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CHAPTER 18 REACTIVE EXTENSIONS FOR .NET
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Figure 18 10. Sample output of WeatherRx application for zip code 32202
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Handling Errors in Rx.NET
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In the world of asynchronous programming, and especially in the world of distributed asynchronous programming, errors are a fact of life and should be expected. Rx.NET Observers provide a separate OnError event handler to deal with unforeseen errors that may arise. For instance, to make the WeatherRx application more robust, let s add an OnError handler to the weather.Subscribe call. The resulting code would look like this: weather.ObserveOn(Deployment.Current.Dispatcher).Subscribe(evt => { if (evt.EventArgs.Result.Details != null) { lblWeatherFahrenheit.Text = "Current Weather, Fahrenheit: " + evt.EventArgs.Result.Details[0].MinTemperatureF.ToString() + " - " + evt.EventArgs.Result.Details[0].MaxTemperatureF.ToString(); lblCelsius.Text = "Current Weather, Celsius: " + evt.EventArgs.Result.Details[0].MinTemperatureC.ToString() + " - " + evt.EventArgs.Result.Details[0].MaxTemperatureC.ToString();
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CHAPTER 18 REACTIVE EXTENSIONS FOR .NET
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imgWeather.Source = new BitmapImage(new Uri(evt.EventArgs.Result.Details[0].WeatherImage, UriKind.Absolute)); } }, ex => { Deployment.Current.Dispatcher.BeginInvoke(() => lblStatus.Text = ex.Message); } ); Note the somewhat cryptic (it s a lamda expression and it uses a lambda expression within its own body) use of the Deployment.Current.Dispatcher.BeginInvoke statement to get around cross-thread access issues discussed previously. In the preceding code, the OnError handler simply displays the exception text, but there is nothing stopping you from dissecting an error thoroughly and providing a possible corrective action. For instance, if the web service is not available at the address specified, you may retry your call to a different location of the web service. Rx.NET also has exception handling operators Catch, Finally, OnErrorResumeNext, and Retry, which aid in recovering from errors. You will explore some of those in the next section as we discuss some potential ways of handling intermittently available data connections on the phones.
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Handling Data Connection Issues with Rx.NET
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On a phone, slow or lost data connections are a fact of everyday life. Ideally, phone applications should detect such connections and provide a recovery mechanism to deal with them. Two potential ways to deal with slow or lost connectivity on the phone are: (1) let the user decide whether the application should retry what it was doing before the connection timed out or lost, and (2) provide an automated retry mechanism. Rx.NET can aid in both of those scenarios. Furthermore, Rx.NET includes a special Timeout operation that will generate a timeout error if it does not receive data, such as a web service callback, from its Observable within a user-specified interval. Let s take a look at the Timeout operation in action. Let s change the WireUpWeatherEvents function to time out if it does not get any data for two seconds:
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Replace the WireUpEvents() function of the WeatherRx application with the following code:
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private void WireUpWeatherEvents() { var weather = GetWeatherSubject(); weather.ObserveOn(Deployment.Current.Dispatcher) .Timeout(TimeSpan.FromSeconds(2)) .Subscribe(evt => { if (evt.EventArgs.Result.Details != null) { lblWeatherFahrenheit.Text = "Current Weather, Fahrenheit: " + evt.EventArgs.Result.Details[0].MinTemperatureF.ToString() + " - " + evt.EventArgs.Result.Details[0].MaxTemperatureF.ToString(); lblCelsius.Text = "Current Weather, Celsius: " + evt.EventArgs.Result.Details[0].MinTemperatureC.ToString() + " - " + evt.EventArgs.Result.Details[0].MaxTemperatureC.ToString(); imgWeather.Source = new BitmapImage(new Uri(evt.EventArgs.Result.Details[0].WeatherImage, UriKind.Absolute)); } },
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CHAPTER 18 REACTIVE EXTENSIONS FOR .NET
ex => { Deployment.Current.Dispatcher.BeginInvoke(() => lblStatus.Text = ex.Message); } ); } Now run the application and notice how after two seconds, it immediately times out and displays the timeout exception text on the emulator. What happened You did not even get a chance to specify the zip code to show the weather for. Our code needs a little refactoring, or changing around. In the code so far, you subscribed to the web service s events immediately on application launch, and since you did not get any data two seconds after the launch of the application, that subscription timed out. The change that you need to make is to subscribe to the web service s events right before you invoke that web service, yet you have to be careful to create this subscription just once. 2. Remove the call to WireUpWeatherEvents from the MainPage constructor and place it within the WireUpKeyEvents function to make that function look like the following:
private void WireUpKeyEvents() { var keys = Observable.FromEvent<KeyEventArgs>(txtZipCode, "KeyUp").Throttle(TimeSpan.FromSeconds(1)).DistinctUntilChanged(); keys.ObserveOn(Deployment.Current.Dispatcher).Subscribe(evt => { if (txtZipCode.Text.Length >= 5) { WireUpWeatherEvents(); weatherClient.GetWeatherByZipCodeAsync(txtZipCode.Text); } }); } Now the timeout feature should work properly. Notice, however, that it will most likely take slightly more than two seconds to return a valid response from the Weather service. Rx.NET also provides a Retry method that optionally takes a parameter for the number of times to retry to re-subscribe to the Observable collection. If you don t specify that parameter, Rx.NET will try to re-subscribe to the Observable collection indefinitely. One way to deal with an absent or slow connection is to retry the subscription two or three times, and then, if unsuccessful, give the user the option to either retry once more or cancel. You will see how to give the user that option in the next section.
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