HTML5 Database API : Synchronous request

Question

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

I like this question. It states the problem very well.

Phillip Senn
– Phillip Senn

2011-10-12 18:58:11 +00:00
Commented Oct 12, 2011 at 18:58 — Phillip Senn
– Phillip Senn, Commented Oct 12, 2011 at 18:58

bobince · Accepted Answer · 2010-10-29 14:50:46Z

8

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);
    }
});

answered Oct 29, 2010 at 14:50

bobince

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1 Comment

Phillip Senn Over a year ago

Are you returning callback, or calling it?

Julien · Accepted Answer · 2010-12-23 04:05:41Z

8

I am the only one to find this asynchronous request ridiculous ? More over, it seems that Safari implements only the asynchronous model right now... I wonder how we efficiently code like that...

I would enjoy any link to serious programing with the async db driver.

answered Dec 23, 2010 at 4:05

Julien

811 bronze badge

Comments

ivanceras · Accepted Answer · 2011-11-19 22:34:48Z

-1

You have to block the next execution when you intend to retrieve the results synchronously, The price you have to pay is the UI getting blocked during the execution.

var ret = null;
var finished = false;

cfunction isStarted(oDB) {
         oDB.query(sql,params,function(transaction,result) {
                ret = result;
                finished = true;
         });

    while(!finished){
    ;//block next execution, while result is being fetched
    }
    return ret;
}

answered Nov 19, 2011 at 22:34

ivanceras

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1 Comment

keppla Over a year ago

This seems not to work, at least in Chromium 17.0.963.56. It's also what i would expect, since JS doesnt really have Threads, an so, not only the UI is blocked during the while-loop, but the actual fetching, too.

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