elem_expect: Test one or more conditions on HTML elements
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elem_expect

R Documentation

Test one or more conditions on HTML elements

Description

elem_expect() waits for a set of conditions to return TRUE. If, after a certain period of time (by default 4 seconds), this does not happen, an informative error is thrown. Otherwise, the original element is returned.

elem_wait_until() does the same, but returns a logical value (whether or not the test passed), allowing you to handle the failure case explicitly.

Usage

elem_expect(x, ..., testthat = NULL, timeout = NULL)

elem_wait_until(x, ..., timeout = NULL)

Arguments

`x`	A `selenider_element`/`selenider_elements` object, or a condition.
`...`	<`dynamic-dots`> Function calls or functions that must return a logical value. If multiple conditions are given, they must all be `TRUE` for the test to pass.
`testthat`	Whether to treat the expectation as a `testthat` test. You do not need to explicitly provide this most of the time, since by default, we can use `testthat::is_testing()` to figure out whether `elem_expect()` is being called from within a `testthat` test.
`timeout`	The number of seconds to wait for a condition to pass. If not specified, the timeout used for `x` will be used, or the timeout of the local session if an element is not given.

Value

elem_expect() invisibly returns the element(s) x, or NULL if an element or collection of elements was not given in x.

elem_wait_for() returns a boolean flag: TRUE if the test passes, FALSE otherwise.

Conditions

Conditions can be supplied as functions or calls.

Functions allow you to use unary conditions without formatting them as a call (e.g. is_present rather than is_present()). It also allows you to make use of R's anonymous function syntax to quickly create custom conditions. x will be supplied as the first argument of this function.

Function calls allow you to use conditions that take multiple arguments (e.g. has_text()) without the use of an intermediate function. The call will be modified so that x is the first argument to the function call. For example, has_text("a") will be modified to become: has_text(x, "a").

The and (&&), or (||) and not (!) functions can be used on both types of conditions. If more than one condition are given in ..., they are combined using &&.

Custom conditions

Any function which takes a selenider element or element collection as its first argument, and returns a logical value, can be used as a condition.

Additionally, these functions provide a few features that make creating custom conditions easy:

Errors with class expect_error_continue are handled, and the function is prevented from terminating early. This means that if an element is not found, the function will retry instead of immediately throwing an error.
selenider functions used inside conditions have their timeout, by default, set to 0, ignoring the local timeout. This is important, since elem_expect() and elem_wait_until() implement a retry mechanic manually. To override this default, manually specify a timeout.

These two features allow you to use functions like elem_text() to access properties of an element, without needing to worry about the errors that they throw or the timeouts that they use. See Examples for a few examples of a custom condition.

These custom conditions can also be used with elem_filter() and elem_find().

Examples


html <- "
<div class='class1'>
<button id='disabled-button' disabled>Disabled</button>
<p>Example text</p>
<button id='enabled-button'>Enabled</button>
</div>

<div class='class3'>
</div>
"
session <- minimal_selenider_session(html)

s(".class1") |>
  elem_expect(is_present)

s("#enabled-button") |>
  elem_expect(is_visible, is_enabled)

s("#disabled-button") |>
  elem_expect(is_disabled)

# Error: element is visible but not enabled
s("#disabled-button") |>
  elem_expect(is_visible, is_enabled, timeout = 0.5) |>
  try() # Since this condition will fail

s(".class2") |>
  elem_expect(!is_present, !is_in_dom, is_absent) # All 3 are equivalent

# All other conditions will error if the element does not exist
s(".class2") |>
  elem_expect(is_invisible, timeout = 0.1) |>
  try()

# elem_expect() returns the element, so can be used in chains
s("#enabled-button") |>
  elem_expect(is_visible && is_enabled) |>
  elem_click()
# Note that elem_click() will do this automatically

s("p") |>
  elem_expect(is_visible, has_exact_text("Example text"))

# Or use an anonymous function
s("p") |>
  elem_expect(\(elem) identical(elem_text(elem), "Example text"))

# If your conditions are not specific to an element, you can omit the `x`
# argument
elem_1 <- s(".class1")
elem_2 <- s(".class2")

elem_expect(is_present(elem_1) || is_present(elem_2))

# We can now use the conditions on their own to figure out which element
# exists
if (is_present(elem_1)) {
  print("Element 1 is visible")
} else {
  print("Element 2 is visible")
}

# Use elem_wait_until() to handle failures manually
elem <- s(".class2")
if (elem_wait_until(elem, is_present)) {
  elem_click(elem)
} else {
  reload()
}

# Creating a custom condition is easiest with an anonymous function
s("p") |>
  elem_expect(
    \(elem) elem |>
      elem_text() |>
      grepl(pattern = "Example .*")
  )

# Or create a function, to reuse the condition multiple times
text_contains <- function(x, pattern) {
  text <- elem_text(x)

  grepl(pattern, text)
}

s("p") |>
  elem_expect(text_contains("Example *"))

# If we want to continue on error, we need to use the
# "expect_error_continue" class.
# This involves making a custom error object.
error_condition <- function() {
  my_condition <- list(message = "Custom error!")
  class(my_condition) <- c("expect_error_continue", "error", "condition")
  stop(my_condition)
}

# This is much easier with rlang::abort() / cli::cli_abort():
error_condition_2 <- function() {
  rlang::abort("Custom error!", class = "expect_error_continue")
}

# This error will not be caught
try(elem_expect(stop("Uncaught error!")))

# These will eventually throw an error, but will wait 0.5 seconds to do so.
try(elem_expect(error_condition(), timeout = 0.5))
try(elem_expect(error_condition_2(), timeout = 0.5))

selenider documentation built on April 3, 2025, 5:51 p.m.