Visual Basic.NET How to Program (PDF)

1076 ASP .NET and Web Services Chapter 21
Method UpdateWeatherConditions, which gathers weather data from a Web
page, is the first WebMethod that a client must call from the Web service. The service also
provides the WebMethods Cities, Descriptions and Temperatures, which
return different kinds of forecast-related information.
When UpdateWeatherConditions (line 32–93) is invoked, the method connects
to a Web site containing the traveler’s forecasts from the National Weather Service (NWS).
Line 37 creates a WebClient object, which we use because the WebClient class is
designed for interaction with a source specified by a URL. In this case, the URL for the
NWS page is http://iwin.nws.noaa.gov/iwin/us/traveler.html. Lines
43–54 call WebClient method OpenRead; the method retrieves a Stream from the
URL containing the weather information and then uses this Stream to create a Stream-
Reader object. Using a StreamReader object, the program can read the Web page’s
HTML markup line-by-line.
The section of the Web page in which we are interested starts with the String
“TAV12.” Therefore, lines 50–53 read the HTML markup one line at a time until this
String is encountered. Once the string “TAV12” is reached, the Do/Loop While structure
(lines 64–67) continues to read the page one line at a time until it finds the header line
(i.e., the line at the beginning of the forecast table). This line starts with either day-
Format, indicating day format, or nightFormat, indicating night format. Because the
line could be in either format, the structure checks for both. Line 70 reads the next line from
the page, which is the first line containing temperature information.
The While structure (lines 72–85) creates a new CityWeather object to represent
the current city. It parses the String containing the current weather data, separating the
city name, the weather condition and the temperature. The CityWeather object is added
to cityList (an ArrayList that contains a list of the cities, their descriptions and their
current temperatures); then, the next line from the page is read and is stored in inputLine
for the next iteration. This process continues until the length of the String read from the
Web page is less than or equal to 28. This signals the end of the temperature section. Line
91 adds the ArrayList cityList to the Session object so that the values are maintained
between method calls.
Method Cities (line 95–113) creates an array of Strings that can contain as many
Strings as there are elements in cityList. Lines 99–100 obtain the list of cities from
the Session object. Lines 106–110 iterate through each CityWeather object in cityList
and insert the city name into the array that is returned in line 109. Methods
Descriptions (lines 115–133) and Temperatures (lines 135–154) behave similarly,
except that they return weather descriptions and temperatures, respectively.
Figure 21.21 contains the code listing for the CityWeather class. The constructor
takes three arguments: The city’s name, the weather description and the current temperature.
The class provides the properties CityName, Temperature and Description,
so that these values can be retrieved by the Web service.
1 ' Fig. 21.21: CityWeather.vb
2 ' Class representing the weather information for one city.
3
4 Imports System
Fig. 21.21 Class that stores weather information about a city (part 1 of 2).