如何在用户点击闪亮的操作按钮时验证输入值

Question

我正在开发一个闪亮的仪表板，用户可以在点击send email按钮时发送电子邮件。当用户点击发送电子邮件时，需要在各种输入字段上执行验证。这是我的ui.R文件看起来像

tabItem("email",
          fluidRow(
            box(
              width = 5, status = "info",solidHeader = TRUE,
              title = "Send Emails",
              helpText(tags$b("If no selection is made, emails will be sent to all the registrants with today's date")),

              dateRangeInput('dateRange',
                             label = 'Date Range Input',
                             start = c("",Sys.Date() - 2), end = c("",Sys.Date() + 2)

              ),
              selectizeInput("email_select", 
                             "Select Email Content",
                             choices = c("Price" = "price",
                                         "Services" = "service"
                                         ),
                             options = list(
                             placeholder = 'Please select',
                             onInitialize = I('function() { this.setValue(""); }')
                          )
                          ),
              uiOutput("city_dropdown")
         )
         )

uiOutput是一个下拉框，一旦上传数据就会填充。这是我的server.R文件，看起来像

shinyServer(function(input, output, session) {

    output$city_dropdown <- renderUI({

       city <- reg_city(final_data)
       city <- city$registrant_city
       city <- as.list(city)

       selectizeInput("city_select", 
                      "Select City",
                      choices = city,
                      options = list(
                      placeholder = 'Please select',
                      onInitialize = I('function() { this.setValue("");}')
                     )

            )


   })

  observeEvent(input$send_email, {
     city <- input$city_select

     if(city == ""){
       validate(
       need(input$city_select != '', 'Please choose a city')

    )}

但是当我点击操作按钮时没有任何反应，如果没有从下拉框中选择城市，则不会显示错误消息。 请帮忙。