Why is IListDataAdapter.getCount asynchronous

Question

In WinJS the only way to get the count of items in a ListView object is with the method getCount().

But this method is asynchronous.

This make it very difficult to be used in a for loop for example when there is a need to loop through the items of the list.

var listView = document.getElementById("listView").winControl;
listView.itemDataSource.getCount().done(
    function (numItems) {
        for (var i = 0; i < numItems; i++) {
            //do your stuff here
        }
    });

If I put this in any part of my code I can't return the value I read in the loop from any function because the getCount() return a promise, making my function also return a promise and so on...

So my question is why? Isn't the number of items in a list already known when the method is called?

Answer 1

The ListView's data contract allows for asynchronous data sources, and we include a base class VirtualizedDataSource that you can use for fancy scenarios like that. If you are using a WinJS.Binding.List as your data source that API is in fact synchronous and you should be able to say:

listView.itemDataSource.list.length

However, if you're writing generic code that deals with ListView's and doesn't know what kind of data source it will

Answer 2

Have you tried joining promises? If your concern is to iterate all of the items in a ListView by selecting each item by index and then performing some work on them, you can use WinJS.Promise.join to create a single promise that contains the results of all the operations.

For example:

var listView = document.getElementById("listView").winControl;
listView.itemDataSource.getCount().then(
    function (numItems) {
        var joinedPromises = [];
        for (var i = 0; i < numItems; i++) {
            joinedPromises.push(listView.itemDataSource.itemsFromIndex(i, 0, 0));
        }
        return WinJS.Promises.join(joinedPromises);
    }).done(
    function (results) {
        // Operate on each item in the ListView's data source.
    },
    function (err) { 
        // Handle any errors from the joined promises.
    });