RungHung Gau

1
Networking

• Rung-Hung Gau
• Department of Computer Science and Engineering
• National Sun Yat-Sen University
• Kaohsiung, Taiwan

2
Outline

• Web Browser in Android
• Networking in Android
• HTTP Operations via Apache HttpComponents
• Parsing Responses (DOM)

3
Web Browser in Android

• WebView (Browser in Android)
• loadUrl()
• loadData() and loadDataWithBaseURL()
• The WebViewClient Interface and
  WebView.setWebViewClient()
• shouldOverrideUrlLoading(WebView view, String url)

4
Webkit-based Browser

• You can embed the built-in Web browser as a
  widget in your own activities for displaying HTML
  or full-fledged browsing.
• The Android browser is based on WebKit, the same
  engine that powers Apple's Safari Web browser.
• The Android browser gets its own Java package
  (android.webkit)
• Here is an example that shows up a webpage

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• lt?xml version"1.0" encoding"utf-8"?gt
• ltLinearLayout xmlnsandroid"http//schemas.androi
  d.com/apk/res/android"
• androidorientation"vertical"
• androidlayout_width"fill_parent"
• androidlayout_height"fill_parent"
• gt
• ltWebView androidid"_at_id/webkit
• androidlayout_height"fill_parent"
• /gt
• lt/LinearLayoutgt

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• package com.commonsware.android.webkit
• import android.app.Activity
• import android.os.Bundle
• import android.webkit.WebView
• public class BrowserDemo1 extends Activity
• WebView browser
• _at_Override
• public void onCreate(Bundle icicle)
• super.onCreate(icicle)
• setContentView(R.layout.main)

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• / get the WebView widget from the
  LinearLayout /
• browser(WebView)findViewById(R.id.webkit)
• / use the WebView widget to display a webpage
  /
• browser.loadUrl("http//commonsware.com")

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• ltmanifest xmlnsandroid"http//schemas.android.co
  m/apk/res/android"
• package"com.commonsware.android.webkit"gt
• ltuses-permission androidname"android.permissio
  n.INTERNET" /gt
• ltapplicationgt
• ltactivity androidname".BrowserDemo1"
  androidlabel"BrowserDemo1"gt
• ltintent-filtergt
• ltaction androidname"android.intent.actio
  n.MAIN" /gt
• ltcategory androidname"android.intent.cat
  egory.LAUNCHER" /gt
• lt/intent-filtergt
• lt/activitygt
• lt/applicationgt
• lt/manifestgt

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Webkit-based Browser

• By default Javascript is turned off in WebView
  widgets.
• If you want to enable Javascript, call
  getSettings().setJavaScriptEnabled(true) on the
  WebView instance.

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Webkit-based Browser

• Besides loadUrl, you can use loadData() to supply
  the HTML for the browser to view.
• There are two flavors of loadData().
• The simpler one allows you to provide the
  content, the MIME type, and the encoding, all as
  strings.
• browser.loadData("lthtmlgtltbodygtHello,
  world!lt/bodygtlt/htmlgt", "text/html", "UTF-8")

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Webkit-based Browser

• WebView offers ways to perform browser
  navigation, including
• reload() to refresh the currently-viewed Web page
• goBack() to go back one step in the browser
  history, and canGoBack() to determine if there is
  any history to go back to
• goForward() to go forward one step in the browser
  history, and canGoForward() to determine if there
  is any history to go forward to

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Webkit-based Browser

• goBackOrForward() to go backwards or forwards in
  the browser history, where negative numbers
  represent a count of steps to go backwards, and
  positive numbers represent how many steps to go
  forwards
• canGoBackOrForward() to see if the browser can go
  backwards or forwards the stated number of steps
• clearCache() to clear the browser resource cache
  and clearHistory() to clear the browsing history

15
Webkit-based Browser

• Your hook into the WebView activity is via
  setWebViewClient(), which takes an instance of a
  WebViewClient implementation as a parameter.
• The supplied callback object will be notified of
  a wide range of activities such as
• onPageStarted()
• onTooManyRedirects()
• onReceivedHttpAuthRequest()
• Here is a browser-based equivalent of our
  original example an application that, upon a
  click, shows the current time.

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• public class BrowserDemo3 extends Activity
• WebView browser
• _at_Override
• public void onCreate(Bundle icicle)
• super.onCreate(icicle)
• setContentView(R.layout.main)
• browser(WebView)findViewById(R.id.webkit)
• / set the callback object that deals with
  browser events /
• browser.setWebViewClient(new Callback())
• loadTime()

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• void loadTime()
• String page"lthtmlgtltbodygtlta href\"clock\"gt
  new Date().toString() "lt/agtlt/bodygtlt/htmlgt"
• browser.loadDataWithBaseURL("x-data//base",
  page, "text/html", "UTF-8", null)
• private class Callback extends WebViewClient
• public boolean shouldOverrideUrlLoading(WebVie
  w view, String url)

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• / your callback is passed a URL (plus the
  WebView itself) and you return true if you will
  handle the request or false if you want default
  handling (e.g., actually fetch the Web page
  referenced by the URL). /
• / for some URLs you can override, while for
  some other URLs, you can use the default
  handling. /
• / The differentiation could depends on the URL
  and the WebView instance /
• loadTime()
• return(true)

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WebSettings

• Control the font sizing via setDefaultFontSize()
  (to use a point size) or setTextSize() (to use
  constants indicating relative sizes like LARGER
  and SMALLEST)
• Control Javascript via setJavaScriptEnabled() (to
  disable it outright) and setJavaScriptCanOpenWindo
  wsAutomatically() (to merely stop it from opening
  pop-up windows)
• The settings you change are not persistent!!

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WebSettings

• Control Web site rendering via setUserAgent()
• 0 means the WebView gives the Web site a
  user-agent string that indicates it is a mobile
  browser,
• 1 results in a user-agent string that suggests it
  is a desktop browser

22
Internet Access in Android

• The expectation is that most, if not all, Android
  devices will have built-in Internet access.
• Networking APIs
• raw sockets
• WebView
• HTTP, SMTP and so on (both on-device and from
  3rd-party JARs)

23
Internet Access in Android

• Android does not have built-in SOAP or XML-RPC
  client APIs.
• However, it does have the Apache HttpComponents
  library baked in.
• REST-style Web services is defined as simple
  HTTP requests for ordinary URLs over the full
  range of HTTP verbs, with formatted payloads
  (XML, JSON, etc.) as responses".

24
HTTP Operations via Apache HttpComponents

• The first step to using HttpClient is to create
  an HttpClient object.
• The client object handles all HTTP requests on
  your behalf.
• Since HttpClient is an interface, you will need
  to actually instantiate some implementation of
  that interface, such as DefaultHttpClient.

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HTTP Operations via Apache HttpComponents

• First, you create an HttpRequest implementation
  instance, with different HttpRequest
  implementations for each different HTTP verb.
• HttpGet for HTTP GET requests
• HttpPost for HTTP POST requests
• Second, you fill in the URL to retrieve and other
  configuration data (e.g., form values for
  HttpPost)
• Last, you pass the method to the HttpClient
  instance to actually make the HTTP request via
  HttpClient.execute().

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HTTP Operations via Apache HttpComponents

• You can get an HttpResponse object back by
• public abstract HttpResponse execute (HttpHost
  target, HttpRequest request)
• Or, you can get the response object generated by
  the response handler by
• public abstract T execute (HttpUriRequest
  request, ResponseHandlerlt? extends Tgt
  responseHandler)
• There are a variety of execute methods in the
  HttpClient interface.

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An Example of Web Service

• The updateForcast() method is called in
  onResume().
• The onResume() method is called just before your
  activity comes to the foreground.
• The updateForecast() method takes a Location as a
  parameter, obtained from the location update
  process.
• The getLatitude() and getLongitude() methods
  return the latitude and longitude of the device's
  position, respectively.
• We hold the URL to the National Weather Service
  XML in a string resource, and pour in the
  latitude and longitude at runtime.

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An Example of Web Service

• The HttpClient object is created in onCreate().
• Given the resulting XML from the REST service, we
  build the forecast HTML page (see below) and pour
  that into the WebKit widget.
• If the HttpClient blows up with an exception, we
  provide that error as a Toast.

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• private void updateForecast(Location loc)
• / format a URL based on the current location /
• String urlString.format(format,
  loc.getLatitude(), loc.getLongitude())
• / use the HTTP GET method for web service /
• HttpGet getMethodnew HttpGet(url)
• try
• / The ResponseHandler will handle the HTTP
  response /
• ResponseHandlerltStringgt responseHandler new
  BasicResponseHandler()
• / There are many flavors of HttpClient.execute()
  methods. /
• / You get the response object generated by the
  response handler. In this case, a String /
• String responseBodyclient.execute(getMethod,
  responseHandler)

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• / build the forecasts collection based on the
  data retrieved from REST web service /
• buildForecasts(responseBody)
• / generate the HTML file/table to be display
  in Browser /
• String pagegeneratePage()
• / show the HTML file/table in the Browser /
• browser.loadDataWithBaseURL(null, page,
  "text/html", "UTF-8", null)
• catch (Throwable t)
• Toast.makeText(this, "Request failed
  "t.toString(), 4000).show()

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Parsing Responses

• The response you get will be formatted using some
  system HTML, XML, JSON, and etc.
• In the case of the WeatherDemo, we need to
  extract the forecast time, temperature, and icon
  (indicating sky conditions and precipitation) and
  generate an HTML page from it.
• We have to parse the response data to extract
  useful information.

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Parsing Responses

• Android includes three XML parsers
• the traditional W3C DOM (org.w3c.dom),
• a SAX parser (org.xml.sax),
• and the XML pull parser
• Android also includes a JSON parser (org.json)
• For WeatherDemo, we use the W3C DOM parser in our
  buildForecasts() method.

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Parsing Responses

• The HTML comes in as an InputStream and is fed
  into the DOM parser.
• From there, we scan for the start-valid-time
  elements and populate a set of Forecast models
  using those start times.
• Then, we find the temperature value elements and
  icon-link URLs and fill those in to the Forecast
  objects.
• In turn, the generatePage() method creates a
  rudimentary HTML table based on the Forecast
  object.

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• void buildForecasts(String raw) throws Exception
  
• DocumentBuilder builderDocumentBuilderFactory.n
  ewInstance().newDocumentBuilder()
• / Use DOM to create a tagged tree from the raw
  string /
• Document docbuilder.parse(new InputSource(new
  StringReader(raw)))
• NodeList timesdoc.getElementsByTagName("start-v
  alid-time")
• for (int i0ilttimes.getLength()i)
• Element time(Element)times.item(i)
• Forecast forecastnew Forecast()
• forecasts.add(forecast)
• forecast.setTime(time.getFirstChild().getNodeV
  alue())

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• NodeList tempsdoc.getElementsByTagName("value")
  
• for (int i0ilttemps.getLength()i)
• Element temp(Element)temps.item(i)
• Forecast forecastforecasts.get(i)
• forecast.setTemp(new Integer(temp.getFirstChil
  d().getNodeValue()))
• NodeList iconsdoc.getElementsByTagName("icon-li
  nk")
• for (int i0ilticons.getLength()i)
• Element icon(Element)icons.item(i)
• Forecast forecastforecasts.get(i)
• forecast.setIcon(icon.getFirstChild().getNodeV
  alue())

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• String generatePage()
• / The method creates a rudimentary HTML table
  based on the forecasts collection /
• StringBuffer bufResultnew StringBuffer("lthtmlgtlt
  bodygtlttablegt")
• bufResult.append("lttrgtltth width\"50\"gtTimelt/th
  gt" "ltthgtTemperaturelt/thgtltthgtForecastlt/thgtlt/trgt")
  
• / forecasts is a collection /
• for (Forecast forecast forecasts)
• bufResult.append("lttrgtlttd align\"center\"gt")
  
• bufResult.append(forecast.getTime())
• bufResult.append("lt/tdgtlttd align\"center\"gt")
  
• bufResult.append(forecast.getTemp())
• bufResult.append("lt/tdgtlttdgtltimg src\"")

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• bufResult.append(forecast.getIcon())
• bufResult.append("\"gtlt/tdgtlt/trgt")
• bufResult.append("lt/tablegtlt/bodygtlt/htmlgt")
• return(bufResult.toString())

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Reference

• http//developer.android.com/reference/packages.ht
  ml