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MonoGame Tests

The MonoGame Tests run against XNA on Windows and MonoGame on Windows, Mac OS X and Linux. They serve as an assurance that MonoGame conforms as closely as possible to XNA.

Simple unit tests make assertions about MonoGame's core class properties, methods and behavior to guarantee compatibility with XNA in those regards. Additionally, visual tests verify via frame capture and comparison that MonoGame renders equivalently to XNA.

Currently, on Windows, the tests can be run using NUnit and target either XNA or MonoGame. On Mac OS X and Linux, the tests target MonoGame and are implemented in an executable assembly that can be run and debugged directly. After execution using the custom test runner, and HTML report of the results will be loaded in your default browser, and a log of stdout can be found in bin$(Configuration)\stdout.txt.

Note: Currently there is no way to skip or select certain tests to run using the custom runner. This functionality is coming soon.

Rendering Tests

Most rendering tests do not require a game loop, but just need a GraphicsDevice to be able to render things. These tests can inherit from GraphicsDeviceTestFixtureBase and use the supplied GraphicsDevice 'gd' to render. Tests that require rendering were formerly implemented with the VisualTestFixtureBase (we call these "visual tests"), but this is no longer recommended unless the test requires an actual Game loop or tests functionality of the Game class itself because these tests are slower and harder to implement. When creating a new rendering test, the first run will fail because there is no reference image. Running the test will capture and save the result. Run the test with the XNA test project to get a reference image that checks XNA compatibility, or with MG to make sure no regression occurs. After adding the captured frame as a reference image, the test should pass.

GraphicsDeviceTestFixtureBase

There are 3 methods in GraphicsDeviceTestFixtureBase that can be used to capture a frame and compare it to a reference image.

  • PrepareFrameCapture: Call this before rendering, so a RenderTarget can be prepared and set as the target for the GraphicsDevice. Optionally pass in the amount of frames you expect to capture. This is used for naming the captures when saving them (i.e. frame1.png or frame001.png) and the amount of captured frames will also be checked explicitly unless the ExactNumberSubmits flag is set to false.
  • SubmitFrame: Store the content of the render target in a list of submitted frames
  • CheckFrames: Call this when all frames are submitted. Checks all submitted frames against reference images if they can be found. Will also write out the captured image and a diff of captured image vs reference image if required by the WriteCapture and WriteDiffs settings. Compared frames get a similarity score between 0 and 1. If this score is higher than the set Similarity the test will pass. For ease of use, when no frame was submitted but PrepareFrameCapture has been called, this will submit a frame. Because of this tests that only submit one frame just need to be wrapped in PrepareFrameCapture/CheckFrames calls and can render as usual in between.

If you still think you need a visual test to properly test a feature, read the paragraphs below to get a sense of how to get started.

MonoGame's visual tests are implemented as Games and GameComponents whose output is captured and compared to known-good reference renderings. Performance is ignored in these tests: the focus here is correct drawing.

Workflow for Implementing a Visual Test

A good visual test, like any good test, should perform the minimum work necessary to verify that the functionality under test is correct. As much as possible, drawing and test code should be made modular by inheriting from GameComponent, DrawableGameComponent, VisualTestGameComponent, or VisualTestDrawableGameComponent to encourage reuse, rather than duplication, in other tests.

Here is one possible workflow for implementing a visual test (assuming the test fixture is already set up):

Before you start:
Examine the existing visual tests to get a sense of how components are being composed into a test and what components may already satisify part of the requirements of the new test.

  1. Implement any new drawing logic needed in a new subclass of one of the *Component base classes.

  2. Compose your test Game in a new [Test] method. As this stage, you can run the new test directly to visually verify the rendering.

  3. Add a FrameCompareComponent to your test Game with at least one IFrameComparer implementation. Use the FrameCompareComponent's predicate to capture and compare frames.

  4. Pass the FrameCompareComponent.Results to the diffing, logging and assertion utility methods provided by VisualTestFixtureBase

  5. The first time a visual test is run, it will fail for lack of reference images to compare the captured images to. However, it will write the captured frames to bin$(Configuration)\CapturedFrames{TestDir}.

  6. Proof the images generated from the first run to ensure that they are correct, then add them to the test project in Assets\ReferenceImages$TestDir. Be sure to add them in the projects for all platforms! These files should have their build actions set to "Compile" and "Copy if newer".

  7. Rerun the visual test. The reference images should be copied into place and the test should now pass.

  8. XOR diffs between the reference images and captured frames are output into bin$(Configuration)\Diffs{TestDir} for debugging purposes.

Notes For Implementing Correct Visual Tests

  • Visual tests must be marked as [RequiresSTA] to work correctly on all platforms
  • Try not to rely on GameComponent.Update, since capturing frames can make the game run slowly and result in multiple Update calls per Draw. Instead, inherit from VisualTestGameComponent or VisualTestDrawableGameComponent and override UpdateOncePerDraw.
  • For similarity thresholds, prefer Constants.StandardRequiredSimilarity unless there is a very good reason to choose another value. This will allow the strictness of all the tests to be adjusted together in the future, as requirements change.
  • Use the Paths static class to reduce typos in resource paths.

Special Considerations

  • For new test fixtures, call Paths.SetStandardWorkingDirectory() in [SetUp] (VisualTestBase does this for you) to ensure that the ContentManager can find your assets on all platforms.
  • Note that all platforms are forced to run in Synchronous mode and that this doesn't always work perfectly on all platforms yet.

NUnit Configuration

There are a few things to know about running these tests under NUnit:

  • You must run the -x86 versions of NUnit, because XNA won't work with the 64-bit versions.
  • You must disable shadow copying because having it enabled makes it impossible for the ContentManager to find any assets.
    • GUI: Tools > Settings > Test Loader > Advanced
    • CLI: /noshadow
  • For debugger support, Run tests directly in the NUnit process, (note that this may cause a few-seconds-long hang when exiting NUnit after running a visual test) otherwise choose 'single separate process'
    • This setting can be found in: Tools > Settings > Test Loader > Assembly Isolation