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Home Getting Started with webOS webOS HTML5 Database Storage Tutorial - The Constructor Function

webOS HTML5 Database Storage Tutorial - The Constructor Function

Article Index
webOS HTML5 Database Storage Tutorial
The Constructor Function
Prototyping in webOS
Finishing up
All Pages

Here we're setting up a few variables that will help us manage the database during the use of our application:

var captured = null;
var highestZ = 0;
var highestId = 0;

The Constructor

function Note()
{
    var self = this;

    var note = document.createElement('div');
    note.className = 'note';
    note.addEventListener('mousedown', function(e) { return self.onMouseDown(e) }, false);
    note.addEventListener('click', function() { return self.onNoteClick() }, false);
    this.note = note;

    var close = document.createElement('div');
    close.className = 'closebutton';
    close.addEventListener('click', function(event) { return self.close(event) }, false);
    note.appendChild(close);

    var edit = document.createElement('div');
    edit.className = 'edit';
    edit.setAttribute('contenteditable', true);
    edit.addEventListener('keyup', function() { return self.onKeyUp() }, false);
    note.appendChild(edit);
    this.editField = edit;

    var ts = document.createElement('div');
    ts.className = 'timestamp';
    ts.addEventListener('mousedown', function(e) { return self.onMouseDown(e) }, false);
    note.appendChild(ts);
    this.lastModified = ts;

    document.body.appendChild(note);
    return this;
}

This is the constructor function for a Note object.  (For information on Javascript constructors, check out this article.) This function sets up an actual note object by creating each of the elements that make up a note and hooking up the various interactions that a user might make with each element.  Let's take a closer look.

    var self = this;

This sets the variable self to refer to this particular instance of the Note object, identified by the this keyword.  For more info on the this keyword, check out this great article.

    var note = document.createElement('div');
    note.className = 'note';
    note.addEventListener('mousedown', function(e) { return self.onMouseDown(e) }, false);
    note.addEventListener('click', function() { return self.onNoteClick() }, false);
    this.note = note;

These lines create the note div element and gives it the CSS class note, which was defined earlier.  It then adds event listener functions for two mouse events: mousedown and click, which call functions onMouseDown and onNoteClick, respectively.  We'll see what these functions do a bit later. 

    var close = document.createElement('div');
    close.className = 'closebutton';
    close.addEventListener('click', function(event) { return self.close(event) }, false);
    note.appendChild(close);

This section sets up the close element and adds a click event listener that calls the function close().  It assigns it a css class (closebutton) and then appends the close element as a child of the Note object instance.

    var edit = document.createElement('div');    
    edit.className = 'edit';
    edit.setAttribute('contenteditable', true);
    edit.addEventListener('keyup', function() { return self.onKeyUp() }, false);
    note.appendChild(edit);
    this.editField = edit;

This section sets up the edit object.  The HTML5 contenteditable attribute is set on this div so that the user can type into the div to change its contents.  An event listener is added onKeyUp, probably to save the contents of the note in real-time (i.e. each time a key is pressed).  We'll see later.

    var ts = document.createElement('div');
    ts.className = 'timestamp';
    ts.addEventListener('mousedown', function(e) { return self.onMouseDown(e) }, false);
    note.appendChild(ts);
    this.lastModified = ts;

This section sets up the timestamp object (shortened to ts).  This function sets up the timestamp that is displayed at the bottom of the note.

The next section defines additional methods for Note objects through the use of prototyping.  Prototyping is an important concept in the Javascript world because it allows us to add additional methods to objects that have already been defined (in our case, the Note object).  Let's see how this is done.



 

18 Comments

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  1. Really bad tutorial, the subject, the HTML 5 databse, is drowning in a load of unrelated code. Any code not strictly related to the topic should be keept to a minimum.
  2. Couldn't agree more with Jacob C.M.A.
  3. Contrary to the other comments left above ^ I thought this tutorial was well thought out and well executed. It covered the basics as well as some advanced techniques for working with HTML5 databases.

    The above comments are clearly the product of uneducated users. If they understood HTML5 db's and the webOS they would know that javascript (that so called "unrelated code") does everything as far as the database interaction and user interaction goes and therefore is, not only related, but necessary to make proper use of the database structure and apply it to both the webOS platform and the web in general.

    Kudos to Mr. Ken Young for a great tutorial, I have learned many new things from this article and appreciate the time he took to write it. Also, I apologize for the aforementioned flaming of users, though quite rude I deemed it necessary in order to maintain my sanity and vent some frustration...
  4. Though this is hardly worth being called a "tutorial" and has very little to do with WebOS, it is interesting to read. I also tried just copy-pasting the complete HTML from the webkit app in a WebOS SDK application, simply in the index.html, and it works. Kindof. Can't create new notes, but the one note works.
  5. Unfortunately, this is not a tutorial. In my opinion, this is just a copy and paste off of webkit Sticky Notes Demo. There is nothing here that documents any HTML 5 Database methods and usage. And to make it worse you call it aq "Web OS" tutorial? Like you said it yourself, you haven't even tried WebOS
  6. Hi
    is there any import files such as jar or any files for using html5 Palm WebOS. ?

    where will use the above code

    var db;

    try {
    if (window.openDatabase) {
    db = openDatabase("NoteTest", "1.0", "HTML5 Database API example", 200000);
    if (!db)
    alert("Failed to open the database on disk. This is probably because the version was bad or there is not enough space left in this domain's quota");
    } else
    alert("Couldn't open the database. Please try with a WebKit nightly with this feature enabled");
    } catch(err) { }

    html file or js file..?
    how to call and access database connection ..?
    is there any sample application plz send me some samples...
  7. How about updating this? Ken, your note at the end said you are going to do that....I'd love to see it. Thanks!
  8. I think that the tutorial explains properly the webstorage standard, thanks for it, also i have just one question, where is the database stored ? could i copy it to other place with the webkit enabled ?

    is possible to access the temporally database and copy to other place?

    thanks
  9. I really liked this tutorial. It helped me understand how to make a database work. I have a couple recomendations for the author:
    you left out just a couple of lines that a beginner might not see that you left out. By the way the code at the beginner was placed it was not clear that you need this between the styles and the javascript at the beginning:


    and also left this out at the end before the paragraph:



    plus 2 more lines:


    I would like to know what the tx is that is being passed in this part:
    db.transaction(function(tx) {
    it is not clear from the tutorial.
    also explain what this line means:
    if (!("_saveTimer" in this))
    it looks for a method called _saveTimer attached to self ( or this )
    everything else made sense. And what does this mean:
    ++highestZ;
    I would guess that it does the same as highestZ++; I just never knew you could reverse it.
    Thanks for the great lesson on html5!
  10. remove the coment that is above. It was a mistake because i needed to encode the html.
    I really liked this tutorial. It helped me understand how to make a database work. I have a couple recomendations for the author:
    you left out just a couple of lines that a beginner might not see that you left out. By the way the code at the beginner was placed it was not clear that you need this between the styles and the javascript at the beginning:


    and also left this out at the end before the paragraph:



    plus 2 more lines:


    I would like to know what the tx is that is being passed in this part:
    db.transaction(function(tx) {
    it is not clear from the tutorial.
    also explain what this line means:
    if (!("_saveTimer" in this))
    it looks for a method called _saveTimer attached to self ( or this )
    everything else made sense. And what does this mean:
    ++highestZ;
    I would guess that it does the same as highestZ++; I just never knew you could reverse it.
    Thanks for the great lesson on html5!
  11. sorry, remove the 2 posts above so this one should show the html correctly
    I really liked this tutorial. It helped me understand how to make a database work. I have a couple recomendations for the author:
    you left out just a couple of lines that a beginner might not see that you left out. By the way the code at the beginner was placed it was not clear that you need this between the styles and the javascript at the beginning:

    and also left this out at the end before the paragraph:

    plus 2 more lines:


    I would like to know what the tx is that is being passed in this part:
    db.transaction(function(tx) {
    it is not clear from the tutorial.
  12. the comment I was trying to post wouldn't allow me to show any kind of html code so i will leave the angle brackets out. remove all comments above this please.

    I really liked this tutorial. It helped me understand how to make a database work. I have a couple recomendations for the author:
    you left out just a couple of lines that a beginner might not see that you left out. By the way the code at the beginner was placed it was not clear that you need this between the styles and the javascript at the beginning:
    end style tag (no angle bracket because the synax is not allowed here in comments)
    script tag
    and also left this out at the end before the paragraph:
    script tag
    end head tag
    body tag
    plus 2 more lines at the very end of code:
    end body tag
    end html tag

    I would like to know what the tx is that is being passed in this part:
    db.transaction(function(tx) {
    it is not clear from the tutorial.
  13. it won't allow a comment too long so I split it into a second part
    also explain what this line means:
    if (!("_saveTimer" in this))
    it looks for a method called _saveTimer attached to self ( or this )
    everything else made sense. And what does this mean:
    ++highestZ;
    I would guess that it does the same as highestZ++; I just never knew you could reverse it.
    Thanks for the great lesson on html5!
  14. Hey Mike, thanks for the comments and sticking with it even though the comments system doesn't seem to want to allow HTML code. I'll need to look into that, because the ability to post sample code is pretty huge on a site like this.

    I've been way behind updating the site because of other things going on (e.g. getting a job that pays the bills!), but I'm hoping to get back to it soon!
  15. I like window.openDatabase(). A partly-developed demo is at: https://www.thebrentc.net/programs/db.html
  16. Thank you Ken. I feel as though I have been on the far outskirts of the coding vortex for some time now.
    Lately I've felt as if I am closing in on the center, slowly but surely. Your tutorial has given me a hefty nudge in that direction and the broad brush was exactly what I needed.
    You cleared up a number of those 'I think I know, sorta, kinda...' things that have been bugging me.
    Sincerely,
    Michael
  17. OK, this is a fairly terrible "tutorial." It skips vital steps like putting in script tags, it ignores errors if it won't window.openDatabase, over all poor quality.
  18. I think it might be better to create the table using: "CREATE TABLE IF NOT EXISTS table_name". An exception might be thrown for reasons other than the table not existing.

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