Runner Overview

The runner is how Fox executes properties, seeds data generation, and triggers shrinking. Although you can create and use the runner directly via FOXRunner, it is more common to use wrappers – such as FOXAssert.

FOXAssert takes a property assertion generator (only FOXForAll for now). An exception is raised when the property fails to hold.

A more flexible FOXAssertWithOptions can be used to provide parameters that is normally accepted by the runner.

Configuring Test Generation

The primary operation of Fox’s runner is to create and execute tests. There are three parameters to configure Fox’s test generation:

  • The seed allows you to seed the random number generator Fox uses. This allows you to set the PRNG to help reproduce failures that Fox may have generated during a test run. Setting this to the default (0) makes Fox generate a seed based on the current time.
  • The number of tests indicates the number of tests Fox generates for this particular property. More tests generated will more thoroughly cover the property at the cost of time. Setting this to the default (0) makes Fox run 500 tests.
  • The maximum size indicates the maximum size factor Fox uses when generating tests. Generators use this size factor as a hint to produce data of the appropriate sizes. For example, FOXInteger generates integers within the range of 0 to maximumSize and FOXArray generates arrays whose number of elements are in the range of 0 to maximumSize. Setting this to the default (0) makes Fox run with a maximumSize of 200. If you know this property’s data generation can tolerate larger sizes, feel free to increase this. Large collection generation can be prohibitively expensive.

Please note that seed, number of tests, and maximum size should all be recorded to reproduce a failure that Fox may have generated.

Per Assertion Configuration

By default, Fox generates 500 tests per assertion with a maximum size of 200 and a random seed. By changing FOXAssert to FOXAssertWithOptions we can provide optional configuration by using the FOXOptions:

FOXAssertWithOptions(FOXForAll(...), (FOXOptions){
    seed=5,              // default: time(NULL)
    numberOfTests=1000,  // default: 500
    maximumSize=100,     // default: 200

Global Configuration

Values can be overridden using environment variables to globally change the defaults.


Note that as of this time of writing, xcodebuild test (command line) does not properly pass environment variables to test bundles for iOS. Use the setter style listed after the environment variables instead.

It is fine to set environment variables via Xcode.

  • Setting FOX_SEED can specify a specific seed to run for all properties.
  • Setting FOX_NUM_TESTS sets the number of tests to generate for each property.
  • Setting FOX_MAX_SIZE sets the maximum size factor Fox uses to when generating tests.

If you cannot use environment variables, Fox also allows you to manually override the values via setters:

  • FOXSetSeed(NSUInteger seed) sets the random seed.
  • FOXSetNumberOfTests(NSUInteger numTests) sets the number of tests.
  • FOXSetMaximumSize(NSUInteger maxSize) sets the maximum size.

An easy way to use these setters would be in an +[initialize] or +[load] method:

@interface TestHelper : NSObject

@implementation TestHelper

+ (void)initialize {
  if (self == [TestHelper class]) {


Using compile-time settings or macros, you can conditionally customize the values as needed.

Configuration Priority

Even though Fox accepts many different ways of setting global configuration, there is a specific ordering Fox checks for configuration values.

The priority that Fox takes for these configurations are in order:

  1. The per-assertion configuration if provided.
  2. The environment variable overrides if provided.
  3. The setter function values are used if provided.
  4. Fox’s internal default values are used.

Random Number Generators

Fox provides a hook for custom pseudo-random number generation. This is also what generators receive as their first argument.

The runner uses FOXDeterministicRandom which uses C++ random by default. This keeps randomization state isolated from any other potential system that uses a global PRNG. But you can implement the FOXRandom protocol to support custom random schemes.

Another implementation Fox provides out of the box is FOXConstantRandom, which always generates a constant number. This can be useful for testing generators with example-based tests.


The runner also provides a way to observe its operation via a reporter. Reporters are a delegate to the runner. They are invoked synchronously, so be careful about its performance impact on execution.

Delegates cannot influence the execution of the runner, but can provide useful user-facing output about the progress of Fox’s execution.

By default, Fox runners do not have a reporter assigned to it. But Fox does provide a couple reporters:

  • FOXStandardReporter reports in a rspec-like dot reporter.
  • FOXDebugReporter reports more information about the execution.

The default reporter can be changed by setting it from the instance FOXAssert uses: [[FOXRunner assertInstance] setReporter:reporter].