A number of JavaScript events have been supported in Shiny after v0.12.2. These events can be used to keep track of the app progress, or even manipulate the values of inputs/outputs. All event names have the prefix shiny:, e.g., shiny:connected. We can listen to these events using .on() in jQuery, e.g.,

$(document).on('shiny:connected', function(event) {
  alert('Connected to the server');
});

When an event is triggered in Shiny, the event object may have some additional properties that can be used to query or modify the information in Shiny, as we will see later in this document. Some events can cancel the process in Shiny, e.g., stop the propogation of an input or output change to the server. Such events include shiny:inputchanged, shiny:message, shiny:value, shiny:error, shiny:updateinput. To cancel the Shiny process, you can use event.preventDefault(), e.g.,

// no outputs will be updated since we canceled the output changes
$(document).on('shiny:value', function(event) {
  event.preventDefault();
});

All events currently supported in Shiny are listed below. You can find a live example at https://gallery.shinyapps.io/107-events (source).

Initial Connection and Disconnection

The events shiny:connected and shiny:disconnected are triggered when an initial connection to server is established, and when a session is ended or the connection is lost for some reason, respectively.

A property socket in the event object is used to store the web socket that is used to communicate between R and JavaScript. For example, you may query the state of the web socket via event.socket.readyState.

Server Status: Busy/Idle

The event shiny:busy is triggered when something is happening on the server (e.g. an observer is running), and the event shiny:idle indicates when the server is idle. The event object does not carry any special properties related to Shiny.

Messages

The event shiny:inputchanged is triggered when an input has a new value, e.g., when you click an action button, or type in a text input. The event object has properties name (the id of the input), value (the value of the input), and inputType (the type of the input, e.g. shiny.action).

For example, suppose you have a numeric input with id foo, you may double its value through this event:

$(document).on('shiny:inputchanged', function(event) {
  if (event.name === 'foo') {
    event.value *= 2;
  }
});

The shiny:message is triggered when any messages are received from the server. The event has a property message, which is the message object (a JavaScript object).

Conditional Panels

When conditional panels (see ?shiny::conditionalPanel) are updated, the event shiny:conditional is triggered on the document.

Binding/Unbinding Inputs/Outputs

All the events above are triggered on the document. There are a few events triggered on specific HTML elements, including the events in the following sections on input and output elements.

When an input or output is bound to Shiny, the event shiny:bound is triggered. Similarly, there is a shiny:unbound event after an input/output is unbound. In these events, the event object has properties binding (the input/output binding object) and bindingType (may be 'input' or 'output' depending on the binding is for an input or output).

Output Events

The shiny:value event is triggered when an output receives a value from the server. The event object has three properties: name (output id), value (output value), and binding (output binding).

The shiny:error event is triggered when an error is propogated to an output. The event also has three properties like the shiny:value event: name, error (the error message), and binding.

The shiny:recalculating and shiny:recalculated events are triggered before and after an output value is recalculated, respectively. Please note shiny:recalculated is triggered after the output value has been recalculated in R, but that does not imply the output value has been displayed on the page. Use shiny:value instead if you want to do something when the output value is rendered.

The shiny:visualchange event is triggered when an output is resized, hidden, or shown. The event object has properties visible (true or false) and binding (the output binding).

Since these events are triggered specifically on an output element, you may add the listener on the output element instead of the document, although the latter also works, e.g.

$('#foo').on('shiny:value', function(event) {
  // append a character string to the output value
  event.value += ' Oh that is nice!';
});

// use event.target to obtain the output element
$(document).on('shiny:value', function(event) {
  // cancel the output of the element with id 'foo'
  if (event.target.id === 'foo') {
    event.preventDefault();
  }
});

Input Events

The shiny:updateinput event is triggered when an input is updated, e.g., when you call updateTextInput() in R to update the label or value of a text input. The event object has properties message (the update message sent from the server) and binding (the input binding).

Summary

Here is a summary of the events. The ones that are cancelable can also be modified by users, e.g., you can change event.value in the shiny:inputchanged event, and the new event.value will be used as the input value (to be passed to R).