如何基于反应式if语句执行多个反应式函数

Question

我正在构建我的第一个闪亮的交互式应用程序，来自https://www.kaggle.com/c/bike-sharing-demand的自行车共享数据。用户可以用单选按钮选择“起点站点”或“终点站点”，然后在地图上接收圆形标记，半径是某个自行车站点在特定日期旅行的开始/结束计数。

我有一个包含trip数据的数据帧(df.trip)和一个包含每个站点的坐标(df.station)的数据帧。根据用户的选择，我将不得不使用'from_station_id‘或'to_station_id’向左连接df.station，以便找到起点/终点站的坐标。

我已经在UI中实现了单选按钮，并且已经有了日期功能。但是，我尝试了许多方法来获得一个响应式的if语句，该语句根据用户的选择在不同的键上连接数据帧。在if中，还有其他的反应式语句。下面提供的是服务器的一部分。

我收到了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.)

    station_type <- reactive({
     if (toString(input$station)=="Start Station") {
       return(TRUE)
     } else {
       return(FALSE)
     }
   })


   if (station_type() == TRUE) {
     filtered <- reactive({
       df.trip %>%
         filter(format(df.trip$starttime,"%Y-%m-%d") == input$date) %>%
         group_by(from_station_id) %>%
         summarize(n_trips = n())
     )}

     tmp.df <- reactive({
         left_join(filtered(), df.station, by = c("from_station_id" = "station_id"))
       })

     leaflet(tmp.df()) %>%
       addTiles(group="OSM") %>%#OSM is default tile providor
       addProviderTiles(providers$CartoDB.Positron) %>%
       setView(
         lng=-122.335167,
         lat=47.608013,
         zoom=12
       )%>%
       addCircleMarkers(lng = ~long, lat = ~lat, weight = 1,label=~name,
                        radius = ~n_trips)
   }

   else {
     filtered <- reactive({
       df.trip %>%
         filter(format(df.trip$starttime,"%Y-%m-%d") == input$date) %>%
         group_by(to_station_id) %>%
         summarize(n_trips = n())
     )}

     tmp.df <- reactive({
       left_join(filtered(), df.station, by = c("to_station_id" = "station_id"))
     })

     leaflet(tmp.df()) %>%
       addTiles(group="OSM") %>%#OSM is default tile providor
       addProviderTiles(providers$CartoDB.Positron) %>%
       setView(
         lng=-122.335167,
         lat=47.608013,
         zoom=12
       )%>%
       addCircleMarkers(lng = ~long, lat = ~lat, weight = 1,label=~name,
                        radius = ~n_trips)
   }



   })````

Answer 1

该错误是由于您试图在renderLeaflet函数外部呈现小叶贴图造成的。您还可以将if部分减少到仅相关部分，并将其全部包含在您的renderLeaflet调用中。

因此，如果您已经在使用leafletOutput(outputId = "map")的UI中设置了leaflet地图，那么在服务器中应该可以这样做：

output$map <- renderLeaflet({

  from_to <- input$station

  df <- df.trip %>%
    filter(format(df.trip$starttime,"%Y-%m-%d") == input$date)

  if (from_to == "Start Station") {
    df <- df %>% 
      group_by(from_station_id) %>%
      summarize(n_trips = n()) %>% 
      left_join(df.station, by = c("from_station_id" = "station_id"))
  } else {
    df <- df %>% 
      group_by(to_station_id) %>%
      summarize(n_trips = n()) %>% 
      left_join(df.station, by = c("to_station_id" = "station_id"))
  }

  leaflet(df) %>%
    addTiles(group="OSM") %>%#OSM is default tile providor
    addProviderTiles(providers$CartoDB.Positron) %>%
    setView(
      lng=-122.335167,
      lat=47.608013,
      zoom=12
    )%>%
    addCircleMarkers(lng = ~long, lat = ~lat, weight = 1,
                     label=~name, radius = ~n_trips)
})