chain of Responsibility pattern is in lines 35 and then 20. Line 35 calls a handler, and line 20 can pass the request on to the next in line. The handler makes this decision based on conditions it can evaluate. Lines 18 and 21 show the other two outcomes: handling the request and handling it by default if the handler is the last in the chain. The Chain of Responsibility pattern has an inherent limitation in that the request can get lost if the chain is not set up properly or if no handler is appropriate for the passed-in data. For this reason, the last handler has a default action. An alternative implementation, as mentioned earlier, would be to define a ChainException that the Handler can throw and the Client can catch. This approach is shown in the following example.
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Example: Trusty Bank
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To further explore the Chain of Responsibility pattern, we ll model the action of a bank that is careful about allowing withdrawals of large amounts. Trusty Bank s business rules for handling withdrawals are:
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8: Behavioral Patterns: Chain of Responsibility and Command
Clerks can handle withdrawals of up to $1,000. Supervisors can handle withdrawals of up to $4,000. The bank manager can handle withdrawals of up to $9,000. Any amount larger than $9,000 has to be divided into several withdrawals. The bank has several clerks on duty at any one time (up to 10), and usually 3 supervisors; there is only one manager. In this example of the Chain of Responsibility pattern, we distinguish between levels in the chain and the number of object instances at each of those levels. The organizational structure we want is shown in Figure 8-4.
Manager Limit $9000
Supervisor Limit $4000
Clerk Limit $1000
We reflect this structure in two C# collections: one for the information about the limits and the number of positions, and one for the actual Handler objects used at runtime. The collections are:
static Dictionary <Levels,Structure> structure = new Dictionary <Levels, Structure> { {Levels.Manager, new Structure {Limit = 9000, Positions =1}}, {Levels.Supervisor, new Structure {Limit = 4000, Positions =3}}, {Levels.Clerk, new Structure {Limit = 1000, Positions =10}}}; static Dictionary <Levels, List<Handler>> handlersAtLevel = new Dictionary <Levels, List<Handler>> { {Levels.Manager, new List <Handler>( )}, {Levels.Supervisor, new List <Handler>( )}, {Levels.Clerk, new List <Handler>( )}};
Chain of Responsibility Pattern |
This excerpt uses two interesting C# features: initializing and enumerated types. We looked at initializing in 3, but I ll repeat the information here, more specifically for collections.
C# 3.0 Feature Initializing Collections
In initializing a collection, we match the values with the required structure. If the items in any dimension of the structure are objects, new is required, and we open another set of brackets. If values are available, they can be inserted, as in:
{Levels.Clerk, new Structure {Limit = 500, Positions =10}},
Otherwise, an object constructor will do, as in:
{Levels.Clerk, new List <Handler>( )},
If the structure is local to a method, its type can be inferred from the initialization and the var declaration is permitted, as in:
var structure = new Dictionary <Levels, Structure> {
cf. C# Language Specification Version 3.0, September 2007, Section 7.5.10.1-3
Both of the collections use an enumerated type: Levels. It is declared as:
enum Levels {Manager, Supervisor, Clerk}
Because enum constants can be associated with values, we could include the limits for the handler types with them, as in:
enum Levels (Manager = 9000, Supervisor = 4000, Clerk = 1000}
We ll revisit this idea in the upcoming Exercises section. Having set up the structure, we then create a collection of three lists called handlersAtLevel. Each list contains the people who are at the level indicated, exactly as in Figure 8-4. The loop to create the handlers (requiring less writing) is:
foreach (Levels level in Enum.GetValues(typeof(Levels))) { for (int i=0; i<structure[level].Positions; i++) { handlersAtLevel[level].Add(new Handler(i, level)); } }
Thus, for each level, we pick up from the structure chart the number of positions required and instantiate that number of handlers, passing through an identifier and the level. Notice that this way of setting up the handlers differs from the theory code in Example 8-1 because there is no direct link to another handler; the link is deduced from the level plus a random number and found in the lists of handlersAtLevel. The next step is to run the bank, accepting various requests, as in the theory example. Here s the loop:
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