dccDropdown: Dropdown component
In dashCoreComponents: Core Interactive UI Components for 'Dash'

Description Usage Arguments Value Examples

Dropdown is an interactive dropdown element for selecting one or more items. The values and labels of the dropdown items are specified in the 'options' property and the selected item(s) are specified with the 'value' property. Use a dropdown when you have many options (more than 5) or when you are constrained for space. Otherwise, you can use RadioItems or a Checklist, which have the benefit of showing the users all of the items at once.

dccDropdown(id=NULL, options=NULL, value=NULL, optionHeight=NULL,
className=NULL, clearable=NULL, disabled=NULL, multi=NULL,
placeholder=NULL, searchable=NULL, search_value=NULL,
style=NULL, loading_state=NULL, persistence=NULL,
persisted_props=NULL, persistence_type=NULL)

`id`	Character. The ID of this component, used to identify dash components in callbacks. The ID needs to be unique across all of the components in an app.
`options`	List of lists containing elements 'label', 'value', 'disabled', 'title'. those elements have the following types: - label (character \| numeric; required): the dropdown's label - value (character \| numeric; required): the value of the dropdown. this value corresponds to the items specified in the 'value' property. - disabled (logical; optional): if true, this option is disabled and cannot be selected. - title (character; optional): the html 'title' attribute for the option. allows for information on hover. for more information on this attribute, see https://developer.mozilla.org/en-us/docs/web/html/global_attributes/titles. An array of options label: [string\|number], value: [string\|number], an optional disabled field can be used for each option
`value`	Character \| numeric \| list of character \| numerics. The value of the input. If 'multi' is false (the default) then value is just a string that corresponds to the values provided in the 'options' property. If 'multi' is true, then multiple values can be selected at once, and 'value' is an array of items with values corresponding to those in the 'options' prop.
`optionHeight`	Numeric. height of each option. Can be increased when label lengths would wrap around
`className`	Character. className of the dropdown element
`clearable`	Logical. Whether or not the dropdown is "clearable", that is, whether or not a small "x" appears on the right of the dropdown that removes the selected value.
`disabled`	Logical. If true, this dropdown is disabled and the selection cannot be changed.
`multi`	Logical. If true, the user can select multiple values
`placeholder`	Character. The grey, default text shown when no option is selected
`searchable`	Logical. Whether to enable the searching feature or not
`search_value`	Character. The value typed in the DropDown for searching.
`style`	Named list. Defines CSS styles which will override styles previously set.
`loading_state`	Lists containing elements 'is_loading', 'prop_name', 'component_name'. those elements have the following types: - is_loading (logical; optional): determines if the component is loading or not - prop_name (character; optional): holds which property is loading - component_name (character; optional): holds the name of the component that is loading. Object that holds the loading state object coming from dash-renderer
`persistence`	Logical \| character \| numeric. Used to allow user interactions in this component to be persisted when the component - or the page - is refreshed. If 'persisted' is truthy and hasn't changed from its previous value, a 'value' that the user has changed while using the app will keep that change, as long as the new 'value' also matches what was given originally. Used in conjunction with 'persistence_type'.
`persisted_props`	List of a value equal to: 'value's. Properties whose user interactions will persist after refreshing the component or the page. Since only 'value' is allowed this prop can normally be ignored.
`persistence_type`	A value equal to: 'local', 'session', 'memory'. Where persisted user changes will be stored: memory: only kept in memory, reset on page refresh. local: window.localStorage, data is kept after the browser quit. session: window.sessionStorage, data is cleared once the browser quit.

named list of JSON elements corresponding to React.js properties and their values

if (interactive() && require(dash)) {
    library(dash)
    library(dashCoreComponents)

    app <- Dash$new()

    app$layout(
      htmlDiv(
        dccDropdown(
          options=list(
            list(label = "New York City", value = "NYC"),
            list(label = "Montreal", value = "MTL"),
            list(label = "San Francisco", value = "SF")
          ),
          value="MTL"
        )
      )
    )

  app$run_server()
}