Creating Custom Widgets

Kendo UI provides options for you to create your own widgets by inheriting from the base widget class.

Getting Started

Extend the base Kendo UI widget class in the kendo.ui namespace.

The following example demonstrates how to create variables to hold values which also helps with the minification. The entire process is wrapped in a self-executing anonymous function so as to protect the global namespace. jQuery is passed in as a reference to make sure $ is jQuery. The widget itself extends the base widget class so it is given the uppercase name of MyWidget—or whatever the name of your widget is for that matter. This is generally considered a best practice when naming classes in JavaScript as opposed to regular objects.
```
(function($) {
    // Shorten references to variables which is better for uglification. kendo = window.kendo,
        ui = kendo.ui,
        Widget = ui.Widget

    var MyWidget = Widget.extend({
        // The initialization code goes here.
    });

})(jQuery);
```
Provide an init method for your widget. This method is called by the framework when the widget is initialized. This init function takes two parameters. The first one is the element on which you are initializing the widget. The second one is a set of options that you are going to specify shortly. These will be configuration values.
```
var MyWidget = Widget.extend({

    init: function(element, options) {

        // The base call to initialize the widget.
        Widget.fn.init.call(this, element, options);

    }
});
```

If you are extending a widget, the call to the base is what translates your widget from declarative initialization or the standard imperative initialization, and merges all the base options and custom options. Declare those options right under the init statement. Anything that you declare in the options object will be available for the user to pass as either a configuration value or as a data attribute.

var MyWidget = Widget.extend({

    init: function(element, options) {

        // The base call to initialize the widget.
        Widget.fn.init.call(this, element, options);
    },

    options: {
        // The name is what it will appear as the kendo namespace(i.e. kendo.ui.MyWidget).
        // The jQuery plugin would be jQuery.fn.kendoMyWidget.
        name: "MyWidget",
        // Other options go here.
        ...
    }

});

Add the widget to Kendo UI. The following example demonstrates the full boilerplate for creating your own Kendo UI widget and making it available like all other Kendo UI widgets are.

(function($) {

    // Shorten the references to variables. This is better for uglification
      var kendo = window.kendo,
        ui = kendo.ui,
        Widget = ui.Widget

    var MyWidget = Widget.extend({

        init: function(element, options) {

            // The base call to the widget initialization.
            Widget.fn.init.call(this, element, options);

        },

        options: {
             // The name is what it will appear as the kendo namespace(i.e. kendo.ui.MyWidget).
             // The jQuery plugin would be jQuery.fn.kendoMyWidget.
             name: "MyWidget",
            // Other options go here.
            ....
        }

    });

    ui.plugin(MyWidget);

})(jQuery);

To make this widget DataSource- or MVVM-aware, implement some additional items. The following section discusses the process of creation a DataSource-aware widget. The MVVM part is tackled later on in this article. The widget that is demonstrated is a simple one that just repeats the data in the DataSource and also allows you to specify your own custom template. You can regard it as an extremely dumbed-down ListView which, for an easier handling, is named the Repeater.

To make your widget aware of a Data Source, use the created convenience method on the DataSource base object. The code snippet offers flexibility in the way you initialize the DataSource for your widget. If you actually create a new DataSource either outside your widget initialization or inline, that.DataSource is returned.
```
that.dataSource = kendo.data.DataSource.create(that.options.dataSource);
```
Create a new DataSource to bind the widget. This step is not a must because you can set the DataSource to an array as demonstrated in the following example. If you pass this array, the kendo.data.DataSource.create method will create a new DataSource based on the data in this array and returns it to that.dataSource.
```
$("#div").kendoRepeater({
    dataSource: ["Item 1", "Item 2", "Item 3"]
});
```
Create a DataSource by specifying its configuration values inline as demonstrated in the following example. The example specifies a DataSource configuration but does not actually create a DataSource instance. The kendo.data.DataSource.create(that.options.dataSource) takes this configuration object and returns a new DataSource instance with the specified configuration.

To replicate the Kendo UI MultiSelect data-binding behavior, explicitly assign the kendo.data.binders.widget.multiSelectCustom = kendo.data.binders.widget.multiselect; binding.
```
$("#div").kendoRepeater({
    dataSource: {
        transport: {
            read: {
                url: "http://mydomain/customers"
            }
        }
    }
});
```

Handling Events

Bind to your DataSource a change event and handle it. This is where you mutate your DOM based on the data read from the DataSource. Typically, this is done in a refresh method. Make it public, so that you or someone else is able to call it on the widget at some point after initialization.

// Bind to the change event to refresh the widget.
that.dataSource.bind("change", function() {
    that.refresh();
});

The way the widget code now looks is demonstrated in the following example. Note that when you bind to the change event on the DataSource, you actually bind to the string value of "change". As a best practice, assign these as constants at the top of the widget, and then refer to the constant. The entire DataSource configuration is also moved into its own method. This is because that signifies the widget since it is the calling object. You can reference all widget properties off of the that object after assigning that to this.

  (function($) {

      var kendo = window.kendo,
          ui = kendo.ui,
          Widget = ui.Widget,
          CHANGE = "change";

      var Repeater = kendo.ui.Widget.extend({
          init: function(element, options) {
              var that = this;
              kendo.ui.Widget.fn.init.call(that, element, options);
              // Initialize or create dataSource.
              that._dataSource();
          },
          options: {
              name: "Repeater"
          },
          _dataSource: function() {
              var that = this;
              // Returns the datasource OR creates one if using an array or a configuration.
              that.dataSource = kendo.data.DataSource.create(that.options.dataSource);

              // Bind to the change event to refresh the widget.
              that.dataSource.bind(CHANGE, function() {
                  that.refresh();
              });
          }
      });

      kendo.ui.plugin(Repeater);

  })(jQuery);

Fetch from the DataSource if necessary by checking for the autoBind configuration value of that.options. Then, call that.dataSource.fetch(). Note that fetch is different from read because it only populates the DataSource if the DataSource is not yet read from. If a read is previously called on DataSource before the widget is initialized, you do not have to make the DataSource read again.

_dataSource: function() {
    var that = this;

    // Returns the datasource OR creates one if using array or configuration.
    that.dataSource = kendo.data.DataSource.create(that.options.dataSource);

    // Bind to the change event to refresh the widget.
    that.dataSource.bind(CHANGE, function() {
        that.refresh();
    });

    // Trigger read on the dataSource if one has not happened yet.
    if (that.options.autoBind) {
        that.dataSource.fetch();
    }
}

Add the autoBind configuration to the options object on the widget and give it a default value of true. All data-bound widgets in Kendo UI do autoBind by default.
```
options: {
    name: "Repeater",
    autoBind: true
}
```

Rendering Widgets with Templates

The HTML that is output by widgets is rendered over the Kendo UI templates. They allow you to pre-compile HTML and inject data or expressions, which are evaluated, into the HTML and a DOM fragment is returned as an HTML string. Nearly all widgets in Kendo UI allow you to specify some kind of a template in addition to the default template that a widget uses. To do this, first add the template to the options object and set its value to an empty string. Contrary to other configuration settings, do not set its default value here.
```
options: {
    name: "Repeater",
    autoBind: true,
    template: ""
}
```
Set the default value by adding a line directly under the call to the base widget initialization. This pre-compiles the template passed in by the user, or uses a default template. In the case of this Repeater, write out strong tags wrapped in paragraphs, and then reference the data object, which will be a string if you pass an array of strings. If you pass objects to the template, the default template renders [object Object].

that.template = kendo.template(that.options.template || "

#= data #
")

Implementing refresh Functions

Since you bound to the change method, you need to implement the refresh public function that will be called when DataSource changes or when it is called directly. Inside the refresh method is where you are going to mutate the DOM.

First, call that.dataSource.view(), which gives you the data from the DataSource. Next, use kendoRender and pass in a template along with the DataSource data—a.k.a. view. This is how Kendo UI widgets mutate the DOM. The render method applies the data to the DataSource and returns the HTML string.
```
  refresh: function() {
      var that = this,
          view = that.dataSource.view(),
          html = kendo.render(that.template, view);
  }
```
Set the HTML of that.element—the element on which you are initializing your widget. If you are handling initialization on an input and you want to translate or wrap that input within a container, add that logic here before setting its HTML. The that.element is a jQuery wrapped element, so you can simply call the html method directly off of it. The final refresh method looks like the one demonstrated in the following example.
```
refresh: function() {
    var that = this,
        view = that.dataSource.view(),
        html = kendo.render(that.template, view);

    that.element.html(html);
}
```

Having added the final touches from the previous step, now you have a fully data-bound widget. The following example demonstrates the complete code for the Repeater widget.

(function() {
    var kendo = window.kendo,
        ui = kendo.ui,
        Widget = ui.Widget,

    CHANGE = "change";

    var Repeater = Widget.extend({
        init: function(element, options) {
            var that = this;

            kendo.ui.Widget.fn.init.call(that, element, options);
            that.template = kendo.template(that.options.template || "<p><strong>#= data #</strong></p>");

            that._dataSource();
        },
        options: {
            name: "Repeater",
            autoBind: true,
            template: ""
        },
        refresh: function() {
            var that = this,
                view = that.dataSource.view(),
                html = kendo.render(that.template, view);

            that.element.html(html);
        },
        _dataSource: function() {
            var that = this;
            // Returns the datasource OR creates one if using array or configuration object.
            that.dataSource = kendo.data.DataSource.create(that.options.dataSource);

            // Bind to the change event to refresh the widget.
            that.dataSource.bind(CHANGE, function() {
                that.refresh();
            });

            if (that.options.autoBind) {
                that.dataSource.fetch();
            }
        }
    });

    ui.plugin(Repeater);

})(jQuery);

The following example uses two widgets that are initialized. The first one takes a simple array as a DataSource. The second uses a remote endpoint, a template, and declarative initialization.

   <div id="repeater"></div>

    <div id="container">
      <div data-role="repeater" data-source="dataSource" data-template="template"></div>
    </div>

    <script type="text/x-kendo-template" id="template">
    <div style="float: left; color: salmon; margin-right: 10px"><h1>#= data.ProductName #</h1></div>
    </script>

    <script>
      (function() {
        var kendo = window.kendo,
            ui = kendo.ui,
            Widget = ui.Widget,

            CHANGE = "change";

        var Repeater = Widget.extend({
          init: function(element, options) {
            var that = this;

            kendo.ui.Widget.fn.init.call(that, element, options);
            that.template = kendo.template(that.options.template || "<p><strong>#= data #</strong></p>");

            that._dataSource();
          },
          options: {
            name: "Repeater",
            autoBind: true,
            template: ""
          },
          refresh: function() {
            var that = this,
                view = that.dataSource.view(),
                html = kendo.render(that.template, view);

            that.element.html(html);
          },
          _dataSource: function() {
            var that = this;
            // Returns the datasource OR creates one if using array or configuration object.
            that.dataSource = kendo.data.DataSource.create(that.options.dataSource);

            // Bind to the change event to refresh the widget.
            that.dataSource.bind(CHANGE, function() {
              that.refresh();
            });

            if (that.options.autoBind) {
              that.dataSource.fetch();
            }
          }
        });

        ui.plugin(Repeater);

      })(jQuery);

      var dataSource = new kendo.data.DataSource({
        type: "odata",
        transport: {
          read: "https://demos.telerik.com/kendo-ui/service/Northwind.svc/Products"
        }
      });

      kendo.bind($("#container"));

      $("#repeater").kendoRepeater({
        dataSource: [ "item1", "item2", "item3" ]
      });
    </script>

Using MVVM

To make this widget MVVM-aware, you need to define some events. Specifically, expose the dataBinding event and the dataBound event.

The dataBinding event is what you are going to call before you mutate the DOM with your widget. This enables MVVM to traverse the fragment that you are about to mutate, and unbind anything that is currently bound. The second event is the dataBound event, which allows MVVM to go back through the fragment, and re-bind what is necessary. These events are exposed through the events object on the widget. These events are strings, so define them as constants in the head of the widget as part of the pattern Kendo UI uses when developing Kendo UI widgets.

By exposing these as events for MVVM to listen to, you have loose coupling between your widget and the MVVM core engine. This means that if you do not expose these events, MVVM will not be aware of the lifecycle of the widget. This is a very good architecture as it ensures that your widget will not break other MVVM bindings of which it is not aware.
```
  var DATABINDING = "dataBinding",
      DATABOUND = "dataBound",
      CHANGE = "change"

  var Repeater = kendo.ui.Widget.extend({

      init: function(element, options) {
          ...
      },
      options{
          ...
      },

      // The events are used by other widgets or developers - API for other purposes.
      // These events support MVVM bound items in the template for loose coupling with MVVM.
      events: [
          // Call before mutating DOM.
          // MVVM will traverse DOM, unbind any bound elements or widgets.
          DATABINDING,
          // Call after mutating DOM.
          // Traverses DOM and binds ALL THE THINGS.
          DATABOUND
      ]
  });
```

MVVM expects you to expose the DOM fragments from your widget, which represents each row or each repeated data element. You must return the outermost element for MVVM to work with. While it varies, this is typically just this.element.children. Since each rendered template item is a DOM fragment attached to the bound element, this is all you need. Expose it for MVVM by making it available off of the items object.

var DATABINDING = "dataBinding",
    DATABOUND = "dataBound",
    CHANGE = "change"

var Repeater = kendo.ui.Widget.extend({

    init: function(element, options) {
        ...
    },
    options{
        ...
    },

    // The events are used by other widgets or developers - API for other purposes.
    // These events support MVVM bound items in the template. for loose coupling with MVVM.
    events: [
        // Call before mutating DOM.
        // MVVM will traverse DOM, unbind any bound elements or widgets.
        DATABINDING,
        // Call after mutating DOM.
        // Traverses DOM and binds ALL THE THINGS.
        DATABOUND
    ],

    // MVVM expects an array of DOM elements that represent each item of the datasource.
    // Has to be the children of the outermost element.
    items: function() {
        return this.element.children();
    }
});

Since it is possible to change the DataSource by using MVVM, you need to implement the setDataSource function. MVVM calls this when the DataSource is set inside a ViewModel. Set your internal DataSource reference equal to the one passed in by MVVM and then rebuild the DataSource by using the already defined dataSource() function.
```
// For supporting changing the datasource over MVVM.
setDataSource: function(dataSource) {
    // Set the internal datasource equal to the one passed in by MVVM.
    this.options.dataSource = dataSource;
    // Rebuild the datasource if necessary or just reassign.
    this._dataSource();
}
```
You need to make some small tweaks to your method which assigns or builds the DataSource. The _dataSource method that you call in init does three things:
- Assigns the DataSource, or builds on from an array or configuration object.
- Reads from the DataSource, if autoBind is enabled, and the DataSource is not yet read from.
- Binds the change event on the DataSource to an internal refresh method that you handle manually.
Since you have already bound the change event on the DataSource possibly once, make sure you unbind it if necessary. If this is not done, the widget retains a list of all bindings and executes the refresh function numerous times. Also, MVVM will be listening to the internal _refreshHandler function which is not yet defined. You need to point the internal _refreshHandler to your publicly exposed refresh method. First though, check and see if a connection exists between the public refresh, which is bound to the change event on the DataSource, and the internal _refreshHandler. If a connection exists, remove just the binding to the change event. If a connection does not exist between your internal _refreshHandler and the public refresh function, you need to create it. This is done by the $.proxy jQuery method, which calls the public refresh with the correct context, which is the widget itself. Finally, rebind to the change event of the DataSource.

The following example implements the proxy jQuery function. proxy informs that when the _refreshHandler is called, it should execute the public refresh widget function; inside that refresh function, this will be a reference to the widget itself, and not something else, such as a window. Due to the fact that the value of the this keyword is always changing in JavaScript, this approach is a good way to ensure that the scope is correct when the refresh function executes.
```
_dataSource: function() {

    var that = this;

    // If the DataSource is defined and the _refreshHandler is wired up,
  // unbind because you need to rebuild the DataSource.
    if ( that.dataSource && that._refreshHandler ) {
        that.dataSource.unbind(CHANGE, that._refreshHandler);
    }
    else {
        that._refreshHandler = $.proxy(that.refresh, that);
    }

    // Returns the datasource OR creates one if using array or configuration object.
    that.dataSource = kendo.data.DataSource.create(that.options.dataSource);
    // Bind to the change event to refresh the widget.
    that.dataSource.bind( CHANGE, that._refreshHandler );

    if (that.options.autoBind) {
        that.dataSource.fetch();
    }
}
```

Trigger the dataBinding and dataBound events in the public refresh. Note that dataBinding happens before you mutate the DOM and dataBound happens directly after that.

refresh: function() {
    var that = this,
        view = that.dataSource.view(),
        html = kendo.render(that.template, view);

    // Trigger the dataBinding event.
    that.trigger(DATABINDING);

    // Mutate the DOM (AKA build the widget UI).
    that.element.html(html);

    // Trigger the dataBound event.
    that.trigger(DATABOUND);
}

Now, you have a fully enabled MVVM in your widget. Define the widget as demonstrated in the following example.

  <div data-role="repeater" data-bind="source: dataSource"></div>
  <script>
      var viewModel = kendo.observable({
          dataSource:  new kendo.data.DataSource({
              transport: {
                  read: "Customers/Orders",
                  dataType: "json"
              }
          })
      });
      kendo.bind(document.body.children, viewModel);
  </script>

Notice that the widget is now bound to the dataSource variable inside of the ViewModel through data-bind. This means that if you add an item to the DataSource client-side, your widget will reflect the change immediately without you having to re-render anything.

In the following complete example, note that when you add an item to the DataSource, it is immediately reflected in the Repeater widget.

```dojo
        <label for="newItem">Enter A New Item</label>
        <input id="newItem" data-bind="value: newItem" class="k-input" />
        <button class="k-button" data-bind="click: add">Add Item</button>

        <div data-role="repeater" data-bind="source: items" data-template="template"></div>

        <script type="text/x-kendo-template" id="template">
            <div style="color: salmon; margin-right: 10px'"><h1>#= data #</h1></div>
        </script>

<script>
var viewModel = kendo.observable({
    items: ["item1", "item2", "item3"],
    newItem: null,
    add: function(e) {
        if (this.get("newItem")) {
            this.get("items").push(this.get("newItem"));
        }
    }
});

kendo.bind(document.body, viewModel);
</script>

```

Working with Value-Bound Widgets

In order for a widget to support value binding, you need to:

Add a value method to the widget, which sets the current widget value and returns the current value if no arguments are passed.
Trigger the widget change event when the widget value is changed.

The following examples demonstrate how to create a simple input widget that selects the value on focus.

Create the widget and implement the functionality that you are looking for.

  (function ($) {
      var kendo = window.kendo;

      var SelectedTextBox = kendo.ui.Widget.extend({
        init: function (element, options) {
          kendo.ui.Widget.fn.init.call(this, element, options);

          this.element.on("focus", this._focus);
        },

        options: {
          name: "SelectedTextBox"
        },

        _focus: function () {
          this.select();
        },

        destroy: function () {
          this.element.off("focus", this._focus);
        }
      });

      kendo.ui.plugin(SelectedTextBox);
  })(jQuery);

Add a value method.

  var SelectedTextBox = kendo.ui.Widget.extend({
    ...
    value: function (value) {
      if (value !== undefined) {
        this.element.val(value);
      } else {
         return this.element.val();
      }
    }
  });

Trigger the change event.

  var SelectedTextBox = kendo.ui.Widget.extend({
    init: function (element, options) {
      ...
      this._changeHandler = $.proxy(this._change, this);
      this.element.on("change", this._changeHandler);
    },

    ...

    _change: function () {
      this.trigger("change");
    },

    destroy: function () {
      this.element.off("change", this._changeHandler);
      this.element.off("focus", this._focus);
    }
  });

The following example combines the snippets and exhibits the full code.

    <script>
      (function ($) {
        var kendo = window.kendo;

        var SelectedTextBox = kendo.ui.Widget.extend({
          init: function (element, options) {
            kendo.ui.Widget.fn.init.call(this, element, options);

            this._changeHandler = $.proxy(this._change, this);
            this.element.on("change", this._changeHandler);
            this.element.on("focus", this._focus);
          },

          options: {
            name: "SelectedTextBox"
          },

          _change: function () {
            this._value = this.element.val();
            this.trigger("change");
          },

          _focus: function () {
            this.select();
          },

          value: function (value) {
            if (value !== undefined) {
              this.element.val(value);
            } else {
              return this.element.val();
            }
          },

          destroy: function () {
            this.element.off("change", this._changeHandler);
            this.element.off("focus", this._focus);
          }
        });

        kendo.ui.plugin(SelectedTextBox);
      })(jQuery);
    </script>

    <input type="text" data-role="selectedtextbox" data-bind="value:foo" />

    <script>
      var viewModel = kendo.observable({
        foo: "bar"
      });

      kendo.bind(document.body, viewModel);
    </script>

Note on Technical Support

Custom widgets that inherit from Kendo UI widgets are not subject to technical support service, unless the question or issue can be discussed in the context of the originating widget that is provided in the Kendo UI installer.

Creating Custom Widgets

Getting Started

Handling Events

Rendering Widgets with Templates

Implementing refresh Functions

Using MVVM

Working with Value-Bound Widgets

Note on Technical Support

See Also

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