In R Shiny, how to replace observeEvent with observe? - r

Question

The following almost-MWE code links user inputs into three objects using observeEvent. The image at the bottom also shows and explains these three objects:
First object is a slider input for number of modeled periods, called variable X.
Second object is a 1x1 matrix where the user inputs a single variable Y.
Third object is a vertically expandable matrix where the user inputs a series of values X|Y.
The calculation underlying the plot is a simple sum product, used as a placeholder simplification for this MWE.
User input linkages are shown in the image at the bottom.
How would I replace the two observeEvents used in MWE below with an observe in each case? If possible. (I believe this will offer a solution to another issue I'm having, based on my research of what the observe function does). I've only used observeEvents in my limited Shiny experience.
MWE code:
library(dplyr)
library(ggplot2)
library(shiny)
library(shinyMatrix)
sumProd <- function(a, b) {
c <- rep(NA, a)
c[] <- sum(b[,1], na.rm = T) %*% sum(b[,2],na.rm = T)
return(c)
}
ui <- fluidPage(
sliderInput('periods', 'Modeled periods (X):', min=1, max=10, value=10),
matrixInput("matrix1",
value = matrix(c(5), nrow = 1, ncol = 1, dimnames = list("Base rate (Y)",NULL)),
cols = list(names = FALSE),
class = "numeric"),
matrixInput("matrix2",
value = matrix(c(10,5), nrow = 1, ncol = 2, dimnames = list(NULL,c("X","Y"))),
rows = list(extend = TRUE, delete = TRUE),
class = "numeric"),
plotOutput("plot")
)
server <- function(input, output, session){
observeEvent(input$periods,{
updateMatrixInput(
session, inputId = "matrix2",
value = matrix(c(input$periods,input$matrix2[1,2]),1,2,dimnames=list(NULL,c("X","Y")))
)
})
observeEvent(input$matrix1, {
tmpMat2 <- c(input$matrix2[,1],input$matrix2[,2])
tmpMat2[length(input$matrix2)/2+1] <- input$matrix1[,1]
updateMatrixInput(session,inputId="matrix2",value=matrix(tmpMat2,ncol=2,dimnames=list(NULL,c("X","Y")))
)
})
plotData <- reactive({
tryCatch(
lapply(seq_len(ncol(input$matrix2)/2), # column counter to set matrix index as it expands
function(i){
tibble(
Scenario = colnames(input$matrix2)[i*2-1],
X = seq_len(input$periods),
Y = sumProd(input$periods,input$matrix2[,(i*2-1):(i*2), drop = FALSE])
)
}) %>% bind_rows(),
error = function(e) NULL
)
})
output$plot <- renderPlot({
req(plotData())
plotData() %>% ggplot() +
geom_line(aes(x = X, y = Y, colour = as.factor(Scenario))) +
theme(legend.title=element_blank())
})
}
shinyApp(ui, server)
Image showing user input linkages among the 3 objects:

Answer 1

I reviewed the post How to listen for more than one event expression within a Shiny observeEvent, which does a nice job of illustrating the interchangeability, in certain circumstances, of observe and observeEvent. Based on that post, I came up with the following substitutions of observeEvent with observe:
sumProd <- function(a, b) {
c <- rep(NA, a)
c[] <- sum(b[,1], na.rm = T) %*% sum(b[,2],na.rm = T)
return(c)
}
ui <- fluidPage(
sliderInput('periods', 'Modeled periods (X):', min=1, max=10, value=10),
matrixInput("matrix1",
value = matrix(c(5), nrow = 1, ncol = 1, dimnames = list("Base rate (Y)",NULL)),
cols = list(names = FALSE),
class = "numeric"),
matrixInput("matrix2",
value = matrix(c(10,5), nrow = 1, ncol = 2, dimnames = list(NULL,c("X","Y"))),
rows = list(extend = TRUE, delete = TRUE),
class = "numeric"),
plotOutput("plot")
)
server <- function(input, output, session){
observe({
input$periods
if(input$periods!=0){
isolate(
updateMatrixInput(
session, inputId = "matrix2",
value = matrix(c(input$periods,input$matrix2[1,2]),1,2,dimnames=list(NULL,c("X","Y")))
)
)
}
})
observe({
input$matrix1
if(input$matrix1!=0){
tmpMat2 <- c(input$matrix2[,1],input$matrix2[,2])
tmpMat2[length(input$matrix2)/2+1] <- input$matrix1[,1]
isolate(
updateMatrixInput(
session,inputId="matrix2",
value=matrix(tmpMat2,ncol=2,dimnames=list(NULL,c("X","Y")))
)
)
}
})
plotData <- reactive({
tryCatch(
lapply(seq_len(ncol(input$matrix2)/2), # column counter to set matrix index as it expands
function(i){
tibble(
Scenario = colnames(input$matrix2)[i*2-1],
X = seq_len(input$periods),
Y = sumProd(input$periods,input$matrix2[,(i*2-1):(i*2), drop = FALSE])
)
}) %>% bind_rows(),
error = function(e) NULL
)
})
output$plot <- renderPlot({
req(plotData())
plotData() %>% ggplot() +
geom_line(aes(x = X, y = Y, colour = as.factor(Scenario))) +
theme(legend.title=element_blank())
})
}
shinyApp(ui, server)