java barcode Methods and functions in Java

Creating Denso QR Bar Code in Java Methods and functions

Methods and functions
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are two idioms for doing this that I ve commonly come across. One is clearly superior to the other, in my opinion, but I coded for years using the other one, and it works. One of the aims of this book is to give names to the patterns (and anti-patterns) that we have adopted by habit, so I will present both here. Referencing the Model by name In this solution, we assign a globally unique ID to each instance of our Model object and keep a global array of these objects referenced by the ID. Given a reference to a DOM element, we can then reference its Model object by using part of the ID as a key to the lookup array. Figure B.1 illustrates this strategy. Generating a unique ID for every element is an overhead in this approach, but ID generation can be accomplished fairly automatically. We can use the array length as part of the key, for example, or a database key, if we re generating code on the web server. As a simple example, we re creating an object of type myObj, which has a clickable title bar that invokes a function myObj.foo(). Here is the global array:
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var MyObjects=new Array();
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And here is the constructor function, which registers the Model object with that array:
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Model object DOM element 3. Retrieve reference to Model 2. Query lookup with id fragment Global lookup Event handler 1. Resolve this.id and extract fragment
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Figure B.1 Referencing the Model from an event handler function by name. The DOM element ID is parsed, and the parsed value used as a key to a global lookup array.
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APPENDIX B
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JavaScript for object-oriented programmers
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function myObj(id){ this.uid=id; MyObjects[this.uid]=this; ... this.render(); }
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Here is a method of the myObj object, which does something exciting. We want to invoke this when the title bar is clicked:
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myObj.prototype.foo=function(){ alert('foooo!!! '+this.uid); }
Here is the object s render() method, which creates various DOM nodes:
myObj.prototype.render=function(){ ... this.body=document.createElement("div"); this.body.id=this.uid+"_body"; ... this.titleBar=document.createElement("div"); this.titleBar.id=this.uid+"_titleBar"; this.titleBar.onclick=fooEventHandler; ... }
When we construct any DOM nodes in the view for this Model object, we assign an ID value to them that contains the Model object ID. Note that we refer to a function fooEventHandler() and set it as the onclick property of our title bar DOM element:
function fooEventHandler(event){ var modelObj=getMyObj(this.id); if (modelObj){ modelObj.foo(); } } }
The event handler function will need to find the instance of myObj in order to invoke its foo() method. We provide a finder method:
function getMyObj(id){ var key=id.split("_")[0]; return MyObjects[key]; }
It has a reference to the DOM node and can use its id property to extract a key from which to retrieve the Model object from the global array. And there it is. The Reference Model By Name method served me well for a few years, and it works, but there is a simpler, cleaner way that doesn t pepper your
Methods and functions
DOM tree with lengthy IDs. (Actually, I never reached a decision as to whether
that was good or bad. It was a waste of memory, for sure, but it also made debugging in the Mozilla DOM Inspector very easy.) Attaching a Model to the DOM node In this second approach to DOM event handling, everything is done with object references, not strings, and no global lookup array is needed. This is the approach that has been used throughout this book. Figure B.2 illustrates this approach. This approach simplifies the event handler s job considerably. The constructor function for the Model object needs no specialized ID manipulation, and the foo() method is defined as before. When we construct DOM nodes, we exploit JavaScript s dynamic ability to attach arbitrary attributes to any object and clip the Model object directly onto the DOM node receiving the event:
myObj.prototype.createView=function(){ ... this.body=document.createElement("div"); this.body.modelObj=this; ... this.titleBar=document.createElement("div"); this.titleBar.modelObj=this; this.titleBar.onclick=fooEventHandler; ... }
When we write the event handler, we can then get a direct reference back to the Model:
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