Developing Portable Mobile Web Applications

What if you could write iPhone and iPad applications on Linux? And, what if those applications also ran on Android phones? Well, you can, by writing your application as a portable mobile Web application. Take advantage of HTML5 and JavaScript libraries to write rich Web applications that users can't tell from native applications. You even can have them install from iTunes and Android Market. Plus,
Client-Side Database

WebKit uses SQLite to implement the client-side database APIs for HTML5. The implementation is remarkably complete, including support for transactions with rollback if the transaction does not succeed. Let's look at the JavaScript in Listing 2, webnotes.js.

The first four lines of the file initialize jQTouch and assign an instance to the variable jQT. The parameter “.icon” is one of many that can be defined for jQTouch. It points to a 57x57 pixel icon for the application (Figure 4).

Figure 4. Application Icon

On line five, we declare a variable “db” for the database instance. The block of code that starts $(document).ready is a jQuery function that executes when the browser has finished loading the DOM, even though the page contents may still be loading. The anonymous function first redirects the submit buttons from the “addNote” and “editNote” forms, pointing them at JavaScript functions. Then we use openDatabase() to do just that, passing it four parameters:

  • Short name for the database.

  • Database version number.

  • Display name for the database.

  • Maximum size of the database.

SQLite creates the database if it does not exist. The anonymous function executes an SQLite transaction that creates or opens a table called “notes”. Each row of that table represents a note with the following columns:

  • INTEGER id KEY: unique identifier auto-incremented by SQLite.

  • DATE lastedit: date the note was last edited.

  • TEXT title: title of the note.

  • TEXT note: contents of the note.

Once the table is opened, we call a function, refreshNotes(), described below, that will update the list displayed on the “home” screen.

We next define the “addNote()” function, which gets invoked when the user touches Save Note on the “addNote” screen. The user already has entered the text for title and note, so we now want to insert a suitable record into the “notes” table. We get the date from the Date() function, and use jQuery to locate the title and note input elements. If you're not familiar with jQuery, it uses a CSS-like syntax to identify DOM elements. In this case, it finds the elements with classes “title” and “note”, and the .val() function assigns their values to JavaScript variables “title” and “note”, respectively. Using the client-side database API to start a transaction, transaction.executeSql() takes four parameters:

  • SQL string: template for the SQL to be executed.

  • Array of parameters whose values replace the ? marks in the SQL template.

  • Function that executes if the operation was successful—in this case, an anonymous function.

  • Function that executes if there was an error—in this case, errorHandler().

If successful, we clear out the values in the input elements (ready for the next add), refresh the list of notes so the new one will show up and use a jQTouch function, jQT.goBack() to return to the “home” screen. Since we used “slideup” to show the “addNote” screen, jQTouch is smart enough to slide it down to return to “home”. We then return “false” to the browser, as we don't need it to continue.

We now define the errorHandler() function, which we reuse for all database transactions in the script. It displays an alert box with the error message and returns.

Next is the refreshNotes() function. We use jQuery to find all the <li> elements on the “home” screen and remove them. Then, we execute a database transaction to find all the note records, and use the “home” screen template to create a list item for each note and insert it into the “home” screen. We add a .click function to each list item that will take users to the “editNote” screen when they click on a note title.

The replaceNoteById(), editNoteById() and deleteNoteById() functions are all very similar to addNote(), with the SQL template changed appropriately.

Running the App

The application can be run from the browser on an iPhone or Android device. As expected from looking at HTML5 features on different phones, Webnotes works fine on the iPhone, iTouch, iPad and Droid. It does not work on the G1, because that phone doesn't support client-side database transactions.

______________________

Rick Rogers has been a professional embedded developer for more than 30 years. Now specializing in mobile application software, when Rick isn't writing software for a living, he's writing books and magazine articles like this one.

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Android compatibility.

tuna's picture

I'm getting my feet wet with mobile development and am trying to get this working. I'm developing on an HTC Droid Eris with Android 2.1. All I'm getting is a black screen. Any one have any ideas if it should work? Or where I should be looking?

"well documented elsewhere"

Anonymous's picture

"Installation of the tools is well documented elsewhere. The Resources section for this article gives pointers to the download URLs."

The resources indeed gives the pointers to the download URLs.
Is there also a link available for "well documented elsewhere"?
Is there also a pointer to the installation of the tools?

Regards,
M. Moon

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