for loop not waiting for function to end - microsoft-metro

Question

I have 5 links on a page and i have to check if all are links are working or not. Here is the code
// iterate through each link and check if ti works.
for(var i=0; i < 5; i++) {
var ifLinkWorks = verifyLinkWorks(links[i]);
if(ifLinkWorks){ OK }
else{ error }
}
This is verifyLinkWorks function. It opens a link. After it get opened, it checks if the page is loaded properly
function verifyLinkWorks(link) {
return winjs.Promise(function(complete) {
link.click();
// wait for page to load
return winjs.promise.timeout(4000).then(function () {
// check if page is loaded
var islinkOK = IsPageLoaded();
complete(islinkOK); // i want verifyLinkWorks to return this value
});
});
}
After reaching link.click(), it is not waiting for page to load. Instead it jumps to the if condtion in outer for loop (which makes linkWorks = undefined therefore,gives Error). How to make it wait in the verfifyLinkWorks function.
Thanks in advance...

Answer 1

You'll need to wait for the results of each promise, either all at once, or individually. As the actions are all async in nature, the code can't wait, but it can call a function when it completes all of the work.
Here, I've created an array which will hold each Promise instance. Once the loop has completed, the code waits for all to complete, and then using the array that is passed, checking the result at each index.
// iterate through each link and check if it works.
var verifyPromises = [];
for(var i=0; i < 5; i++) {
verifyPromises.push(verifyLinkWorks(links[i]));
}
WinJS.Promise.join(verifyPromise).done(function(results) {
for(var i=0; i < 5; i++) {
var ifLinkWorks = results[i];
if (ifLinkWorks) { /* OK */ }
else { /* error */ }
}
});
In case the link.click() call fails, I've wrapped it in a try/catch block:
function verifyLinkWorks(link) {
return WinJS.Promise(function(complete, error) {
try {
link.click();
} catch (e) {
complete(false); // or call the error callback ...
}
// wait for page to load, just wait .. no need to return anything
WinJS.Promise.timeout(4000).then(function () {
// check if page is loaded
var islinkOK = IsPageLoaded();
// finally, call the outer promise callback, complete
complete(islinkOK);
});
});
}
If you want to check the validity of a URL, I'd suggest you consider using WinJS.xhr method to perform a HEAD request instead (rfc). With each link variable, you can use a timeout to validate that there's a reasonable response at the URL, without downloading the full page (or switch to a GET and check the response body).
WinJS.Promise.timeout(4000,
WinJS.xhr({
type: 'HEAD',
url: link
}).then(function complete(result) {
var headers = result.getAllResponseHeaders();
}, function error(err) {
if (err['name'] === 'Canceled') {
}
if (err.statusText) {
}
})
);

Answer 2

Ok heres the link to the msdn code sample for win js promise object.
Promise winjs
now going through the code
<button id="start">StartAsync</button>
<div id="result" style="background-color: blue"></div>
<script type="text/javascript">
WinJS.Application.onready = function (ev) {
document.getElementById("start").addEventListener("click", onClicked, false);
};
function onClicked() {
addAsync(3, 4).then(
function complete(res) {
document.getElementById("result").textContent = "Complete";
},
function error(res) {
document.getElementById("result").textContent = "Error";
},
function progress(res) {
document.getElementById("result").textContent = "Progress";
})
}
function addAsync(l, r) {
return new WinJS.Promise(function (comp, err, prog) {
setTimeout(function () {
try {
var sum = l + r;
var i;
for (i = 1; i < 100; i++) {
prog(i);
}
comp(sum);
}
catch (e) {
err(e);
}
}, 1000);
});
}
</script>
you will see the addAsync(3,4).then() function. So all the code is to be kept inside that function in order to have a delayed response . Sorry m using a tab so cannot write it properly.
Also go through link then for winjs promise