java qr code generator library Some small refactoring case studies in Java

Maker QR Code in Java Some small refactoring case studies

Some small refactoring case studies
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Using multiple event handlers It s common practice when scripting DOM nodes using JavaScript to define the script in the window.onload function, which is executed after the page (and therefore the DOM tree) is fully loaded. Let s say that we have a DOM element on our page that will display dynamically generated data fetched from the server at regular intervals once the page is loaded. The JavaScript that coordinates the data fetching and the display needs a reference to the DOM node, so it gets it by defining a window.onload event:
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window.onload=function(){ displayDiv=document.getElementById('display'); }
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All well and good. Let s say that we now want to add a second visual display that provides alerts from a news feed, for example (see chapter 13 if you re interested in implementing this functionality). The code that controls the news feed display also needs to grab references to some DOM elements on startup. So it defines a window.onload event handler, too:
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window.onload=function(){ feedDiv=document.getElementById('feeds'); }
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We test both sets of code on separate pages and find them both to work fine. When we put them together, the second window.onload function overwrites the first, and the data feed fails to display and starts to generate JavaScript errors. The problem lies in the fact that the window object allows only a single onload function to be attached to it. Limitations of a composite event handler Our second event handler overrides the first one. We can get around this by writing a single composite function:
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window.onload=function(){ displayDiv=document.getElementById('display'); feedDiv=document.getElementById('feeds'); }
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This works for our current example, but it tangles together code from the data display and the news feed viewer, which are otherwise unrelated to each other. If we were dealing with 10 or 20 systems rather than 2, and each needed to get references to several DOM elements, then a composite event handler like this would become hard to maintain. Swapping individual components in and out would become difficult and error prone, leading to exactly the sort of situation that we
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described in the introduction, where nobody wants to touch the code in case it should break. Let s try to refactor a little further, by defining a loader function for each subsystem:
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window.onload=function(){ getDisplayElements(); getFeedElements(); } function getDisplayElements(){ displayDiv=document.getElementById('display'); } function getFeedElements(){ feedDiv=document.getElementById('feeds'); }
This introduces some clarity, reducing our composite window.onload() to a single line for each subsystem, but the composite function is still a weak point in the design and is likely to cause us trouble. In the following section, we ll examine a slightly more complex but more scalable solution to the problem. The Observer pattern It can be helpful sometimes to ask where the responsibility for an action lies. The composite function approach places responsibility for getting the references to DOM elements on the window object, which then has to know which subsystems are present in the current page. Ideally, each subsystem should be responsible for acquiring its own references. That way, if it is present on a page, it will get them, and if it isn t present, it won t. To set the division of responsibility straight, we can allow systems to register for notification of the onload event happening by passing a function to call when the window.onload event is fired. Here s a simple implementation:
window.onloadListeners=new Array(); window.addOnLoadListener(listener){ window.onloadListeners[window.onloadListeners.length]=listener; }
When the window is fully loaded, then the window object need only iterate through its array of listeners and call each one in turn:
window.onload=function(){ for(var i=0;i<window.onloadListeners.length;i++){ var func=window.onlloadListeners[i]; func.call(); } }
Some small refactoring case studies
Register
Observer
Unregister
Observable
Notify Maintain list of registered Observers
Responsibility of Observer
Responsibility of Observable
Figure 3.2 Division of responsibility in the Observer pattern. Objects wishing to be notified of an event, the Observers, can register and unregister themselves with the event source, Observable, which will notify all registered parties when an event occurs.
Provided that every subsystem uses this approach, we can offer a much cleaner way of setting up all the subsystems without tangling them up in one another. Of course, it takes only one rogue piece of code to directly override window.onload and the system will break. But we have to take charge of our codebase at some point to prevent this from happening. It s worth pointing out here that the newer W3C event model also implements a multiple event handler system. We ve chosen to build our own here on top of the old JavaScript event model because implementations of the W3C model aren t consistent across browsers. We discuss this in greater detail in chapter 4. The design pattern into which our code here is refactored is called Observer. Observer defines an Observable object, in our case the built-in window object, and a set of Observers or Listeners that can register themselves with it (figure 3.2). With the Observer pattern, responsibility is apportioned appropriately between the event source and the event handler. Handlers take responsibility for registering and unregistering themselves. The event source takes responsibility for maintaining a list of registered parties and firing notifications when the event occurs. The pattern has a long history of use in event-driven UI programming, and we ll return to Observer when we discuss JavaScript events in more detail in chapter 4. And, as we ll see, it can also be used in our own code objects independently of the browser s mouse and key event processing. For now, let s move on to the next recurring issue that we can solve through refactoring.
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