Writing integration tests for graphics, input, and accessibility


  1. Use TestWithEnvironment to ensure that each test runs with a fresh component topology.
  2. Do not use CMX injected-services to construct component topology. (It shares the same component instances between different test runs.)
  3. Set up fuchsia.hardware.display.Provider to be vended by fuchsia-pkg://fuchsia.com/fake-hardware-display-controller-provider#meta/hdcp.cmx", and bundle fake-hardware-display-controller with your test package.
  4. Bundle Root Presenter‘s component_v1_for_test with the test’s package. It prevents the actual input driver from interacting with the test. Invoke Root Presenter from the test package's URL. E.g., fuchsia-pkg://touch-input-test#meta/root_presenter.cmx.
  5. Bundle Scenic‘s component with the test’s package. It ensures that the test uses the Scenic it was built with. Invoke Scenic from the test package's URL. E.g., fuchsia-pkg://touch-input-test#meta/scenic.cmx.
  6. Don‘t invoke components from another test’s package!
  7. No sleeps or waits, unless the API is deficient. Every action by the test is gated on a logical condition that the test can observe. E.g., inject touch events only when the test observes the child view is actually connected to the view tree and vending content to hit.

Guidelines for writing integration tests

We have Fuchsia-based products built on the Fuchsia platform. As Fuchsia platform developers, we want to ship a solid platform, and validate that the the platform works correctly for all our supported products. Integration tests ensure we uphold correctness and stability of platform functionality that spans two or more components, via our prebuilt binaries (such as Scenic) and API contracts (over FIDL). This is especially valuable in validating our ongoing platform migrations. One example is the set of touch dispatch paths, such as from Input Pipeline to Scenic to Flutter.

Models of production

Integration tests model a specific product scenario by running multiple Fuchsia components together. For example, to ensure that the “touch dispatch path from device to client” continues to work as intended, we have a “touch-input-test” that exercises the actual components involved in touch dispatch, over the actual FIDLs used in production.

Because integration tests are a model, there can (and should) be some simplification from actual production. Obviously, these tests won‘t run the actual product binaries; instead, a reasonable stand-in is crafted for a test. The idea is that it’s the simplest stand-in that can be used in a test, which still can catch serious problems and regressions.

Sometimes, it‘s not straightforward for the test to use an actual platform path used in production; we use a reasonable stand-in for these cases too. For example, we can’t actually inject into /dev/class/input-report, so we have a dedicated API surface on Input Pipeline to accept injections in a test scenario.

The important thing is that the test gives us confidence that evolution of platform code and platform protocols will not break existing product scenarios.

No flakes

When the scenario involves graphics, it's very easy to accidentally introduce flakiness into the test, robbing us of confidence in our changes. Graphics APIs operate across several dimensions of lifecycle, topology, and synchronization/signaling schemes, in the domains of components, graphical memory management, view system, and visual asset placement. Furthermore, these APIs provide the basis for, and closely interact with, the Input APIs and the Accessibility APIs.

The principal challenge is to write tests that set up a real graphics stack, in a way that is robust against elasticity in execution time. We talk about those challenges in “Synchronization challenges”, below. There are other reasons for tests going wrong, and most of them can be dealt with by enforcing hermeticity at various levels. We talk about these first. A final challenge is to author tests that model enough of the interesting complexity on just the platform side, so that we know complex product scenarios don't break with platform evolution.

Precarious stack of stuff

At the bottom we have graphics tests. Input tests build on top of graphics tests. And accessibility tests build on top of input tests. Hence they have all the same problems, just with more components. It is thus critical that a basic graphics test is reasonable to write and understand, because they form the basis for “higher level” tests that inherently have more complexity.

Questions and answers

Why not just rely on product-side e2e tests?

Product owners must write e2e tests to ensure their product is safe from platform changes. E2e tests are big, heavy, and expensive to run; often, they are flaky as well. They are authored in a different repository, and run in their own test automation regime (“CQ”). And they care about the subset of OS functionality that their product relies on.

Given these realities, platform developers cannot rely on these e2e tests to catch problems in platform APIs and platform code.

By authoring platform-side integration tests, platform developers can get breakage signals much faster with less code in the tests, and systematically exercise all the functionality used across the full range of supported products. Product owners benefit by increased confidence in the platform's reliability.

Why all this emphasis on hermeticity?

Deterministic, flake-free tests increase the signal-to-noise ratio from test runs. They make life better.

When your tests rain down flakes every day, we ignore these tests, and they become noise. But when we try to fix the source of flakes, it often reveals a defect in our practices, or APIs, or documentation, which we can fix (think “impact”). Each of these hermeticity goals address a real problem that someone in Fuchsia encountered. When we have hermeticity, everyone benefits, and Fuchsia becomes better.

Why all this emphasis on integration tests?

Fuchsia's platform teams have important migrations in progress that affect products. Integration tests are a critical method of guaranteeing that our platform changes are safe and stable with respect to our product partners. Examples: Components Framework v2, Input API migration, Flatland API migration, etc.

What about CTS tests?

The Fuchsia Compatibility Test Suite ensures that the implementations offered by the Fuchsia platform conform to the specifications of the Fuchsia platform. An effective CTS will have UI integration tests, and so this guidance doc applies to those UI integration tests.

Prefer hermeticity

Various types of hermeticity make our tests more reliable.

Package hermeticity

All components used in the test should come from the same test package. This can be verified by examining the fuchsia-pkg URLs launched in the test; they should reference the test package.

If we don‘t have package hermeticity, and a component C is defined in the universe U, then the C launched will come from U, instead of your locally modified copy of C. This issue isn’t so much a problem in CQ, because it rebuilds everything from scratch. However, it is definitely an issue for local development, where it causes surprises - another sharp corner to trap the unwary. That is, a fix to C won't necessarily run in your test, and hampers developer productivity.

There is a further advantage to package hermeticity. For those components that read from the config-data package, this practice allows a test package to define their own config-data for the components they contain. In fact, this is the only way to define a piece of custom config-data for a test. For example, the display rotation in Root Presenter is conveyed with config-data.

Environment hermeticity

All components in the test should be brought up and torn down in a custom Fuchsia environment. For example, in Components Framework v1, the “TestWithEnvironment” fixture allows the test to construct the precise environment it needs for the test to run properly.

This practice forces component state to be re-initialized on each run of the test, thereby preventing inter-test state pollution.

The advantages of doing so are:

  • The test is far more reproducible, as the initial component state is always known to be good.
  • It's trivial to run the test hundreds of times in a tight loop, thus speeding up flake detection.
  • The test author can adjust the environment more precisely, and more flexibly, than otherwise possible.

No to injected-services

In component framework v1, it‘s possible to declare injected-services in a test’s CMX manifest. Declaring injected-services is somewhat of an anti-pattern. It, too, also constructs a test environment, but all the test executions run in the same environment. If a service component had dirtied state, a subsequent TEST_F execution will inadvertently run against that dirtied state.

Capability hermeticity

All components in the test should not be exposed to the actual root environment. For FIDL protocols, this is not so much an issue. However, there are other types of capabilities where CF v1 has leaks. A good example is access to device capabilities, such as /dev/class/input-report and /dev/class/display-controller. Components that declare access to device capabilities will actually access these capabilities, on the real device, in a test environment.

We can gain capability hermeticity by relying on a reasonable fake. Two examples.

  • The display controller, with some configuration, can be faked out. A subsequent advantage is that graphics tests can be run in parallel!
    • One downside is that it's not easy to physically observe what the graphical state is, because the test no longer drives the real display. So development can be a little harder.
  • The input devices are faked out with an injection FIDL, and that's how tests can trigger custom input. However, the component that receives injections still needs to avoid declaring access to /dev/class/input-report! The recommendation here is to put a /dev-less copy of the component manifest into the test package.

Synchronization challenges

Correct, flake-free inter-component graphics synchronization depends intimately on the specific graphics API being used. The legacy Scenic API, sometimes called “GFX”, has sparse guarantees for when something is “on screen”, so extra care must be taken to ensure a flake free test. As a rule of thumb, if you imagine the timeline of actions for every component stretching and shrinking by arbitrary amounts, a robust test will complete for all CPU-schedulable timelines. The challenge is to construct action gates where the test will hold steady until a desired outcome happens. Sleeps and timeouts are notoriously problematic for this reason. Repeated queries of global state (such as a pixel color test) are another mechanism by which we could construct higher-level gates, but incur a heavy performance penalty and adds complexity to debugging.

Another dimension of complexity is that much of client code does not interact directly with Fuchsia graphics APIs; instead they run in an abstracted runner environment. Flutter and Web are good examples where the client code cannot directly use Scenic APIs. Some facilities can be piped through the runner, but tests generally cannot rely on full API access. Some runners even coarsen the timestamps, which also complicates testing a bit.

One more subtlety. We're interested in the “state of the scene graph”, which is not precisely the same thing as “state of the rendering buffer”. For most purposes, they are loosely equivalent, because the entity taking a visual screenshot is the same entity that holds the scene graph - Scenic. However, specific actions, like accessibility color adjustments, will not be accurately portrayed in a visual screenshot, because the color adjustment takes place in hardware, below Scenic.

GFX is hard

For GFX in particular, nested views are particularly difficult to synchronize. The key difficulty is that a client needs two discrete pieces of information, relative view size and pixel metrics, to construct a correctly scaled content on a particular physical screen, but GFX conveys the view metrics only after the view is already “linked up” to the global scene graph. So from the parent view‘s perspective, child view connectivity cannot imply the child view’s content has rendered to screen.

A workaround signal, fuchsia.ui.gfx.ViewState.is_rendering, tells the parent view that something in the child view started rendering actual content. This is actually sufficient for a single-depth child view, when the child's content is simple. In fact, some input tests rely on this signal to successfully gate input injection. But for a child view that actually consists of a 2+ view hierarchy, the is_rendering signal does not say which views in the child view hierarchy have rendered content, only that some descendant view has rendered content to screen.

For client views that have direct access to the GFX API, it‘s possible to construct a tower of signals along the child view hierarchy, but this is fragile, complex, and subtle. It is also not feasible for clients that employ a runner, like web clients. (The web runner internally constructs a parent view and a child view for security.) From the test’s perspective, such a client will not generate a reliable signal in the GFX API.

Flatland makes it easier

The upcoming Flatland API, in contrast, solves this problem with a sophisticated linkage system, where scene graph connectivity is made independent of view metrics. That is, a parent view can send a child view the correct view metrics prior to the child view actually connecting to the global scene graph. Then, when the child view has finished preparing the rendered content, it can connect to the global scene graph in an atomic step, without revealing intermediate content construction phases (for example, nested content in a view hierarchy).

Test setup - one GFX child view

For a single GFX child view, the test can set up a “interceptor” or “proxy” view, to harvest the fuchsia.ui.gfx.ViewState.is_rendering signal from Scenic. Then subsequent actions, such as injecting touch events, can be reliably performed, with good assumptions about the child's graphical content.

An example is how touch-input-test.cc sets up touch-gfx-client in TouchInputTest.CppGfxClientTap. Here, the test sets up its own Scenic view, links the child's view underneath its own, and waits for the child view to connect, using it as a gate for touch injection.


  • Programmatically correct use of APIs
  • Reliable
  • Fast, minimal computational disturbance; good for performance work


  • Setup and control flow can be delicate to manage

Test setup - child is a GFX view hierarchy

For a stacked view hierarchy, current best practice is still to set up an interceptor view, and gate subsequent actions on the ViewState.is_rendering signal. However, the signal merely indicates that at least one of the child views in the hierarchy started rendering content; for some scenarios, like Chromium, this is not a sufficiently robust gating mechanism. That is, the signal is a little too early, since the test actually needs both child view and grandchild view to be in a rendering state.

To work around this nondeterminism, the subsequent action (touch injection) needs to run in a loop, until all descendant views have published their content to the scene graph. For example, for TouchInputTest.ChromiumTap, the test issues a repeated “tap-tap-tap” until it sees the client respond in an expected way.

Advantages: same as above


  • Can't manage a child view hierarchy in a completely deterministic way.

Test setup - gate on screenshot histogram

An alternate synchronization scheme is to set up the scene with various predefined colors, which may get toggled in response to actions, such as touch input. The test requests screenshots in succession, until the desired color condition is reached, by counting the number of pixels in each color value (a “histogram”). For example, if the child view is expected to present a red rectangle, the test will loop until the histogram returns predominantly red, and then perform a subsequent action.


  • The resulting test code is nicely linear in control flow, and easy to grasp at a high level.
  • Works with arbitrarily deep child hierarchies, assuming a good enough color scheme.


  • The color scheme has to be relatively simple, and the reader needs to understand the scheme.
  • Heavyweight and slow, due to all those screenshots taken.
  • Scenic actually has to work to generate those screenshots, so it can skew tests used for performance analysis.
  • Not completely foolproof. Flakes can still happen.
  • Screenshotting is a grandfathered global-scope capability (see fuchsia.ui.scenic.Scenic), and it will be more tightly controlled in the future.

Example: see FlutterEmbedderTests.Embedding.

Test setup - one Flatland child view (TBD)

Test setup - Realm Builder

The Touch Input Test is constructed using the Realm Builder library. This library is used to construct the test Realm in which the components under test operate. The test suite, hereafter test driver component, is a v2 component. This is in contrast to the components in the constructed realm, e.g. scenic, that are at the moment v1 components. This is so because Realm Builder provides a v1 “bridge” that allows for Realms constructed to include v1 and v2 components.

Realm Builder (and CFv2 in general) allows us to be explicit about what capabilities are routed to and from components. This is crucial for testing because it allows test authors to have fine-grained control over the test environment of their components. Take for example scenic. In touch-input-test, a handle to fuchsia.hardware.display.Provider from fake-hardware-display-controller-provider#meta/hdcp.cmx is routed. By providing a fake hardware display provider, we can write integration tests without having to use the real display controller. This mapping of source and target is explicitly written in the test file during Realm construction.

Since the Realm is populated by v1 components at the moment, it‘s worth mentioning how this works at a high level. Components added to a Realm constructed with Realm Builder work in the same way as typical Realm construction statically. In other words, Realm Builder doesn’t do anything special. It simply allows for the construction of Realms at runtime, instead of statically via manifests. It generates the necessary manifests and clauses, e.g. offer capability foo to #child-a, behind the scene when a user invokes a method like realm_builder->AddRoute(...). For v1 components, this is somewhat still true. The major difference is that Realm Builder constructs a wrapper v2 component to launch a v1 component. When the wrapper component is started, it launches the v1 component by creating a singleton fuchsia.sys.NestedEnvironment. This NestedEnvironment will only contain the v1 component, and won't contain any services except for those explicitly routed to it during Realm construction.

A sample diagram of a Realm constructed with RealmBuilder would look like:

    /    \

scenic.cmx root_presenter.cmx

In production, the environment would look more like:

 /       \

scenic.cmx root_presenter.cmx

Modeling complex scenarios

The graphics API allows each product to generate an arbitrarily complex scene graph. However, the products we have today typically rely on a few “core topologies” that are stable and suitable for the product to build on.

It's a valuable exercise to capture each of these core topologies in our platform integration tests. Some examples:

  • Touch dispatch to the Flutter runner.
    • The intra-component connection between the Flutter runner (C++) and the Flutter framework (Dart) is delicate, and a runner test will catch bad rolls into fuchsia.git. Workstation was once broken for many months due to a problem here.
  • Touch dispatch to the Chromium runner.
    • Chromium employs a two-view topology as part of its JS sandboxing strategy. Having a test ensures that Chromium is correctly using our APIs, and that our changes don't accidentally break Chromium.
  • Touch dispatch from Flutter runner to Chromium runner.
    • This scenario models a critical production path where the product reinjects touch events into its Chromium child view.
  • One parent view and two child views, using assorted runners, to ensure touch dispatch is routed to the correct view.

Developing new models are also how we test new topologies and interaction patterns to make sure the APIs are sensible and usable, and serve as as a foundation for converting an entire product.

  • For example, converting a product to Session Framework has many moving parts, and validating specific graphics scenarios builds confidence in APIs, implementations, and migration strategy.