vega-view

vega-view

View component and transforms for Vega visualizations. A View instantiates an underlying dataflow graph and provides a component for rendering and interacting with a visualization. When initialized with a container DOM element, the View adds a Canvas or SVG-based visualization to a web page. Alternatively, a View can be used either client-side or server-side to export static SVG or PNG (Canvas) images.

View API Reference

• View Construction
• View Configuration
• Dataflow and Rendering
• Signals
• Event Handling
• Image Export
• Data

View Construction

Methods for constructing and deconstructing views. In addition to the methods described below, View instances also inherit all (non-overidden) methods of the vega-dataflow Dataflow parent class.

# vega.View(runtime[, options]) <>

Constructor that creates a new View instance for the provided Vega dataflow runtime specification. If provided, the options argument should be an object with one or more of the following properties:

• loadOptions: Default options for loading data files or images. See the load method.
• logLevel: Initial log level to use. See the logLevel method.
• renderer: The type of renderer to use ('canvas' or 'svg'). See the renderer method.

The View constructor call is typically followed by a chain of method calls to setup the desired view configuration. At the end of this chain, the run method evaluates the underlying dataflow graph to update and render the visualization.

var view = new vega.View(runtime)
  .logLevel(vega.Warn) // set view logging level
  .initialize(document.querySelector('#view')) // set parent DOM element
  .renderer('svg') // set render type (defaults to 'canvas')
  .hover() // enable hover event processing
  .run(); // update and render the view

# view.finalize() <>

Prepares the view to be removed from a web page. To prevent unwanted behaviors and memory leaks, this method removes any event listeners the visualization has registered on external DOM elements.

View Configuration

Methods for configuring the view state. These methods are often (but not always) invoked immediately after the View constructor, prior to the first invocation of the run method.

# view.initialize([container]) <>

Initializes internal rendering and event handling, then returns this view instance. If the DOM element container is provided, visualization elements (such as Canvas or SVG HTML elements) will be added to the web page under this containing element. If container is not provided, the view will operate in headless mode, and can still generate static visualization images using the image export methods.

# view.logLevel(level) <>

Sets the current log level and returns this view instance. This method controls which types of log messages are printed to the JavaScript console, and is inherited from the Dataflow parent class. The valid level values are vega.None (the default), vega.Warn, vega.Info, vega.Debug. See the logger method in vega-util for more.

# view.renderer(type) <>

Sets the renderer type (e.g., to 'canvas' (the default) or 'svg') and returns this view instance. While typically invoked immediately upon view creation, this method can be called at any time to change the renderer.

Additional renderer types may be used if registered via the renderModule method exported by vega-scenegraph; for an example see the vega-webgl-renderer.

# view.hover([hoverSet, updateSet]) <>

Enables hover event processing and returns this view instance. The optional arguments specify which named encoding sets to invoke upon mouseover and mouseout. The hoverSet defaults to 'hover', corresponding to the "hover" set within a Vega mark specification "encode" block. The updateSet defaults to 'update', corresponding to the "update" set within a Vega mark specification "encode" block. If this method is never invoked, the view will not automatically handle hover events. Instead, the underlying dataflow definition will have to explicitly set up event streams for handling mouseover and mouseout events.

# view.background([color]) <>

Gets or sets the view background color. If no arguments are provided, returns the current background color. If color is specified, this method sets the background color and returns this view instance. This method does not force an immediate update to the view: invoke the run method when ready.

# view.width([width]) <>

Gets or sets the view width, in pixels. If no arguments are provided, returns the current width value. If width is specified, this method sets the width and returns this view instance. This method does not force an immediate update to the view: invoke the run method when ready. This method is equivalent to view.signal('width'[, width]).

# view.height([height]) <>

Gets or sets the view height, in pixels. If no arguments are provided, returns the current height value. If height is specified, this method sets the height and returns this view instance. This method does not force an immediate update to the view: invoke the run method when ready. This method is equivalent to view.signal('height'[, height]).

# view.padding([padding]) <>

Gets or sets the view padding, in pixels. Padding objects take the form {left: 5, top: 5, right: 5, bottom: 5}. If no arguments are provided, returns the current padding value. If padding is specified, this method sets the padding and returns this view instance. This method does not force an immediate update to the view: invoke the run method when ready. This method is equivalent to view.signal('padding'[, padding]).

Dataflow and Rendering

Methods for invoking dataflow evaluation and view rendering.

# view.run([encode]) <>

Evaluates the underlying dataflow graph and returns this view instance. The optional encode argument is a String value indicating the name of a custom "encode" set to run in addition to the standard "update" encoder. If one or more data sets have been queued to be loaded from external files, this method will function asynchronously: the method will initiate file loading and return immediately, and the dataflow graph will be evaluated when file loading completes. Any scenegraph elements modified during dataflow evaluation will automatically be re-rendered in the view.

Internally, this method invokes the run method of the Dataflow parent class, and then additionally performs rendering.

# view.render([update]) <>

Renders the scenegraph and returns this view instance. If no arguments are provided, the entire scenegraph is redrawn. If provided, the update argument should be an array of "dirty" scenegraph items to redraw. Incremental rendering will be performed to redraw only damaged regions of the scenegraph.

During normal execution, this method is automatically invoked by the run method. However, clients may explicitly call this method to (re-)render the scene on demand (for example, to aid debugging).

# view.enqueue(items) <>

Enqueues "dirty" scenegraph items to be re-drawn the next time dataflow evaluation completes. This method is typically invoked by dataflow operators directly to populate a dirty list for incremental rendering.

# view.scenegraph() <>

Returns the Vega scenegraph instance for this view.

Signals

Methods for accessing and updating dataflow signal values.

# view.signal(name[, value]) <>

Gets or sets a dataflow signal. If only the name argument is provided, returns the requested signal value. If value is also specified, updates the signal and returns this view instance. If the signal does not exist, an error will be raised. This method does not force an immediate update to the view: invoke the run method when ready.

# view.state([state]) <>

Gets or sets the state of signals in the dataflow graph. If no arguments are specified, returns an object of name-value mappings for all signals in the dataflow. If the state argument is provided, this method updates the value of all signals included in the input state object, invokes the run method, and returns this view instance.

# view.addSignalListener(name, handler) <>

Registers a listener for changes to the signal with the given name. If the signal does not exist, an error will be raised. When the signal value changes, the handler function is invoked with two arguments: the name of the signal and the new signal value. Listeners will be invoked when the signal value changes during pulse propagation (e.g., after view.run() is called).

To remove a listener, use the removeSignalListener method.

view.addSignalListener('width', function(name, value) {
  console.log('WIDTH: ' + value);
});
view.width(500).run(); // listener logs 'WIDTH: 500'

# view.removeSignalListener(name, handler) <>

Removes a signal listener registered with the addSignalListener method. If the signal does not exist, an error will be raised. If the signal exists but the provided handler is not registered, this method has no effect.

Event Handling

Methods for generating new event streams and registering event listeners. See also the hover method.

# view.events(source, type[, filter]) <>

Returns a new EventStream for a specified source, event type, and optional filter function. The source should be one of "view" (to specify the current view), "window" (to specify the browser window object), or a valid CSS selector string (that will be passed to document.querySelectorAll). The event type should be a legal DOM event type. If provided, the optional filter argument should be a function that takes an event object as input and returns true if it should be included in the produced event stream.

Typically this method is invoked internally to create event streams referenced within Vega signal definitions. However, callers can use this method to create custom event streams if desired. This method assumes that the view is running in a browser environment, otherwise invoking this method may have no effect.

# view.addEventListener(type, handler) <>

Registers an event listener for input events. The event type should be a string indicating a legal DOM event type supported by vega-scenegraph event handlers. Examples include "mouseover", "click", "keydown" and "touchstart". When events occur, the handler function is invoked with two arguments: the event instance and the currently active scenegraph item (which is null if the event target is the view component itself).

All registered event handlers are preserved upon changes of renderer. For example, if the View renderer type is changed from "canvas" to "svg", all listeners will remain active. To remove a listener, use the removeEventListener method.

view.addEventListener('click', function(event, item) {
  console.log('CLICK', event, item);
});

# view.removeEventListener(type, handler) <>

Removes an event listener registered with the addEventListener method.

Image Export

Methods for exporting static visualization images. These methods can be invoked either client-side or server-side.

# view.toCanvas(items) <>

Returns a Promise that resolves to a canvas instance containing a rendered bitmap image of the view. If invoked in a browser, the returned Promise resolves to an HTML5 canvas element. If invoked server-side in node.js, the Promise resolves to a node-canvas Canvas instance.

# view.toSVG(items) <>

Returns a Promise that resolves to an SVG string, providing a vector graphics image of the view.

# view.toImageURL(type) <>

Returns a Promise that resolves to an image URL for a snapshot of the current view. The type argument must be one of 'svg', 'png' or 'canvas'. Both the png and canvas types result in a PNG image. The generated URL can be used to create downloadable visualization images.

// generate a PNG snapshot and then download the image
view.toImageURL('png').then(function(url) {
  var link = document.createElement('a');
  link.setAttribute('href', url);
  link.setAttribute('target', '_blank');
  link.setAttribute('download', 'vega-export.png');
  link.dispatchEvent(new MouseEvent('click'));
}).catch(function(error) { /* error handling */ });

Data

Methods for accessing data sets and performing streaming updates.

# view.data(name) <>

Returns the data set with the given name. The returned array of data objects is a live array used by the underlying dataflow. Callers that wish to modify the returned array should first make a defensive copy, for example using view.data('name').slice().

# view.change(name, changeset) <>

Updates the data set with the given name with the changes specified by the provided changeset instance. This method does not force an immediate update to the view: invoke the run method when ready.

view.change('data', vega.changeset().insert([...]).remove([...]))
    .run()

Internally, this method takes the provided ChangeSet and invokes Dataflow.pulse. See vega-dataflow for more.

# view.insert(name, tuples) <>

Inserts an array of new data tuples into the data set with the given name, then returns this view instance. The input tuples array should contain one or more data objects that are not already included in the data set. This method does not force an immediate update to the view: invoke the run method when ready. Insert can not be used in combination with the remove method on the same pulse; to simultaneously add and remove data use the change method.

Internally, this method creates a ChangeSet and invokes Dataflow.pulse. See vega-dataflow for more.

# view.remove(name, tuples) <>

Removes data tuples from the data set with the given name, then returns this view instance. The tuples argument can either be an array of tuples already included in the data set, or a predicate function indicating which tuples should be removed. This method does not force an immediate update to the view: invoke the run method when ready. Remove can not be used in combination with the insert method on the same pulse; to simultaneously add and remove data use the change method.

For example, to remove all tuples in the 'table' data set with a count property less than five:

view.remove('table', function(d) { return d.count < 5; }).run();

Internally, this method creates a ChangeSet and invokes Dataflow.pulse. See vega-dataflow for more.