Returning value from file read with WinJS for use in page

Question

I currently have an issue with a file read in a Windows 8/WinRT application. I have a simple navigation style app, several pages have access to the same data and I have a data.js file that defines a namespace (Data) with a number of members. One part of the application saves items to a txt file stored in the applications local data folder. But on some of the other pages I need to read this in or check for the existence of an item within the list of previously saved items. To do this I added another method into the data.js file. The trouble is, when I call this method to check for the existence of an item, it doesn't return the value straight away due to the async nature, but the rest of code in the page specific js file still seems to execute before it jumps back into the parsing. This means that the logic to check for an item doesn't seem to work. I have a feeling it's down to my use of either .done or .then but my code is as follows:

  DATA.JS
   var doesItemExist= function(item_id){

    var appFolder = Windows.Storage.ApplicationData.current.localFolder;
   //note I've tried this with and without the first "return" statement
   return  appFolder.getFileAsync(dataFile).then(function (file) {
        Windows.Storage.FileIO.readTextAsync(file).done(function (text) {
            try {
                var json = JSON.parse(text);
                if (json) {
                    for (var i = 0; i < json.items.length; i++) {
                        var temp_item = json.items[i];
                        if (temp_item.id === item_id) {
                            return true;
                            break;
                        }
                    }
                } else {
                    return false;
                }
            } catch (e) {
                return false;
                console.log(e);
            }
        }, function (e) { return false;console.log(e); });
    }, function (e) { // error handling
        return false;
        console.log(e);

    });
}
 WinJS.Namespace.define("Data", {
    doesItemExist: doesItemExist
}); //all of the above is wrapped in a self executing function

Then on Page.js I have the following:

    var add = document.getElementById('add');
        if (Data.doesItemExist(selected_item.id)) {
            add.style.display = 'block';
        } else {
            add.style.display = 'none';
        }

All the variables here are assigned and debugging doesn't produce any errors, control just appears to go back to the if/else statement after it hits the getFileAsync but before it even goes through the for loop. But subsequently it does go in to the for loop but after the if statement has finished. I'm guessing this is down to the async nature of it all, but I'm not sure how to get around it. Any ideas?

thanks

Answer 1

A Promise should work here.

I created a new Navigation app, and added a Data.js file containing the following code:

(function () {
    var appData = Windows.Storage.ApplicationData;

    function doesItemExist(item_id) {
        return new WinJS.Promise(
            function (completed, error, progress) {
                var exists = false;
                appData.current.localFolder.createFileAsync("data.txt", Windows.Storage.CreationCollisionOption.openIfExists).then(
                    function (file) {
                        Windows.Storage.FileIO.readTextAsync(file).then(
                            function (fileContents) {
                                if (fileContents) {
                                    if (fileContents = "foo!") {
                                        completed(true);
                                    }
                                    else {
                                        completed(false);
                                    }
                                }
                                else {
                                    completed(false);
                                }
                            }
                        );
                    },
                    function (e) {
                        error(e);
                    }
                );
            }
        );
    }

    WinJS.Namespace.define("Data", {
        doesItemExist: doesItemExist
    });
})();

Note that I've simplified the code for retrieving and parsing the file, since that's not really relevant to the problem. The important part is that once you've determined whether the item exists, you call completed(exists) which triggers the .then or .done of the Promise you're returning. Note that you'd call error(e) if an exception occurs, as I'm doing if there's an exception from the call to createFileAsync (I use this call rather than getFileAsync when I want to be able to either create a file if it does not exist, or return the existing file if it does, using the openIfExists option).

Then, in Home.js, I added the following code to the ready handler:

var itemExists;

var itemExistsPromise = Data.doesItemExist(42);
itemExistsPromise = itemExistsPromise.then(function (exists) {
    itemExists = exists;
    var content = document.getElementById("content");
    content.innerText = "ItemExists is " + itemExists;
});

itemExistsPromise.done(function () {
    var a = 42;
});

var b = 0;

The code above sets the variable itemExistsPromise to the returned promise from the function in Data.js, and then uses an anonymous function in the .then function of the Promise to set the variable itemExists to the Boolean value returned from the doesItemExist Promise, and grabs the <p> tag from Home.html (I added an id so I could get to it from code) and sets its text to indicate whether the item exists or not). Because I'm calling .then rather than .done, the call returns another promise, which is passed into the itemExistsPromise variable.

Next, I call itemExistsPromise.done to do any work that has to wait until after the work performed in the .then above it.

If you set a breakpoint on the lines "var a = 42" and "var b = 0" (only included for the purpose of setting breakpoints) as well as on the line "itemExists = exists", you should find that this gives you the control you need over when the various parts are executed.

Hope that helps!