HTML5 Web SQL Transactions Skipped Without Error When Touch Triggered In IOS
Solution 1:
Our testing showed that this behaviour would also occur whenever the keyboard was displaying and would prevent transactions in an onblur or onfocus event (although, not onkey{down|up|press}). Using setTimeout would cause terrible performance issues. We had found that the .transaction() success callback would be triggered, even if the transaction callback was not called, so came up with this method that works well for us:
var oldOpenDatabase = window.openDatabase;
window.openDatabase = function() {
var db = oldOpenDatabase.apply(window, arguments);
var oldTrans = db.transaction;
db.transaction = function(callback, err, suc) {
var db = this;
var params = arguments;
var hasRun = false;
oldTrans.call(db, function(tx){
hasRun = true; callback(tx);
}, err||function(){}, function(tx) {
if(hasRun){
if (suc != undefined)
suc(tx);
return;
}
else {
oldTrans.apply(db, params);
}
});
}
return db;
}
This code checks that the transaction callback has fired in the success handler and, if not, gives it another chance.
Solution 2:
Web Workers and OpenDatabaseSync fix the same problem on my site. Except web workers are not supported on iOS4.x and Android so we can implement the old way for those and web workers on iOS5.
The very basic example below uses a loop to do a single insert 500 times writing to the screen after each insert. At the end a query is made to return the number of rows in the table. Now we can see transactions are not skipped even with a lot of scrolling.
This example is designed so you can test the resilience to scrolling during the inserts on iPhone or iPad.
Two pages are required, an html page and separate javascript file for the worker. So for the html page;
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0" name="viewport"/>
<meta name="apple-mobile-web-app-capable" content="yes" />
<meta name="apple-mobile-web-app-status-bar-style" content="black" />
<title></title>
<script src="../scripts/jquery-1.6.2.min.js" type="text/javascript"></script>
<style type="text/css">
body
{
background-color:Black;
}
.number
{
color:#00FF00;
font-family:Courier New;
font-size:12pt;
}
button
{
position:fixed;
right:5px;
top:5px;
padding:15px;
}
</style>
<script type="text/javascript">
var worker;
$(document).ready(function () {
worker = new Worker('worker1.js');
worker.addEventListener('message', function (e) {
$("#test").html(new Date().toLocaleTimeString() + " " + e.data.message + "<BR/>" + $("#test").html());
if (e.data.complete == true) {
worker.terminate();
}
}, false);
});
function stop() {
worker.terminate();
}
</script>
</head>
<body>
<form id="form1" onsubmit="return false">
<div id="test" class="number">
<button onclick="stop()">Stop</button>
</div>
</form>
</body>
</html>
and worker1.js ;
var wdb = {};
wdb.db = openDatabaseSync('test', '1.0', 'DB test', 1024);
doQuery("CREATE TABLE IF NOT EXISTS test(abc INTEGER PRIMARY KEY);");
doQuery("DELETE FROM test"); // optional
for (var i = 0; i < 500; i++) {
doQuery("INSERT INTO test(abc) VALUES( " + i + " );");
postMessage({ message: "Inserted " + i.toString(), complete: false });
}
var summary= doQuery("SELECT COUNT(*) AS Total FROM test;");
postMessage({ message: "Found " + summary.item(0).Total.toString() + " rows", complete: true });
function doQuery(sql) {
wdb.db.transaction(function (tx) {
rs = tx.executeSql(sql, []);
});
return rs.rows;
}
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