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Call a REST Web Service

This recipe shows how to call a RESTful service using HTTP, parse JSON results, and display the results on the screen.

Recipe

In this recipe, we'll create an app that reports weather information, such as temperature, humidity, and current conditions. This app issues a request to a REST service to get weather data, parses the response, and displays the results on the screen. The user keys in global latitude and longitude coordinates before pressing a Get Weather button to view information about the weather at that location.

Our example app will send latitude and longitude information to geonames.org in an HTTP request. The resulting weather information is returned in JSON format. When the HTTP response is received, our app will parse the results and display a subset of the weather information in TextBox fields in the lower half of the screen (as shown in the above screenshot). A more advanced version of this app could use GPS to determine the current latitude and longitude of the device, but that is beyond the scope of this article.

Follow these steps to create an app that issues a REST request to an HTTP server, parses the results, and displays the results on the screen.

  • First, we'll create a new Xamarin.Android app. The example screenshots in this article were taken using the Android L Developer Preview, but an earlier Android version such as KitKat (API level 19) can be used. If you are new to Android development with Xamarin, see Hello, Android to learn how to create Xamarin.Android applications.

  • Next, we'll use the Xamarin.Android Designer to create a UI (see Designer Overview if you are not familiar with the Android interface designer). Let's start by replacing the contents of Resources/layout/Main.axml with the XML example below. This code begins the GUI layout with two EditText elements. These elements allow the user to enter longitude and latitude values. Let's use the id's latText and longText for these EditBox elements:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <LinearLayout
        android:orientation="horizontal"
        android:layout_marginTop="10dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:id="@+id/latSection">
        <TextView
            android:text="Enter Latitude:"
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_width="140dp"
            android:layout_height="40dp"
            android:gravity="right"
            android:id="@+id/latLabel" />
        <EditText
            android:text="47.7"
            android:layout_width="150dp"
            android:layout_height="50dp"
            android:id="@+id/latText"
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_marginLeft="5dp" />
    </LinearLayout>
    <LinearLayout
        android:orientation="horizontal"
        android:layout_marginTop="10dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:id="@+id/longSection">
        <TextView
            android:text="Enter Longitude:"
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_width="140dp"
            android:layout_height="40dp"
            android:gravity="right"
            android:id="@+id/longLabel" />
        <EditText
            android:text="-122.5"
            android:layout_width="150dp"
            android:layout_height="50dp"
            android:id="@+id/longText"
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_marginLeft="5dp" />
    </LinearLayout>
  • Next, let's add a button that the user can press to get the weather. Let's give this button the id getWeatherButton. Append the following XML to layout in Resources/layout/Main.axml (started above):
<LinearLayout
        android:orientation="vertical"
        android:layout_marginTop="28dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:id="@+id/getSection">
        <Button
            android:id="@+id/getWeatherButton"
            android:layout_width="300dp"
            android:layout_height="wrap_content"
            android:gravity="center_horizontal"
            android:textAppearance="?android:attr/textAppearanceLarge"
            android:text="Get Weather" />
    </LinearLayout>
  • Let's add four TextView elements for reporting location name, temperature, humidity, and current conditions; let's assign the id's locationText, tempText, humidText, and condText, respectively. Append the following XML to Resources/layout/Main.axml:
<LinearLayout
        android:orientation="horizontal"
        android:layout_marginTop="30dp"
        android:layout_marginLeft="40dp"
        android:layout_marginRight="40dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/locSection">
        <TextView
            android:text="Location:"
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_width="120dp"
            android:layout_height="30dp"
            android:gravity="left"
            android:id="@+id/locLabel" />
        <TextView
            android:text=""
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_width="wrap_content"
            android:layout_height="60dp"
            android:gravity="left"
            android:id="@+id/locationText" />
    </LinearLayout>
    <LinearLayout
        android:orientation="horizontal"
        android:layout_marginTop="10dp"
        android:layout_marginLeft="40dp"
        android:layout_marginRight="40dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/tempSection">
        <TextView
            android:text="Temperature:"
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_width="120dp"
            android:layout_height="30dp"
            android:gravity="left"
            android:id="@+id/tempLabel" />
        <TextView
            android:text=""
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_width="wrap_content"
            android:layout_height="30dp"
            android:gravity="left"
            android:id="@+id/tempText" />
    </LinearLayout>
    <LinearLayout
        android:orientation="horizontal"
        android:layout_marginTop="10dp"
        android:layout_marginLeft="40dp"
        android:layout_marginRight="40dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/humidSection">
        <TextView
            android:text="Humidity:"
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_width="120dp"
            android:layout_height="30dp"
            android:gravity="left"
            android:id="@+id/humidLabel" />
        <TextView
            android:text=""
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_width="wrap_content"
            android:layout_height="30dp"
            android:gravity="left"
            android:id="@+id/humidText" />
    </LinearLayout>
    <LinearLayout
        android:orientation="horizontal"
        android:layout_marginTop="10dp"
        android:layout_marginLeft="40dp"
        android:layout_marginRight="40dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/condSection">
        <TextView
            android:text="Conditions:"
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_width="120dp"
            android:layout_height="30dp"
            android:gravity="left"
            android:id="@+id/condLabel" />
        <TextView
            android:text=""
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:layout_width="wrap_content"
            android:layout_height="60dp"
            android:gravity="left"
            android:id="@+id/condText" />
    </LinearLayout>
</LinearLayout>
  • Let's save our layout and exit the designer. Here is an example of how the UI looks after we add the above XML code sections:

  • In our MainActivity subclass, let's add the following code to the OnCreate method. This code gets the user-entered latitude and longitude values from the UI, packages these values in an HTTP request, fetches the data, parses the result, and updates the screen. We'll create the FetchWeatherAsync and ParseAndDisplay methods in subsequent steps.

// Set our view from the "main" layout resource
SetContentView(Resource.Layout.Main);

// Get the latitude/longitude EditBox and button resources:
EditText latitude = FindViewById<EditText>(Resource.Id.latText);
EditText longitude = FindViewById<EditText>(Resource.Id.longText);
Button button = FindViewById<Button>(Resource.Id.getWeatherButton);

// When the user clicks the button ...
button.Click += async (sender, e) => {

    // Get the latitude and longitude entered by the user and create a query.
    string url = "http://api.geonames.org/findNearByWeatherJSON?lat=" +
                 latitude.Text +
                 "&lng=" +
                 longitude.Text +
                 "&username=demo";

    // Fetch the weather information asynchronously, 
    // parse the results, then update the screen:
    JsonValue json = await FetchWeatherAsync (url);
    // ParseAndDisplay (json);
};
  • Before we send the HTTP request to the server, we must be prepared to wait for the HTTP response, which – depending on the latency of the network connection – could come back quickly or could take many seconds to return. During this wait, we don't want the application UI to freeze up while the UI thread waits for a response. Therefore, the HTTP request must be an asynchronous HTTP request. We use async/await to keep the UI thread responsive to user input while the app waits for the HTTP request and response to complete. For more about asynchronous programming techniques, see Writing Reponsive Applications.

    Let's add the following method after the OnCreate method. This method issues the HTTP request, waits for a response, and returns the response to the caller:

// Gets weather data from the passed URL.
private async Task<JsonValue> FetchWeatherAsync (string url)
{
    // Create an HTTP web request using the URL:
    HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create (new Uri (url));
    request.ContentType = "application/json";
    request.Method = "GET";

    // Send the request to the server and wait for the response:
    using (WebResponse response = await request.GetResponseAsync ())
    {
        // Get a stream representation of the HTTP web response:
        using (Stream stream = response.GetResponseStream ())
        {
            // Use this stream to build a JSON document object:
            JsonValue jsonDoc = await Task.Run (() => JsonObject.Load (stream));
            Console.Out.WriteLine("Response: {0}", jsonDoc.ToString ());

            // Return the JSON document:
            return jsonDoc;
        }
    }
}
  • The above example converts the HTTP response stream into a JsonValue object — this makes it easier for us to extract the weather information that we will need in later steps. Because we will be using System.Json to parse the response, we must add a reference to System.Json to our project:

    • In Xamarin Studio, right-click the project's References node, select Edit References…, click the All tab, and enter System.Json into the search field. Next, select the System.Json assembly when it appears and then click OK.

    • In Visual Studio, right-click the project's References node, select Add Reference…, and enter System.Json into the search field. Next, select the System.Json assembly when it appears and click OK.

    Note that, for clarity, this example omits exception handlers. Parsing operations can throw exceptions (if, for example, the expected field name is not found during a name/value lookup). Also, network connection errors can generate exceptions. In a production app, these code paths should be wrapped with exception handlers to handle real-life error conditions.

  • Next, we'll build and run our app, testing it to see if we can get a response to our HTTP request. When the app launches, we'll enter latitude and longitude information into its UI (or use the default coordinates) and press the Get Weather button. If the HTTP request and response succeeds, we should get a JSON-formatted HTTP response that resembles the following example (the JSON response string should be visible in the IDE debug output):

{
    "weatherObservation": {
        "ICAO": "KBFI",
        "clouds": "broken clouds",
        "cloudsCode": "BKN",
        "countryCode": "US",
        "datetime": "2014-12-05 22:53:00",
        "dewPoint": "8.3",
        "elevation": 4,
        "humidity": 79,
        "lat": 47.55,
        "lng": -122.31666666666666,
        "observation": "KBFI 052253Z 32003KT 10SM BKN070 BKN090 12/08 A2989 RMK AO2 SLP122 T01170083",
        "seaLevelPressure": 1012.2,
        "stationName": "Seattle, Seattle Boeing Field",
        "temperature": "11.7",
        "weatherCondition": "n/a",
        "windDirection": 320,
        "windSpeed": "03"
    }
}
  • Now that we have successfully sent an HTTP response that elicits a valid response, we'll parse the JsonValue object to extract the weather information that we want to display. In the example JSON response shown above, we want to extract the stationName, temperature, humidity, clouds, and weatherCondition name/value pairs, using this information to fill out the bottom portion of the weather app display. Let's add the following method after the FetchWeatherAsync method:
// Parse the weather data, then write temperature, humidity, 
// conditions, and location to the screen.
private void ParseAndDisplay (JsonValue json)
{
    // Get the weather reporting fields from the layout resource:
    TextView location = FindViewById<TextView>(Resource.Id.locationText);
    TextView temperature = FindViewById<TextView>(Resource.Id.tempText);
    TextView humidity = FindViewById<TextView>(Resource.Id.humidText);
    TextView conditions = FindViewById<TextView>(Resource.Id.condText);

    // Extract the array of name/value results for the field name "weatherObservation". 
    JsonValue weatherResults = json["weatherObservation"];

    // Extract the "stationName" (location string) and write it to the location TextBox:
    location.Text = weatherResults["stationName"];

    // The temperature is expressed in Celsius:
    double temp = weatherResults["temperature"];
    // Convert it to Fahrenheit:
    temp = ((9.0 / 5.0) * temp) + 32;
    // Write the temperature (one decimal place) to the temperature TextBox:
    temperature.Text = String.Format("{0:F1}", temp) + "° F";

    // Get the percent humidity and write it to the humidity TextBox:
    double humidPercent = weatherResults["humidity"];
    humidity.Text = humidPercent.ToString() + "%";

    // Get the "clouds" and "weatherConditions" strings and 
    // combine them. Ignore strings that are reported as "n/a":
    string cloudy = weatherResults["clouds"];
    if (cloudy.Equals ("n/a"))
        cloudy = "";
    string cond = weatherResults["weatherCondition"];
    if (cond.Equals ("n/a"))
        cond = "";

    // Write the result to the conditions TextBox:
    conditions.Text = cloudy + " " + cond;
}
  • In the OnCreate method, let's uncomment the call to ParseAndDisplay so that it will run. Next, let's build and run our app again, testing it to see if the results are successfully parsed and written to the screen. If everything works as expected, we should see a result that is similar to the first screenshot, above.

    Note that this sample uses a geonames.org demo account, which supports a limited number of accesses. If an exception is generated or the display does not report weather information, check your IDE output window to view the HTTP response. If the response contains a "daily limit has been exceeded" message, you will have to try the test again later.

Additional Information

The HttpWebRequest class allows apps to interact directly with servers using HTTP. The System.Json class can be used to parse JSON results from an HTTP request. Because HTTP interaction is handled asynchronously via async/await, the app is responsive to user input during network operations.

For more information about integrating with web services, Introduction to Web Services explains how to integrate REST, WCF, and SOAP web service technologies with Xamarin mobile applications.