HTML5 Database API : Synchronous request

Question

Hi, i currently use the client side database on an html5 iphone webapp. In my code i need to check if a row is present in the local DB :

function isStarted(oDB) {
 var ret = null;
 oDB.query(sql,params,function(transaction,result) {
    if(result.rows.length > 0 ) {
        ret = true;
    } else {
        ret = false;
    }
 });

return ret;

}

Unfortunately the return of isStarted() occurs before the callback function and i always get a "null" value. In the W3c spec we can see an "synchronous-database-api" but how can i use it ? Is there a trick to get the good "ret" value with asynchronus requets ?

Thanks for your help

Answer 1

To get an object implementing DatabaseSync you have to call openDatabaseSync(...) instead of openDatabase(...). I don't know about the iPhone, or what the oDB object you have is, but according to spec you only get the openDatabaseSync method in a WebWorker and not in the normal web browser window. Certainly XMLHttpRequest has demonstrated that potentially-length synchronous operations in the UI thread are not a good idea.

It's not possible to run asynchronous code synchronously, or vice versa. To do so you'd need language-level features like threads or co-routines that JavaScript doesn't have. You have to exit your functions and return control to the browser to allow it to perform the HTTP request or database query, and call you back on the handler function you gave it.

So you will have to rewrite your code ‘inside-out’ to pass callback functions instead of expecting return values, every time you do something involving database IO.

function tellMeWhenIsStarted(oDB, callback) {
    oDB.query(sql,params,function(transaction,result) {
        callback(result.rows.length>0);
    }
});