As a block developer, you may want to write unit tests for any blocks you create. Testing blocks is more complex than writing unit tests for standard R code. For blocks, we need methods to both interact with the reactive shiny values in the blocks, and to capture the state and expressions returned by our blocks.

In both instances, we can take advantage of `shiny::testServer()`. This function allows us to simulate a blockr application, without having to spin up a full app (which would be time consuming to create and result in slow tests). This vignette will show how to apply `shiny::testServer()` to a suite of common test cases. For more details on how `shiny::testServer()` works, we recommend you see this article in the official Shiny docs.

## Common test cases

#### Test class of object

Before interacting with a block, it is often a good idea to test that the block constructor has worked as intended and created the correct class of object. For this example, let’s create a pretend “test_block” class:

    test_that("block constructor", {
        expect_s3_class(new_test_block(), "test_block")
    })

#### Test input widgets work

Next, you may want to test that the input widgets to your block work as intended. That is, that changes in inputs are reflected by changes in reactive values in the block. For this example, let’s imagine out test block has an input widget that allows the user to select a color:

    test_that("test block server input widgets update reactive values", {
        testServer(
            app = new_test_block()$expr_server, # Capture the expression server
            expr = {
                session$setInputs(color = "green") # Set a color
                expect_equal(color(), "green")

                session$setInputs(color = "red") # Change color
                expect_equal(color(), "red")
            }
        )
    })

#### Test that state is correctly returned

After confirming our input widgets work as expected, we may then want to check that the “state” of our block is returned correctly. Let’s expand our previous example to add a second input widget, number, which allows the user to select a numeric value. Here we access the state returned by our block by indexing into the `session$returned` object returned by `shiny::testServer()`:

    test_that("state is correctly returned", {
        testServer(
            app = new_test_block()$expr_server,
            expr = {
                # First check default values
                expect_equal(session$returned$state$number(), numeric())
                expect_equal(session$returned$state$color(), character())

                # Then toggle the inputs and recheck state is updated correctly
                session$setInputs(number = 1)
                expect_equal(session$returned$state$number(), 1)
                session$setInputs(color = "pink")
                expect_equal(session$returned$state$color(), "pink")
            }
        )
    })

#### Test that expressions are correctly returned

In addition to checking the “state” returned by our block, we can also check that the “expr” is correctly evaluated. To evaluate our expressions, we can call `base::eval()` on our returned expression, and then check that this evaluated expressions matches our expression. In this example, let’s assume that our input widgets combine in an expression to build a new “colored_number” class which just prints a colored number to the screen:

    test_that("expr evaluates correctly", {
        testServer(
            app = new_test_block()$expr_server,
            expr = {
                session$setInputs(number = 1)
                session$setInputs(color = "cyan")

                # Call `base::eval()` on our expression
                evaluated_expr <- eval(session$returned$expr())
                expect_s3_class(evaluated_expr, "colored_number")
            }
        )
    })

#### Test that expressions correctly throw errors

We may also want to test that errors are correctly thrown for incorrect expressions:

    test_that("incorrect colours throw an error", {
        testServer(
            app = new_test_block()$expr_server,
            expr = {
                session$setInputs(number = 2)

                # Set invalid colour
                session$setInputs(color = "Ooops")

                # Check that an error is thrown
                expect_error(eval(session$returned$expr()))
            }
        )
    })

#### Passing additional args

It is likely that some of your blocks require data or other blocks to be passed in as arguments to the block. To do this in our test suite, we can pass any such objects in a list to the `args` object of `testServer()`. For instance, let’s assume our test block requires a data.frame to be passed to the argument “df”:

    test_that("test block server input widgets update reactive values", {
        testServer(
            app = new_test_block()$expr_server,
            # Set block arguments with the "args" argument
            args = list(df = reactive(gt::gt(mtcars))),
            expr = {
                session$setInputs(color = "green")
                expect_equal(color(), "green")
            }
        )
    })