https://github.com/calderonsamuel/sseparser

Parse Server-Sent Events in R

https://github.com/calderonsamuel/sseparser

Keywords

Repository

Parse Server-Sent Events in R

README.Rmd

---
output: github_document
---



```{r, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  fig.path = "man/figures/README-",
  out.width = "100%"
)
```

# SSEparser


[![Lifecycle: experimental](https://img.shields.io/badge/lifecycle-experimental-orange.svg)](https://lifecycle.r-lib.org/articles/stages.html#experimental)
[![CRAN status](https://www.r-pkg.org/badges/version/SSEparser)](https://CRAN.R-project.org/package=SSEparser)
[![R-CMD-check](https://github.com/calderonsamuel/SSEparser/actions/workflows/R-CMD-check.yaml/badge.svg)](https://github.com/calderonsamuel/SSEparser/actions/workflows/R-CMD-check.yaml)
[![Codecov test coverage](https://codecov.io/gh/calderonsamuel/SSEparser/branch/main/graph/badge.svg)](https://app.codecov.io/gh/calderonsamuel/SSEparser?branch=main)
[![CRAN RStudio mirror downloads](https://cranlogs.r-pkg.org/badges/last-month/SSEparser?color=blue)](https://r-pkg.org/pkg/SSEparser)


The goal of SSEparser is to provide robust functionality to parse Server-Sent Events and to build on top of it.

## Installation

You can install `SEEparser` from CRAN like so:

```r
install.packages("SSEparser")
```

Alternatively, you can install the development version like so:

``` r
pak::pak("calderonsamuel/SSEparser")
```

## Example

The `parse_sse()` function takes a string containing a server-sent event and converts it to a R list.

```{r example}
library(SSEparser)

event <- "data: test\nevent: message\nid: 123\n\n"

parse_sse(event)

```

Comments are usually received in a line starting with a colon. They are not parsed.

```{r}
with_comment <- "data: test\n: comment\nevent: example\n\n"

parse_sse(with_comment)
```

## Use in HTTP requests

`parse_sse()` wraps the `SSEparser` R6 class, which is also exported to be used with real-time streaming data. The following code handles a request with MIME type "text/event-stream".

```{r}
parser <- SSEparser$new()
response <- httr2::request("https://postman-echo.com/server-events/3") %>%
	httr2::req_body_json(data = list(
		event = "message",
		request = "POST"
	)) %>%
	httr2::req_perform_stream(callback = \(x) {
		event <- rawToChar(x)
		parser$parse_sse(event)
		TRUE
	})

str(parser$events)
```

## Extending SSEparser

Following the previous example, it should be useful to parse the content of every `data` field to be also an R list instead of a JSON string. For that, we can create a new R6 class which inherits from `SSEparser`. We just need to overwrite the `append_parsed_sse()` method.

```{r}
CustomParser <- R6::R6Class(
	classname = "CustomParser",
	inherit = SSEparser,
	public = list(
		initialize = function() {
			super$initialize()
		},
		append_parsed_sse = function(parsed_event) {
			parsed_event$data <- jsonlite::fromJSON(parsed_event$data)
			self$events = c(self$events, list(parsed_event))
			invisible(self)
		}
	)
)
```

Notice that the only thing we are modifying is the parsing of the data field, not the parsing of the event itself. This is the original method from `SSEparser`:

```{r}
SSEparser$public_methods$append_parsed_sse
```


`CustomParser` uses `jsonlite::fromJSON()` to parse the data field of every chunk in the event stream. We can now use our custom class with the previous request[^1].

[^1]: This endpoint returns random event field names for each chunk in every request, so the response will not be exactly the same.

```{r}
parser <- CustomParser$new()
response <- httr2::request("https://postman-echo.com/server-events/3") %>%
	httr2::req_body_json(data = list(
		event = "message",
		request = "POST"
	)) %>%
	httr2::req_perform_stream(callback = \(x) {
		event <- rawToChar(x)
		parser$parse_sse(event)
		TRUE
	})

str(parser$events)
```

Now instead of a JSON string we can have an R list in the data field **while the stream is still in process**.

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