# DataView

## Contents

- Overview
- Example
- Construction
- Methods
- Properties
- Getting Data
- Subscriptions

## Overview

A DataView offers a filtered and/or formatted view on a DataSet. One can subscribe to changes in a DataView, and easily get filtered or formatted data without having to specify filters and field types all the time.

## Example

The following example shows how to use a DataView.

    // create a DataSet
    var data = new vis.DataSet();
    data.add([
      {id: 1, text: 'item 1', date: new Date(2013, 6, 20), group: 1, first: true},
      {id: 2, text: 'item 2', date: '2013-06-23', group: 2},
      {id: 3, text: 'item 3', date: '2013-06-25', group: 2},
      {id: 4, text: 'item 4'}
    ]);

    // create a DataView
    // the view will only contain items having a property group with value 1,
    // and will only output fields id, text, and date.
    var view = new vis.DataView(data, {
      filter: function (item) {
        return (item.group == 1);
      },
      fields: ['id', 'text', 'date']
    });

    // subscribe to any change in the DataView
    view.on('*', function (event, properties, senderId) {
      console.log('event', event, properties);
    });

    // update an item in the data set
    data.update({id: 2, group: 1});

    // get all ids in the view
    var ids = view.getIds();
    console.log('ids', ids); // will output [1, 2]

    // get all items in the view
    var items = view.get();

## Construction

A DataView can be constructed as:

    var data = new vis.DataView(dataset, options)

where:

- `dataset` is a DataSet or DataView.
- `options` is an object which can contain the following properties. Note that these properties are exactly the same as the properties available in methods `DataSet.get` and `DataView.get`.

  Name | Type | Default | Description
  --- | --- | --- | ---
  convert | Object.<String, String> | none | An object containing field names as key, and data types as value. By default, the type of the properties of an item are left unchanged. When a field type is specified, this field in the items will be converted to the specified type. This can be used for example to convert ISO strings containing a date to a JavaScript Date object, or convert strings to numbers or vice versa. The available data types are listed in section Data Types.
  fields | String[ ] | Object.<String, String> | none | An array with field names, or an object with current field name and new field name that the field is returned as. By default, all properties of the items are emitted. When `fields` is defined, only the properties whose name is specified in `fields` will be included in the returned items.
  filter | function | none | Items can be filtered on specific properties by providing a filter function. A filter function is executed for each of the items in the DataSet, and is called with the item as parameter. The function must return a boolean. All items for which the filter function returns true will be emitted. See also section Data Filtering.

## Methods

DataView contains the following methods.

Method | Return Type | Description
--- | --- | ---
get([options] [, data]) get(id [,options] [, data]) get(ids [, options] [, data]) | Object | Array | Get a single item, multiple items, or all items from the DataView. Usage examples can be found in section Getting Data, and the available `options` are described in section Data Selection. When no item is found, `null` is returned when a single item was requested, and and empty Array is returned in case of multiple id's.
getDataSet() | DataSet | Get the DataSet to which the DataView is connected.
getIds([options]) | Number[] | Get ids of all items or of a filtered set of items. Available `options` are described in section Data Selection, except that options `fields` and `type` are not applicable in case of `getIds`.
off(event, callback) | none | Unsubscribe from an event, remove an event listener. See section Subscriptions.
on(event, callback) | none | Subscribe to an event, add an event listener. See section Subscriptions.
refresh() | none | Refresh the filter results of a DataView. Useful when the filter function contains dynamic properties, like:
    var data = new vis.DataSet(...);
    var view = new vis.DataView(data, {
      filter: function (item) {
        return item.value > threshold;
      }
    });
In this example, `threshold` is an external parameter. When the value of `threshold` changes, the DataView must be notified that the filter results may have changed by calling `DataView.refresh()`.
setData(data) | none | Replace the DataSet of the DataView. Parameter `data` can be a DataSet or a DataView.

## Properties

DataView contains the following properties.

Property | Type | Description
--- | --- | ---
length | Number | The number of items in the DataView.

## Getting Data

Data of the DataView can be retrieved using the method `get`.

    var items = view.get();

Data of a DataView can be filtered and formatted again, in exactly the same way as in a DataSet. See sections Data Manipulation and Data Selection for more information.

    var items = view.get({
      fields: ['id', 'score'],
      filter: function (item) {
        return (item.score > 50);
      }
    });

## Subscriptions

One can subscribe on changes in the DataView. Subscription works exactly the same as for DataSets. See the documentation on subscriptions in a DataSet for more information.

    // create a DataSet and a view on the data set
    var data = new vis.DataSet();
    var view = new vis.DataView({
      filter: function (item) {
        return (item.group == 2);
      }
    });

    // subscribe to any change in the DataView
    view.on('*', function (event, properties, senderId) {
      console.log('event:', event, 'properties:', properties, 'senderId:', senderId);
    });

    // add, update, and remove data in the DataSet...