What is Fox?

Fox is a port of the property-based testing tool test.check for Objective-C and Swift.

QuickCheck inspired Property-Based Testing Tools are test generators. These tools allow you to describe a property of your program that should always hold true instead of writing hand-crafted test cases. A pseudo-code example would be:

// pseudocode: A property for the sort() function
property := ForAll(xs where xs is an Array of Unsigned Integers){
    // perform action
    sorted_numbers := sort(xs)
    // verify that sorted_numbers is in ascending order
    i := 0
    for n in sorted_numbers {
        if i <= n {
            i = n
        } else {
            return FAILURE
    return SUCCESS

This example tests a sort function. Fox will generate tests based on the requirements of xs (any array of unsigned integers) to find a failing example that causes a FAILURE.

In the mathematical sense, Fox is a weak proof checker of a property where the tool tries to assert the property is valid by randomly generating data to find a counter-example.

Shrinking Failures

A benefit of Fox over purely random data generation is Shrinking. An informed random generation is done by size. This allows the tool to reduce the counter-example to a smaller data set without having a user manually separate thes signal from the noise in the randomly generated data.

For example, if a sort() implementation failed with an exception when reading 9s. Fox might generate this value to provoke the failure:

xs = @[@1, @5, @9, @3, @5] // fails

And then proceed to shrink xs by trying smaller permutations:

xs = @[@5, @9, @3, @5] // still fails
xs = @[@9, @3, @5] // fails
xs = @[@3, @5] // passed
xs = @[@9, @5] // fails
xs = @[@9] // fails

Fox does this automatically whenever a failure occurs. This is valuable instead of having to manually debug a failure when random data generation is used.

Ready to get started? Install Fox.