Reactive context for leaflet() in dynamic Rmarkdown document (runtime:shiny)

Untitled.Rmd

---
title: "Reactive Leaflet"
author: "David Holstius"
date: "April 16, 2015"
output: html_document
runtime: shiny
---

## Problem Description

**How should one provide a reactive context to `leaflet()`?**

This naïve attempt fails with:

> Quitting from lines 14-37 (Untitled.Rmd) 
Error in .getReactiveEnvironment()$currentContext() : 
  Operation not allowed without an active reactive context. (You tried to do something that can only be done from inside a reactive expression or observer.)

... if the line beginning with `subset` is uncommented.

```{r, echo=FALSE}
set.seed(123)
rand_lng = function(n = 10) rnorm(n, -93.65, .01)
rand_lat = function(n = 10) rnorm(n, 42.0285, .01)
rand_z = function(n = 10) rnorm(n, mean = 0, sd = 1)

library(sp)
my_spdf <- SpatialPointsDataFrame(
  coords = cbind(rand_lng(50), rand_lat(50)),
  data = data.frame(z = rand_z(50))) 

library(shiny)
selectizeInput(
  inputId = "threshold", 
  label = "Threshold", 
  choices = c("Zero" = 0.0, "One" = 1.0),
  selected = 0.0)

library(leaflet)
my_spdf %>%
  # subset(z > input$threshold) %>%
  leaflet() %>%
  addTiles() %>%
  addCircles()
```

So, basically do

renderLeaflet(
  my_spdf %>%
    ...
)