Knockout, select all checkboxes and view list of select checkboxes

I have a list with checkboxes, when I click at one of them, it is displayed in theSelectedUsers, works as it should, and checkboxes data-bind="checked: AllChecked" will checked too, but it should not be.

<div data-bind="foreach: SelectedUsers">
    <p>
        <span data-bind="text: userName" ></span>
    </p>
</div>

But, when I use data-bind="checked: AllChecked", nothing is displayed in SelectedUsers, after that, when I click on some checkbox, all items are selected in the list and nothing is displayed in SelectedUsers

What's wrong? example: http://ift.tt/1FPZGuz

<div id="sideB">
    <div data-bind="foreach: AllUsers">
        <label>
            <input type="checkbox" name="checkedUser" data-bind="value: userName, checked:$root.selectedUserNames" />
            <span data-bind="text: userName" ></span>
        </label> 
    </div>
    Selected
    <div data-bind="foreach: SelectedUsers">
        <p>
            <span data-bind="text: userName" ></span>
        </p>
    </div>
        <label><input type="checkbox" data-bind="checked: AllChecked" />Check all</label>
</div>

Knockout-3.3.0:

function User(data) {
this.userName = ko.observable(data.userName);
this.selected = ko.observable(data.selected);

}

var dataSource = [new User({userName: "Bill", selected: false}),
    new User({userName: "Andrii", selected: false})
    ];

function UsersViewModel() {
    var self = this;
    self.AllUsers = ko.observableArray(dataSource);
    self.SelectedUsers = ko.observableArray([]);

self.selectedUserNames = ko.observableArray([]);

self.selectedUserNames.subscribe(function(newValue) {
    var newSelectedUserNames = newValue;
    var newSelectedUsers = [];
    ko.utils.arrayForEach(newSelectedUserNames, function(userName) {
        var selectedUser = ko.utils.arrayFirst(self.AllUsers(), function(user) {
            return (user.userName() === userName);
        });
        newSelectedUsers.push(selectedUser);
    });
    self.SelectedUsers(newSelectedUsers);
});

self.AllChecked = ko.computed({
    read: function() {
        var firstUnchecked = ko.utils.arrayFirst(dataSource, function(item) {
            return self.selectedUserNames() == false;
        });
        return firstUnchecked == null;
    },
    write: function(value) {
        ko.utils.arrayForEach(dataSource, function(item) {
            self.selectedUserNames(value);
        });
    }
})
}

ko.applyBindings(new UsersViewModel(), document.getElementById('sideB'));