Chapter 24 Accessibility 1247 The tag (line 7) encapsulates other CallXML tags. Usually, sets of CallXML tags that perform similar tasks are enclosed within …. The block element in this example encapsulates the , , and tags. A block element also can be nested in other block elements. Lines 20 23 contain some attributes of the tag. The getDigits element obtains the user s touch-tone response and stores it in the variable declared by the var attribute (i.e., ISBN). The maxDigits attribute (line 21) indicates the maximum number of digits that the application can accept. This application accepts only one character. If no maximum is stated, then the application uses the default value, nolimit. The termDigitsattribute (line 22) contains the list of characters that terminate user input. When a user inputs a character from this list, the application is notified that it has received the last acceptable input; any character entered after this point is invalid. These characters do not terminate the call; they simply notify the application to proceed to the next instruction, because the necessary input has been received. In our example, the values for termDigitsare 1, 2, 3 and 4. The default value for termDigits is the null value (”"). The maxTime attribute (line 23) indicates the maximum amount of time that the application will wait for a user response. If the user fails to enter input within the given time frame, then the CallXML application invokes the event handler onMaxSilence. The default value for this attribute is 30 seconds. The onMaxSilence element (lines 27 37) is an event handler that is invoked when attribute maxTime(or maxSilence) expires. The event handler notifies the application of the appropriate action to perform when a user fails to respond. In this case, the application asks the user to enter a value, because the maxTime has expired. After receiving input, getDigits (line 32) stores the entered value in the ISBN variable. The onTermDigit element (lines 39 68) is an event handler that notifies the application of the appropriate action to perform when a user selects one of the termDigits characters. At least one tag must be associated with (i.e., must appear after) the getDigits element, even if the default value (”") is used. We provide four actions that the application can perform in response to the specific termDigits value entered by the user. For example, if the user enters 1, the application reads the ISBN value for the Internet and World Wide Web How to Program: Second Edition textbook. Line 72 contains the event handler, which terminates the telephone call when the user hangs up the telephone. Our event handler is an empty tag (i.e., no action is performed when this tag is invoked). The logging feature (Fig. 24.20) displays the conversation between the application and the user. As in the previous example, the first row specifies the URL of the application and the global variables of the session. The subsequent rows display the conversation : The application asks the caller which ISBN value to read; the caller enters 1(Internet and World Wide Web How to Program: Second Edition), and the application reads the corresponding ISBN. The end of session message states that the application has terminated. We provide brief descriptions of various logic and action CallXML elements in Fig. 24.21. Logic elements assign values to, and clear values from, the session variables; action elements perform specified tasks, such as answering and terminating a telephone call during the current session. A complete list of CallXML elements is available at: www.oasis-open.org/cover/callxmlv2.html
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