Loading external data

Many APIs that load external data use the URLRequest class to define the properties of necessary network request.

The URLLoader class let you send a request to a server and access the information returned. You can also use the URLLoader class to access files on the local file system in contexts where local file access is permitted (such as the Flash Player local-with-filesystem sandbox and the AIR application sandbox). The URLLoader class downloads data from a URL as text, binary data, or URL-encoded variables. The URLLoader class dispatches events such as complete , httpStatus , ioError , open , progress , and securityError .

The URLLoader class provides an alternative to the XMLHttpRequest class. You can use either class to download data via an HTTP request.

Downloaded data is not available until the download has completed. You can monitor the progress of the download (bytes loaded and bytes total) by listening for the progress event to be dispatched. However, if a file loads quickly enough a progress event might not be dispatched. When a file has successfully downloaded, the complete event is dispatched. By setting the URLLoader dataFormat property, you can receive the data as text, raw binary data, or as a URLVariables object.

The URLLoader.load() method (and optionally the URLLoader class’s constructor) takes a single parameter, request , which is a URLRequest object. A URLRequest object contains all of the information for a single HTTP request, such as the target URL, request method ( GET or POST ), additional header information, and the MIME type.

For example, to upload an XML packet to a server-side script, you could use the following code:

var secondsUTC = new Date().time; 
var dataXML = (new DOMParser()).parseFromString( "<time>" + secondsUTC + "</time>", "application/xml" ); 
var request = new air.URLRequest("http://www.example.com/time.cfm"); 
request.contentType = "text/xml"; 
request.data = dataXML; 
request.method = air.URLRequestMethod.POST; 
var loader = new air.URLLoader(); 
loader.load(request);

The previous snippet creates an XML document named dataXML that contains the XML packet to be sent to the server. The example sets the URLRequest contentType property to "text/xml" and assigns the XML document to the URLRequest data property. Finally, the example creates a URLLoader object and sends the request to the remote script by using the load() method.

The URLStream class provides access to the downloading data as the data arrives. The URLStream class also lets you close a stream before it finishes downloading. The downloaded data is available as raw binary data.

When reading data from a URLStream object, use the bytesAvailable property to determine whether sufficient data is available before reading it. An EOFError exception is thrown if you attempt to read more data than is available.

When you build dynamic applications, it can be useful to load data from external files or from server-side scripts. This lets you build dynamic applications without having to edit or recompile your application. For example, if you build a “tip of the day” application, you can write a server-side script that retrieves a random tip from a database and saves it to a text file once a day. Then your application can load the contents of a static text file instead of querying the database each time.

The following snippet creates a URLRequest and URLLoader object, which loads the contents of an external text file, params.txt:

var request = new air.URLRequest("params.txt"); 
var loader = new air.URLLoader(); 
loader.load(request);

var request = new air.URLRequest("http://www.example.com/sendfeedback.cfm"); 
request.method = air.URLRequestMethod.POST;

The external document, params.txt, that is loaded at run time contains the following data:

monthNames=January,February,March,April,May,June,July,August,September,October,November,December&dayNames=Sunday,Monday,Tuesday,Wednesday,Thursday,Friday,Saturday

The file contains two parameters, monthNames and dayNames . Each parameter contains a comma-separated list that is parsed as strings. You can split this list into an array using the String.split() method.

Avoid using reserved words or language constructs as variable names in external data files, because doing so makes reading and debugging your code more difficult.

function completeHandler(event) 
{ 
    var loader2 = URLLoader(event.target); 
    air.trace(loader2.data); 
}

If the remote document contains name-value pairs, you can parse the data using the URLVariables class by passing in the contents of the loaded file, as follows:

function completeHandler(event) 
{ 
    var loader2 = event.target; 
    var variables = new air.URLVariables(loader2.data); 
    air.trace(variables.dayNames); 
}

Each name-value pair from the external file is created as a property in the URLVariables object. Each property within the variables object in the previous code sample is treated as a string. If the value of the name-value pair is a list of items, you can convert the string into an array by calling the String.split() method, as follows:

var dayNameArray = variables.dayNames.split(",");

If you are loading numeric data from external text files, convert the values into numeric values by using a top-level function, such as parseInt() , parseFloat() , and Number() .

Instead of loading the contents of the remote file as a string and creating a new URLVariables object, you could instead set the URLLoader.dataFormat property to one of the static properties found in the URLLoaderDataFormat class. The three possible values for the URLLoader.dataFormat property are as follows:

• URLLoaderDataFormat.BINARY —The URLLoader.data property will contain binary data stored in a ByteArray object.

• URLLoaderDataFormat.TEXT —The URLLoader.data property will contain text in a String object.

• URLLoaderDataFormat.VARIABLES —The URLLoader.data property will contain URL-encoded variables stored in a URLVariables object.

The following code demonstrates how setting the URLLoader.dataFormat property to URLLoaderDataFormat.VARIABLES allows you to automatically parse loaded data into a URLVariables object:

var request = new air.URLRequest("http://www.example.com/params.txt"); 
var variables = new air.URLLoader(); 
variables.dataFormat = air.URLLoaderDataFormat.VARIABLES; 
variables.addEventListener(air.Event.COMPLETE, completeHandler); 
try 
{ 
    variables.load(request); 
}  
catch (error) 
{ 
    air.trace("Unable to load URL: " + error); 
} 
 
function completeHandler(event) 
{ 
    var loader = event.target; 
    air.trace(loader.data.dayNames); 
}

As the following example shows, loading XML from an external file is the same as loading URLVariables. You can create a URLRequest instance and a URLLoader instance and use them to download a remote XML document. When the file has completely downloaded, the complete event is dispatched and the trace() function outputs the contents of the file to the command line.

 
var request = new air.URLRequest("http://www.example.com/data.xml"); 
var loader = new air.URLLoader(); 
loader.addEventListener(air.Event.COMPLETE, completeHandler); 
loader.load(request); 
 
function completeHandler(event) 
{ 
    var dataXML = event.target.data; 
    air.trace(dataXML); 
}

In addition to loading external data files, you can also use the URLVariables class to send variables to a server-side script and process the server’s response. This is useful, for example, if you are programming a game and want to send the user’s score to a server to calculate whether it should be added to the high scores list, or even send a user’s login information to a server for validation. A server-side script can process the user name and password, validate it against a database, and return confirmation of whether the user-supplied credentials are valid.

The following snippet creates a URLVariables object named variables, which creates a new variable called name . Next, a URLRequest object is created that specifies the URL of the server-side script to send the variables to. Then you set the method property of the URLRequest object to send the variables as an HTTP POST request. To add the URLVariables object to the URL request, you set the data property of the URLRequest object to the URLVariables object created earlier. Finally, the URLLoader instance is created and the URLLoader.load() method is invoked, which initiates the request.

var variables = new air.URLVariables("name=Franklin"); 
var request = new air.URLRequest(); 
request.url = "http://www.[yourdomain].com/greeting.cfm"; 
request.method = air.URLRequestMethod.POST; 
request.data = variables; 
var loader = new air.URLLoader(); 
loader.dataFormat = URLLoaderDataFormat.VARIABLES; 
loader.addEventListener(Event.COMPLETE, completeHandler); 
try 
{ 
    loader.load(request); 
} 
catch (error) 
{ 
    air.trace("Unable to load URL"); 
} 
 
function completeHandler(event) 
{ 
    air.trace(event.target.data.welcomeMessage); 
}

The following code contains the contents of the Adobe ColdFusion® greeting.cfm document used in the previous example:

<cfif NOT IsDefined("Form.name") OR Len(Trim(Form.Name)) EQ 0> 
    <cfset Form.Name = "Stranger" /> 
</cfif> 
<cfoutput>welcomeMessage=#UrlEncodedFormat("Welcome, " & Form.name)# 
</cfoutput>