Understanding interfaces and enumerations in VS .NET

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Understanding interfaces and enumerations
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Listing 3.12 Implementing the IComparable interface
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Samples.Temperature = function(value) { this._value = value; } Samples.Temperature.prototype = { compareTo : function(comparand) { if(Samples.Temperature.isInstanceOfType(comparand)) { var thisValue = this.get_value(); var comparandValue = comparand.get_value(); if(thisValue == comparandValue) return 0;
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Values are equal Values are different
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return (thisValue > comparandValue) 1 : -1; } else { throw Error.argumentType(); } }, get_value : function() { return this._value; }, set_value : function(value) { this._value = value; }
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} Samples.Temperature.registerClass('Samples.Temperature', null, Samples.IComparable);
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Implement IComparable
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As soon as you pass the Samples.IComparable interface to the registerClass B method, all the methods defined in the interface are copied in the prototype object of the Temperature constructor. If you forget to specify the interface type, the runtime doesn t raise an error, but the interface isn t recognized as being implemented by the Temperature class. Together with interfaces, enumerations are a feature supported by many object-oriented languages. Because JavaScript doesn t support them, let s see how you can fill this gap using the Microsoft Ajax Library.
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Enumerations
Enumerations are a way to give names to numbers. The Microsoft Ajax Library lets you create enumerations to associate names with integer values. You can also specify values in hexadecimal format and create bit-field flags. Let s start with the pattern for creating a Size enumeration, shown in listing 3.13.
JavaScript for Ajax developers
Listing 3.13 A Size enumeration
Type.registerNamespace('Samples'); Samples.Size = function() { throw Error.notImplemented(); } Samples.Size.prototype = { Small: 1, Medium: 2, Large: 3 } Samples.Size.registerEnum('Samples.Size');
As you did with interfaces, you define an enumeration by starting with a function. Because it makes no sense to instantiate an enumeration, you need to avoid the use of the Samples.Size function as a constructor. To do this, it s enough to raise a client exception of type notImplemented as soon as someone attempts to call the function. The enumeration names are defined in the prototype object, and they must be integers. They can also be specified using the hexadecimal format. The name and value pairs in the prototype object are considered the names and values of the enumeration. In the example, you define three names (Small, Medium, and Large) associated with three integers (1, 2, 3). You call the registerEnum method, passing the fully qualified name of the enumeration as an argument. Let s play a bit with enumerations to illustrate some of the methods available. Creating a variable of type Samples.Size is easy:
var size = Samples.Size.Medium;
At this point, the size variable holds the value 2. To display the name associated with a particular value, you call the toString method on the enumeration type, passing one of the enumeration values:
alert(Samples.Size.toString(size));
If you try to pass an invalid value, a client exception of type ArgumentOutOfRangeException is thrown. You can also parse a string as an enumeration value by calling the parse method on the enumeration type:
var smallSize = Samples.Size.parse('Small');
Keep in mind that the string passed to the parse method is case sensitive. If the enumeration type doesn t contain a matching name, a client exception of type ArgumentException is thrown.
Understanding interfaces and enumerations
You can also define enumerations to use flags. Flags are useful when you need to combine multiple values of an enumeration. Listing 3.14 shows an example that helps you understand the use of flags mode.
Listing 3.14 A FileAccess enumeration that uses flags
Type.registerNamespace('Samples'); Samples.FileAccess = function() { throw Error.notImplemented(); } Samples.FileAccess.prototype = { Read : 1, Write : 2, Execute : 4 } Samples.FileAccess.registerEnum('Samples.FileAccess', true);
In order for flags mode to work correctly, values must be powers of 2. To enable flags, you must pass true as the second argument to the registerEnum method. You can also combine flags by using the bitwise OR operator:
var readWritePermission = Samples.FileAccess.Read | Samples.FileAccess.Write;
To remove a flag, you have to AND-NOT the combined flags with the one you want to remove, as in the following statement:
var readPermission = readWritePermission & ~Samples.FileAccess.Write;
Finally, if you call the toString method when in flags mode, you obtain a string that contains all the combined names, separated by commas. For example, the following statement displays a message box with the string Read, Write:
alert(Samples.FileAccess.toString(readWritePermission));
We ve now covered all the object-oriented constructs provided by the Microsoft Ajax Library. In the next section, you ll see how this enhanced type system can be used to perform reflection on JavaScript objects.
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