How to handle pagination with Knockout

Question

I have a div that is setup to bind to a observeableArray ,but I only want to show at most 50 items from that observeableArray at any given time. I want to handle this with pagination with a previous and next button along with indices on the page to allow users to cycle through pages of items from the collection.
I know I could probably do this with a computedObservable and a custom data binding but I'm not sure how to do it (I'm still a Knockout neophyte).
Can anyone point me in the right direction?

Here is my code (the JS is in TypeScript):

<div class="container-fluid">
    <div class="row-fluid">
        <div class="span12">
            <%=
            if params[:q]
              render 'active_search.html.erb'
            else
              render 'passive_search.html.erb'
            end
            %>
            <%= form_tag("/search", method: "get", :class => "form-search form-inline") do %>
            <%= label_tag(:q, "Search for:") %>
            <%= text_field_tag(:q, nil, class:"input-medium search-query") %>
            <%= submit_tag("Search", :class=>"btn") %>
            <% end %>

            <div class="media" data-bind="foreach: tweetsArray">
                <%= image_tag('twitter-icon.svg', :class=>"tweet_img", :style=>"display:inline;") %>
                <div class="media-body" style="display:inline;">
                    <h4 class="media-heading" data-bind="text: user.screen_name" style="display:inline;"></h4>
                    <span data-bind="text:text" style="display:inline;"></span> <br />
                    <span data-bind="text:'Created at '+created_at"></span> <br />
                </div>
            </div>

            <div class="pagination pagination-centered">
                <ul>
                    <li>
                        <a href="#">Prev</a>
                    </li>
                    <li>
                        <a href="#">1</a>
                    </li>
                    <li>
                        <a href="#">Next</a>
                    </li>
                </ul>
            </div>

        </div>
    </div>
</div>

<script>
    var viewModel = new twitterResearch.TweetViewModel();
    ko.applyBindings(viewModel);

    //TODO: notes to self, use custom binding for pagination along with a computed observable to determine where at in the list you are

    //document.onReady callback function
    $(function() {
        $.getJSON('twitter', {}, function(data) {
            viewModel.pushTweet(data);
            console.log(data.user);
        });
    });
</script>

declare var $: any;
declare var ko: any;

module twitterResearch {
    class Tweet {
        text: string;
        created_at: string;
        coordinates: string;
        user: string;
        entities: string;
        id: number;
        id_str: string;

        constructor(_text: string, _created_at: string, _coordinates: any, _user: any,
                    _entities: any, _id_str: string, _id: number){

            this.text = _text;
            this.created_at = _created_at;
            this.coordinates = _coordinates;
            this.user = _user;
            this.entities = _entities;
            this.id_str = _id_str;
            this.id = _id;
        }
    }

    export class TweetViewModel{

        tweetsArray: any;
        constructor()
        {
            this.tweetsArray = ko.observableArray([]);
        }

        //tweet is going to be the JSON tweet we return
        //from the server
        pushTweet(tweet)
        {
            var _tweet = new Tweet(tweet.text, tweet.created_at, tweet.coordinates,
                                    tweet.user, tweet.entities, tweet.id_str, tweet.id);
            this.tweetsArray.push(_tweet);
            this.tweetsArray.valueHasMutated();
        }
    }
}

Answer 1

Pagination is quite simple with Knockout. I would personally achieve it this way:

• Have an observableArray containing all your elements
• Have an observable containing the current page (initialized to 0)
• Have a variable declaring the number of elements per page
• Have a computed that returns the number of pages, calculated thanks to the number of elements per page and the total number of elements.
• Finally, add a computed that slices the array containing all the elements.

Given that, you can now add a function that increments (next) or decrements (previous) the current page.

Here is a quick example:

var Model = function() {
    var self = this;
    this.all = ko.observableArray([]);
    this.pageNumber = ko.observable(0);
    this.nbPerPage = 25;
    this.totalPages = ko.computed(function() {
        var div = Math.floor(self.all().length / self.nbPerPage);
        div += self.all().length % self.nbPerPage > 0 ? 1 : 0;
        return div - 1;
    });

    this.paginated = ko.computed(function() {
        var first = self.pageNumber() * self.nbPerPage;
        return self.all.slice(first, first + self.nbPerPage);
    });

    this.hasPrevious = ko.computed(function() {
        return self.pageNumber() !== 0;
    });

    this.hasNext = ko.computed(function() {
        return self.pageNumber() !== self.totalPages();
    });

    this.next = function() {
        if(self.pageNumber() < self.totalPages()) {
            self.pageNumber(self.pageNumber() + 1);
        }
    }

    this.previous = function() {
        if(self.pageNumber() != 0) {
            self.pageNumber(self.pageNumber() - 1);
        }
    }
}

You'll find a simple and complete example here: http://jsfiddle.net/LAbCv/ (might be a bit buggy, but the idea is there).

Answer 2

Actually I am working on a website, which has a lot of tables (most of them need paging).
So actually, I needed some reusable-component for paging to use it in all the cases which I need paging.
Also, I needed more advanced features than which provided in the accepted answer to this question.

So I developed my own component to solving this issue, here it is.

Now on Github

JsFiddle

And for more details, continue reading (Please consider to take the code from GitHub, not from here, as the GitHub code was updated and enhanced since I put it here)

JavaScript

function PagingVM(options) {
    var self = this;

    self.PageSize = ko.observable(options.pageSize);
    self.CurrentPage = ko.observable(1);
    self.TotalCount = ko.observable(options.totalCount);

    self.PageCount = ko.pureComputed(function () {
        return Math.ceil(self.TotalCount() / self.PageSize());
    });

    self.SetCurrentPage = function (page) {
        if (page < self.FirstPage)
            page = self.FirstPage;

        if (page > self.LastPage())
            page = self.LastPage();

        self.CurrentPage(page);
    };

    self.FirstPage = 1;
    self.LastPage = ko.pureComputed(function () {
        return self.PageCount();
    });

    self.NextPage = ko.pureComputed(function () {
        var next = self.CurrentPage() + 1;
        if (next > self.LastPage())
            return null;
        return next;
    });

    self.PreviousPage = ko.pureComputed(function () {
        var previous = self.CurrentPage() - 1;
        if (previous < self.FirstPage)
            return null;
        return previous;
    });

    self.NeedPaging = ko.pureComputed(function () {
        return self.PageCount() > 1;
    });

    self.NextPageActive = ko.pureComputed(function () {
        return self.NextPage() != null;
    });

    self.PreviousPageActive = ko.pureComputed(function () {
        return self.PreviousPage() != null;
    });

    self.LastPageActive = ko.pureComputed(function () {
        return (self.LastPage() != self.CurrentPage());
    });

    self.FirstPageActive = ko.pureComputed(function () {
        return (self.FirstPage != self.CurrentPage());
    });

    // this should be odd number always
    var maxPageCount = 7;

    self.generateAllPages = function () {
        var pages = [];
        for (var i = self.FirstPage; i <= self.LastPage() ; i++)
            pages.push(i);

        return pages;
    };

    self.generateMaxPage = function () {
        var current = self.CurrentPage();
        var pageCount = self.PageCount();
        var first = self.FirstPage;

        var upperLimit = current + parseInt((maxPageCount - 1) / 2);
        var downLimit = current - parseInt((maxPageCount - 1) / 2);

        while (upperLimit > pageCount) {
            upperLimit--;
            if (downLimit > first)
                downLimit--;
        }

        while (downLimit < first) {
            downLimit++;
            if (upperLimit < pageCount)
                upperLimit++;
        }

        var pages = [];
        for (var i = downLimit; i <= upperLimit; i++) {
            pages.push(i);
        }
        return pages;
    };

    self.GetPages = ko.pureComputed(function () {
        self.CurrentPage();
        self.TotalCount();

        if (self.PageCount() <= maxPageCount) {
            return ko.observableArray(self.generateAllPages());
        } else {
            return ko.observableArray(self.generateMaxPage());
        }
    });

    self.Update = function (e) {
        self.TotalCount(e.TotalCount);
        self.PageSize(e.PageSize);
        self.SetCurrentPage(e.CurrentPage);
    };

    self.GoToPage = function (page) {
        if (page >= self.FirstPage && page <= self.LastPage())
            self.SetCurrentPage(page);
    }

    self.GoToFirst = function () {
        self.SetCurrentPage(self.FirstPage);
    };

    self.GoToPrevious = function () {
        var previous = self.PreviousPage();
        if (previous != null)
            self.SetCurrentPage(previous);
    };

    self.GoToNext = function () {
        var next = self.NextPage();
        if (next != null)
            self.SetCurrentPage(next);
    };

    self.GoToLast = function () {
        self.SetCurrentPage(self.LastPage());
    };
}

HTML

<ul data-bind="visible: NeedPaging" class="pagination pagination-sm">
    <li data-bind="css: { disabled: !FirstPageActive() }">
        <a data-bind="click: GoToFirst">First</a>
    </li>
    <li data-bind="css: { disabled: !PreviousPageActive() }">
        <a data-bind="click: GoToPrevious">Previous</a>
    </li>

    <!-- ko foreach: GetPages() -->
    <li data-bind="css: { active: $parent.CurrentPage() === $data }">
        <a data-bind="click: $parent.GoToPage, text: $data"></a>
    </li>
    <!-- /ko -->

    <li data-bind="css: { disabled: !NextPageActive() }">
        <a data-bind="click: GoToNext">Next</a>
    </li>
    <li data-bind="css: { disabled: !LastPageActive() }">
        <a data-bind="click: GoToLast">Last</a>
    </li>
</ul>

Features

1. Show on need
   When there is no need for paging at all (for example the items which need to display less than the page size) then the HTML component will disappear.
   This will be established by statementdata-bind="visible: NeedPaging".

2. Disable on need
   for example, if you are already selected the last page, why the last page or the Next button should be available to press?
   I am handling this and in that case I am disabling those buttons by applying the following binding data-bind="css: { disabled: !PreviousPageActive() }"
   

3. Distinguish the Selected page
   a special class (in this case called active class) is applied on the selected page, to make the user know in which page he/she is right now.
   This is established by the binding data-bind="css: { active: $parent.CurrentPage() === $data }"

4. Last & First
   going to the first and last page is also available by simple buttons dedicated to this.

5. Limits for displayed buttons
   suppose you have a lot of pages, for example, 1000 pages, then what will happen? would you display them all for the user? absolutely not you have to display just a few of them according to the current page. for example, showing 3 pages before and other 3 pages after the selected page.
   This case has been handled here <!-- ko foreach: GetPages() -->
   the GetPages function applying a simple algorithm to determine if we need to show all the pages (the page count is under the threshold, which could be determined easily), or to show just some of the buttons.
   you can determine the threshold by changing the value of the maxPageCount variable
   Right now I assigned it as the following var maxPageCount = 7; which mean that no more than 7 buttons could be displayed for the user (3 before the SelectedPage, and 3 after the Selected Page) and the Selected Page itself.
   
   You may wonder, what if there were not enough pages after OR before the current page to display? do not worry I am handling this in the algorithm
   for example, if you have 11 pages and you have maxPageCount = 7 and the current selected page is 10, Then the following pages will be shown
   
   5,6,7,8,9,10(selected page),11
   
   so we always stratifying the maxPageCount, in the previous example showing 5 pages before the selected page and just 1 page after the selected page.

6. Selected Page Validation
   All set operation for the CurrentPage observable which determine the selected page by the user, is going through the function SetCurrentPage. In only this function we set this observable, and as you can see from the code, before setting the value we make validation operations to make sure that we will not go beyond the available page of the pages.

7. Already clean
   I use only pureComputed not computed properties, which means you do not need to bother yourself with cleaning and disposing of those properties. Although, as you will see in the example below, you need to dispose of some other subscriptions which are outside of the component itself

NOTE 1
You may notice that I am using some bootstrap classes in this component, This is suitable for me, but , of course, you can use your own classes instead of the bootstrap classes.
The bootstrap classes which I used here are pagination, pagination-sm, active and disabled
Feel free to change them as you need.

NOTE 2
So I introduced the component for you, It is time to see how it could work.
You would integrate this component into your main ViewModel as like this.

function MainVM() {
    var self = this;

    self.PagingComponent = ko.observable(new Paging({
        pageSize: 10,      // how many items you would show in one page
        totalCount: 100,   // how many ALL the items do you have.
    }));

    self.currentPageSubscription = self.PagingComponent().CurrentPage.subscribe(function (newPage) {
        // here is the code which will be executed when the user changes the page.
        // you can handle this in the way you need.
        // for example, in my case, I am requesting the data from the server again by making an ajax request
        // and then updating the component

        var data = /*bring data from server , for example*/
        self.PagingComponent().Update({

            // we need to set this again, why? because we could apply some other search criteria in the bringing data from the server, 
            // so the total count of all the items could change, and this will affect the paging
            TotalCount: data.TotalCount,

            // in most cases we will not change the PageSize after we bring data from the server
            // but the component allows us to do that.
            PageSize: self.PagingComponent().PageSize(),

            // use this statement for now as it is, or you have to made some modifications on the 'Update' function.
            CurrentPage: self.PagingComponent().CurrentPage(),
        });
    });

    self.dispose = function () {
        // you need to dispose the manual created subscription, you have created before.
        self.currentPageSubscription.dispose();
    }
}

Last but not least, Sure do not forget to change the binding in the HTML component according to your special viewModel, or wrap all the component with the with binding like this

<div data-bind="with: PagingComponent()">
    <!-- put the component here -->
</div>

Cheers

Answer 3

I have created a blogpost with detailed explanation on how to create pagination with the help of a little JQuery plugin (here).

Basically, I have used normal knockout data binding with AJAX and after data has been retrieved from the server, I call the plugin. You can find the plugin here. It's called Simple Pagination.