Creating an Error-Handling Object


An error-handling object is essential when building and releasing large web applications that use heavy amounts of JavaScript. While building, error handling is very useful for identifying bugs in your code so that you can easily track them down and progressively finish building the application. When releasing an application, error handling is even more important because users need to know how to handle issues that may arise and cannot do so without feedback from the application, which can tell them what went wrong. It is also important after releasing because you or your fellow developers will be able to identify what went wrong when a user is contacting you with an issue. To handle errors, let's create an object called ErrorManager as in Listing 16.1.

Listing 16.1. Instantiating ErrorManager (ErrorManager.js)

ErrorManager = {};

The core of this object is JavaScript's intrinsic onerror event. This method listens for errors and fires an event that can point to any custom callback method that you specify. The onerror event also has the capability to pass three parameters to the callback method, to provide detailed information regarding any errors that occur. The three parameters that the event passes are the actual error message, which identifies what error occurred; the URL of the document where the error occurred; and the line number in the document where the error occurred. These parameters can have a number of uses. They can be used to provide feedback to users regarding any errors that occur, or they can be used to provide feedback to the developer so that he is aware of any issues that occur during user interaction after an application has been released or is being tested. We will set this event to a local callback in the ErrorManager object, but first we need to create the observer methods.

Register Observers

The onerror event will be used to notify the subject, which is the ErrorManager object of any errors that occur. ErrorManager will then notify any objects that have been registered and stored in its collection. There are three methods we will create, which we discussed in the previous section. The methods that will handle all the observation functionality are called registerObserver, notifyObservers, and unregisterObserver. These methods must be the first written in the object after the object has been declared. Listing 16.2 shows the registerObserver method.

Listing 16.2. Registering Observers (ErrorManager.js)

ErrorManager.registerObserver = function(_observer) {     ErrorManager.observerCollection.push(o); }

The registerObserver method does exactly that: It registers an observer by adding it to an observerCollection, which is a property of the object and is essentially an array that is specifically used to store observers. Observers that are added to this collection need to specify the object name plus the method they want to use as the notification callback in a codestring. A codestring is a string representation of any code. In this case, it would be a method call in the form of a string:

"Object.notify"


The collection will be declared in an initialize method that we will create as soon as we have finished adding the observer methods to the object so that we keep the scope of the object members intact.

Notify Observers

The next method, called notifyObservers (see Listing 16.3), iterates through the observerCollection and fires the callback methods that the observers specified.

Listing 16.3. Notifying the Observers (ErrorManager.js)

ErrorManager.notifyObservers = function(message, url, line) {     for(var i in ErrorManager.observerCollection)     {         eval(ErrorManager.observerCollection[i] +"('"+message +"','"+ url +"','"+ line+"');");     } }

While iterating through the observerCollection, the notifyObservers method calls the observers by using JavaScript's intrinsic eval method to create a method from the codestring that was passed to the registerObserver method. The eval method determines whether the codestring is valid and executes the code if it is valid. I have also added the additional power of passing the error parameters from the onerror method to the notify methods. This allows the observers to be aware of the error that occurred and act on it as they see fit. For example, an object may register to the ErrorManager and receive notification when an error occurs. Based on the parameters we are passing to the callback method, the line of code where the error occurred may be used to highlight the corresponding issue in the GUI. This allows our objects to provide feedback to the user as to what went wrong. The best part about this pattern is that there can be an unlimited number of objects registered to the subject and they can all handle errors in different ways, depending on what portion of the application they manage. This allows for complete flexibility in your application and keeps the objects extremely decoupled, while allowing them to communicate with each other.

Unregister Observers

The next method is the unregisterObserver method, which is used to remove observers from the collection so that they are no longer notified when an event occurs. Listing 16.4 shows the method as it is used in ErrorManager.

Listing 16.4. Unregistering Observers (ErrorManager.js)

ErrorManager.unregisterObserver = function(_observer) {     for(var observer in ErrorManager.observerCollection)     {         if(_observer == ErrorManager.observerCollection[observer])         {             ErrorManager.observerCollection.splice(observer, 1);         }     }  }

This method receives a codestring as did the registerObserver method. It then uses this string while iterating through the collection by checking to see whether there is a matching codestring in the collection. When and if there is a match, the codestring is removed from the collection. In order to remove the string from the collection, we need to use the JavaScript splice method to remove the specified index in the collection. After the observer has been removed from the collection, it will no longer be notified of any events that it was once registered to, although the object may register again at any time.

Once we have these three methods created, we need to initialize the object and all its properties. Listing 16.5 shows our ErrorManager's initialize method.

Listing 16.5. Initializing the ErrorManager (ErrorManager.js)

ErrorManager.initialize = function() {     ErrorManager.observerCollection = new Array();     ErrorManager.registerObserver("ErrorManager.emailError");     onerror = ErrorManager.notifyObservers; } ErrorManager.initialize();

As you can see, the initialize method is very important to this object. It must be called as soon as it has been created to allow proper scoping of its properties. As I mentioned earlier, it handles the creation of the observerCollection that we used throughout all of the observer methods. The observerCollection is nothing more than a simple array that is used to collect the observers as they are registered. The next two lines of code in this method register two methods from the ErrorManager: a method named emailError and another named alert. These two methods will be notified when any JavaScript error occurs in the application. Now that we have the observer methods created and the objects' properties are available, we can set the local callback method to JavaScript's intrinsic onerror event. This event becomes the core of this object; without it, the object would not function. Anytime a JavaScript error occurs, this event fires the callback, which in this case is the notifyObservers method. It may look as if we are defining a variable, but since this is an intrinsic JavaScript event it knows how to handle the assignment. This method then handles notifying all the observers of the error that occurred, along with all the details, including the message, URL, and exact line of code.

The first notify method that we will create is the emailError method (see Listing 16.6) that we registered in the initialize method. This method contains the functionality to send an email to specified developers listing all the details of the error.

Listing 16.6. Emailing Errors (ErrorManager.js)

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ErrorManager.emailError = function(message, url, line) {     var error = "<b>Error:</b> <font color=red>"+ message +"</font><br/>";     error += "<b>URL:</b> "+ url +"<br/>";     error += "<b>Line:</b> "+ line +"<br/>";     var page = "classes/ErrorManager.class.php";     var subject = "My Ajax Application Error";     AjaxUpdater.Update('POST', page +"?subject="+ subject +"&message="+ error, this .catchResponse); } ErrorManager.catchResponse = function() {     if(Ajax.checkReadyState('loading') == "OK")     {         // Handle the response from the server-side     } }

Take a look at the parameters the emailError method accepts. They are the three parameters that we previously discussed. We will use these parameters to create an HTML-formatted email that we will ultimately send to ourselves or the developer of our application. After we have the error formatted as HTML, we will identify the variables that will be used as the parameters in the email. The first parameter is the page that we are requesting through Ajax, which is an ErrorManager class that we will create with PHP shortly, and the second is the subject of the email. After we have our variables identified for the request, we will send it to the AjaxUpdaters's update method, which we created in Chapter 14, "Singleton Pattern." This request will be made through the POST method to the page that we specify, which in this case is an ErrorManager class that we will create with PHP. The request is sent along with a query string that consists of the variables we defined, such as the subject and HTML error. We will also specify a callback method, named catchResponse, which we can use to handle the response from the server. After all this information has been passed to the AjaxUpdater, it will make the request through the Ajax object to the PHP page specified.

Although we have not covered server-side interaction with Ajax, in this section we will be jumping ahead a bit in order to add the functionality to send the email. Later, in Part V, "Server-Side Interaction," we will take an in-depth look at server-side interaction. The PHP ErrorManager class is a fairly small class, so it will be a good object in which to make our first server-side interaction. Listing 16.7 shows the class in all its glory.

Note

The PHP classes used throughout this book require PHP version 5.0.


Listing 16.7. ErrorManager PHP Class (ErrorManager.class.php)

<?php $errorManager = new ErrorManager(); $errorManager->send($_GET["subject"], $_GET["message"]); class ErrorManager {     public function ErrorManager() {}     public function send($_subject, $_message)     {         $headers = "From: noreply@sample.com\r\n" .                     "Reply-To: noreply@sample.com\r\n" .                    "MIME-Version: 1.0\r\n" .                    "Content-Type: text/html; charset=utf-8\r\n" .                    "Content-Transfer-Encoding: 8bit\r\n\r\n";         mail("you@yourdomain.com", $_subject, $_message, $headers);     } } ?>

As I mentioned, this class is fairly small because it only consists of one method. It is important to notice that we are instantiating the object in the same file. This allows us to make requests directly to the class without having to create an intermediate file that does it for us. Either way is perfectly fine, but I find this way to be a bit cleaner and easier to manage. After the object is instantiated, we call the send method and pass it two parameters, which are the subject, and message parameters. You may recognize these parameters from the ErrorManager object that we created in JavaScript. These are the onerror parameters that are sent to the callback method and ultimately passed through the request to this class. Therefore, this method is invoked as soon as the page is requested. The request is then handled by the send method, which sends an email based on the parameters that are sent to it. However, before it sends the email, it also specifies the headers, such as the from email address, the reply-to email address, and the MIME version, content type, and content-transfer encoding, which are all used to handle the HTML format that we are passing through the request. After the headers have been identified, we will use PHP's intrinsic mail function, which allows you to send an email.

The only requirement is that the server you are using to run the web application must have access to a mail application. In order to get more information about the mail function, you may visit the PHP manual at www.php.net and search for mail in the function list. This page will provide you with additional information regarding the function and samples to show you how to make our ErrorManager class more robust. For example, you may want to add error handling in the ErrorManager class in case the email is not sent for any reason. If an error occurs, you can then respond to the client-side Ajax engine and ultimately the catchResponse method in the JavaScript ErrorManager with the response. The JavaScript ErrorManager can then handle the response by trying to send another email if there was a failure or simply notifying the user of the error that occurred as an alternative to sending the email.

Now that we have a fully functional ErrorManager object, we can use it throughout our application. Let's see how.



Ajax for Web Application Developers
Ajax for Web Application Developers
ISBN: 0672329123
EAN: 2147483647
Year: 2007
Pages: 129
Authors: Kris Hadlock

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