USING ANDROID WITH THE INTERNET. Slide 2 Network Prerequisites The following must be included so as...

USING ANDROID WITH THE INTERNET

Network Prerequisites The following must be included so as to

allow the device to connect to the network The tag should be an immediate child of <manifest>

<uses-permission android:name="android.permission.INTERNET" /><uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

HTTP (Introduction) In most cases the transfer protocol is

HTTP We connect our device to a server and get

data We then process that data somehow

Two HTTP clients HttpClient HttpURLConnection

Both support HTTPS and IPV6 Use HttpURLConnection for post

Lollypop devices

Checking for Network Access The network might be unavailable for

many reasons We may have WiFi service or just cell

service Note that service might be metered so

we need to limit bandwidth

The ConnectivityManager The ConnectivityManager answers

questions about the state of the network

The getSystemService( Context.CONNECTIVITY_SERVICE) method gets an instance of the class

The NetworkInfo Class The getActiveNetworkInfo class

returns an NetworkInfo instance Call the isConnected method to

determine whether there is a network connection

See http://developer.android.com/reference/android/net/NetworkInfo.html

Introduction to Multithreading Many activities can take a while to

complete Some activities might not complete at

all A thread is a concurrent unit of

execution There is a thread for the UI We can create other threads

This is a complicated topic but I’ll provide just enough here for you to create a thread and run it

The AsyncTask Class (Introduction) The android.os.AsyncTask allows

background operations to be performed and returned to the UI thread

It is a helper class that wraps the Thread class

Use it to perform short-lived background tasks

Params, Progress, Result

The AsyncTask Class (Creating) To use the AsyncTask create a subclass

with three generic parameters The first contains the data passed to the

thread The second, if used contains an integer

used to report the thread’s progress The third, if used contains the data type of

the thread value. If the parameter is Void, then it will not be

used

The AsyncTask Class (Example) Extend AsyncTask

A string is passed to the task There is no progress reporting A string is returned from the task

When the background task is executed, it goes through four steps

The AsyncTask Class (Implementing – Step 1) onPreExecute is invoked on the UI

thread before the task begins It is here that you setup the task Initialize a progress meter, for example

Implementing this step is optional

The AsyncTask Class (Implementing – Step 2) doInBackground is the worker It executes immediately after onPreExecute finishes

It is here that you perform the background computation

The parameters are passed to this procedure

The results are returned in this step Call publishProgress(Progress…) to

publish progress results (optional)

The AsyncTask Class (Implementing – Step 2) String… denotes a variable length

array of type String So urls contains the parameters

The AsyncTask Class (Implementing – Step 3) onProgressUpdate is invoked on the UI

thread It gets called as a result of the publishProgress call

Use it to update a progress meter or log Implementing this method is optional

The AsyncTask Class (Implementing – Step 4) onPostExecute is invoked on the UI

thread The parameter contains the result of the

asynchronous method call

STATUS You now know how to set up the

asynchronous infrastructure

Net we will see how to send a HTTP request with a URL and process the result

Android URL and HTTP Related Classes The URL class

The URL Class The java.net.URL class represents a url

Convert strings to URLs Convert URLs to strings

http://developer.android.com/reference/java/net/URL.html




The URL Class

Opening a Connection (1) The URL.openConnectcion() method

establishes a connection to a resource Over this connection, you make the

request and get the response We will use HTTP here but other

protocols are supported

Opening a Connection (2) The setReadTimeout setter defines the

time to wait for data The setConnectTimeout setter the time

to wait before establishing a connection The setRequestMethod defines

whether the request will be a GET or a POST

The setDoInput setter, if true allows receiving of data

Opening a Connection (3) Calling the connect() method opens

the connection getResponseCode() gets the HTTP

response code from the server -1 if there is no response code. Such as 404 not found?

getInputStream() gets the input stream from which you can read data Works just like an open file

Opening A Connection (Example)

Reading the Input

Introduction to XML XML is briefly discussed here as it

provides a generalized means to get URL data Many services return data as XML

What XML Does

Nothing

What XML is Philosophically (1) It’s a language designed to describe

data in a hierarchical way By itself, XML is useless XML does not process the data it describes Simply put, XML elements are collected

together into an XML document It’s up to “something else” to process that

document

What XML is Philosophically (2) Hierarchical XML data can represent

anything Even though XML does nothing with the

data That’s because XML describes data

XML does not describe the meaning of data There are other XML “recommendations”

that give meaning to XML data

A First XML Example The answer (question) to the universe is

42

<?xml version="1.0" encoding="utf-8" ?><Universe> <Answer>42</Answer></Universe>

How XML Works (1) An XML document is processed using a

generic XML parser Companies that support XML have an

implementation of an XML parser The XML Document Object Model (DOM)

represents an XML document in-memory Again, this representation is defined by the W3C .NET supports the “level 2” DOM representation

How XML Works (2) An XML parser processes hierarchical XML

data, which is made up of: markup – Tags, references, and declarations,

which are processed by the XML parser character data – Data appearing between the

markup Character data is not processed by the XML

parser It’s the document’s data

XML Documents (Introduction) XML syntax is simple but rigorous

An XML document must be syntactically correct to be well formed

As opposed to HTML document which can be processed even though the are incorrect

An XML document must adhere to the rules set for by a DTD or XSD to be valid

Formally, an XML document is an instance of a class defined by the DTD or XSD

More on this topic later when we get to document validation

Introduction to XML Syntax (1) XML tags resemble HTML tags but they are not

predefined That is, you give names to tags just as you give

names to variables and other identifiers Starting tags appear as <tag> Ending tags appear as </tag> Empty elements can appear as <tag/>

XML elements MUST have an ending tag unlike HTML A starting and ending tag makes up an element

Introduction to XML Syntax (2) An element can have different types of

content If an element contains other elements, it has

element content If an element contains text and other elements,

the element has mixed content If an element contains only data, then the element

has simple content Elements can also have 0, 1 or more

attributes

Introduction to XML Syntax (3) Tags can be nested

These are hierarchical nodes Tags MUST be nested correctly though

There must be only one root tag White space is preserved in XML documents

This is a big departure from HTML A new line is represented with the LF character Space and Tab characters are also considered

white space

XML Attributes (Introduction) Tags can have one or more attributes

Attributes appear after the tag name Attributes are conceptually similar to properties

An attribute could also be implemented as a nested element

The choice is yours As you will see, a schema restricts and

attribute’s values or occurrences

XML Attributes (Syntax 1) Attributes appear as key=value pairs

An attribute must have both a key and a value Attribute keys do not appear in quotation

marks Attribute values must appear in quotation

marks Remember, single and double quotes are

interchangeable A space separates each key=value pair

XML Attributes (Example) In the following example, id is the

attribute

<?xml version="1.0" encoding="utf-8" ?><Universe id="1" size="Infinite"> <Answer>42</Answer></Universe>

Comparing Attributes and Elements The following 2 documents are (roughly)

equivalent<?xml version="1.0" encoding="utf-8" ?><student id="12345"> <name>Bill</name></student><?xml version="1.0" encoding="utf-8" ?><student> <id>12345</id> <name>Bill</name></student>

Processing XML In Android There are several Android and Java

classes with which we can process an XML document

Generally speaking we can Read the data sequentially using a SAX

parser Read the data into an in-memory XML

document, with which we can do some DOM processing

We will use a DOM parser My demo continues to work with the

weather.

Processing XML All XML nodes

have a typehttp://www.w3schools.com/dom/dom_nodetype.asp

http://www.w3schools.com/dom/dom_nodetype.asp



Processing XML Create a DocumentBuilder via the

DocumentBuilderFactory Call the parse method on an input

stream to read and parse the document Using the Document object, navigate

and work with the in-memory XML document

Classes We Need DocumentBuilderFactory DocumentBuilder Document NodeList Node InputSource StringReader

The DocumentBuilderFactory Class

It lets applications get a parser that produced XML object trees from XML documents This is pretty much the same DOM with

which you are familiar (JavaScript) As the name implies, we use a factory

class and method to create the

USING ANDROID WITH THE INTERNET. Slide 2 Network Prerequisites The following must be included so as...

Documents

Transcript of USING ANDROID WITH THE INTERNET. Slide 2 Network Prerequisites The following must be included so as...