Add testing platform architecture (#41735)

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Co-authored-by: David Pine <[email protected]>

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* Update docs/core/testing/unit-testing-platform-architecture-services.md

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* Update docs/core/testing/unit-testing-platform-architecture-services.md

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Co-authored-by: David Pine <[email protected]>

Author: Evangelink

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+---
+title: Microsoft.Testing.Platform capabilities overview
+description: Learn about Microsoft.Testing.Platform capabilities concept.
+author: MarcoRossignoli
+ms.author: mrossignoli
+ms.date: 07/11/2024
+---
+
+# Microsoft.Testing.Platform capabilities
+
+In the context of the testing platform, a *capability* refers to the *potential to perform a specific action or provide specific information*. It's a means for the testing framework and extensions to *declare* their *ability* to *operate* in a certain manner or provide specific information to the *requesters*.
+
+The *requesters* can be any component involved in a test session, such as the platform, an extension, or the testing framework itself.
+
+The primary objective of the capability system is to facilitate effective communication among the components involved in a test session, enabling them to exchange information and meet their respective needs accurately.
+
+## Guided example
+
+Let's consider a hypothetical example to demonstrate the necessity of a capability system.
+
+> [!NOTE]
+> This example is purely for illustrative purposes and isn't currently implemented within the testing platform or any testing framework.
+
+Imagine a situation where you have an extension that requires the testing framework to execute no more than one test at a time. Furthermore, after each test, the extension needs to know the CPU usage for that specific test.
+
+To accommodate the preceding scenario, you need to inquire from the testing framework if:
+
+1. It has the capability to execute only one test at a time.
+2. It can provide information regarding the amount of CPU consumed by each test.
+
+How can the extension determine if the testing framework has the ability to operate in this mode and provide CPU usage information for a test session? In the testing platform, this capability is represented by an implementation the `Microsoft.Testing.Platform.Capabilities.ICapability` interface:
+
+```csharp
+// Base capabilities contracts
+
+public interface ICapability
+{
+}
+
+public interface ICapabilities<TCapability>
+    where TCapability : ICapability
+{
+    IReadOnlyCollection<TCapability> Capabilities { get; }
+}
+
+// Specific testing framework capabilities
+
+public interface ITestFrameworkCapabilities : ICapabilities<ITestFrameworkCapability>
+{
+}
+
+public interface ITestFrameworkCapability : ICapability
+{
+}
+```
+
+As you can see, the `ICapability` interface is a *marker* interface because it can represent *any capability*, and the actual implementation will be context dependent. You'll also observe the `ITestFrameworkCapability`, which inherits from `ICapability` to classify the capability. The capability system's generic nature allows for convenient grouping by context. The `ITestFrameworkCapability` groups all the capabilities implemented by the [testing framework](./unit-testing-platform-architecture-extensions.md#create-a-testing-framework). The `ICapabilities<TCapability>` interface reveals the *set* of all capabilities implemented by an extension. Similarly, for the base one, there's a context-specific testing framework called `ITestFrameworkCapabilities`. The `ITestFrameworkCapabilities` is provided to the platform during the [testing framework registration](./unit-testing-platform-architecture-extensions.md#register-a-testing-framework) process.
+
+To create a capability that addresses the aforementioned scenario, you define it as follows:
+
+```csharp
+public interface IDisableParallelismCapability : ITestFrameworkCapability
+{
+    bool CanDisableParallelism { get; }
+    bool CanProvidePerTestCpuConsumption { get; }
+    void Enable();
+}
+```
+
+If the testing framework implements this interface, at runtime, the following can be queried:
+
+* Verify if the testing framework has the ability to turn off parallelism `CanDisableParallelism = true`.
+* Determine if the testing framework can supply CPU usage data `CanProvidePerTestCPUConsumption = true`.
+* Request the testing adapter to activate this mode by invoking the `Enable()` method before the test session commences.
+
+The hypothetical code fragment inside the extension could be something like:
+
+```csharp
+IServiceProvider provider = null; // TODO: Get IServiceProvider.
+var capabilities = serviceProvider.GetRequiredService<ITestFrameworkCapabilities>();
+
+// Utilize the `GetCapability` API to search for the specific capability to query.
+var capability = capabilities.GetCapability<IDisableParallelismCapability>();
+if (capability is null)
+{
+    // Capability not supported...
+}
+else
+{
+    capability.Enable();
+    if (capability.CanDisableParallelism)
+    {
+        // Do something...
+    }
+
+    if (capability.CanProvidePerTestCpuConsumption)
+    {
+        // Do something...
+    }
+}
+```
+
+The preceding example illustrates how the capability infrastructure enables a powerful mechanism for communicating abilities between the components involved in a test session. While the sample demonstrates a capability specifically designed for the testing framework, any component can expose and implement extensions that inherit from `ICapability`.
+
+It's evident that not all details can be communicated through an interface. Considering the previous example, what should the extension expect if the `CanProvidePerTestCpuConsumption` is supported? What kind of custom information is expected to be transmitted via the [IMessageBus](./unit-testing-platform-architecture-services.md#the-imessagebus-service) by the testing framework? The solution is **documentation of the capability**. It's the responsibility of the capability *owner* to design, ship, and document it as clearly as possible to assist implementors who want to effectively *collaborate* with the extension that requires the specific capability.
+
+For instance, the TRX report extension enables the testing framework to implement the necessary capability to accurately generate a TRX report. The extension to register is included in the <https://www.nuget.org/packages/Microsoft.Testing.Extensions.TrxReport> NuGet package, but the capability to implement is found in the *contract only*<https://www.nuget.org/packages/Microsoft.Testing.Extensions.TrxReport.Abstractions> NuGet package.
+
+In conclusion, let's summarize the primary aspects of the capability system:
+
+* It's essential for facilitating clear and stable communication between components.
+* All capabilities should inherit from `ICapability` or an interface that inherits from it, and are exposed through a collection with the `ICapabilities` interface.
+* It aids in the evolution of features without causing breaking changes. If a certain capability isn't supported, appropriate action can be taken.
+* The responsibility of designing, shipping, and documenting the usage of a capability lies with the *capability owner*. The testing platform can also *own* a capability in the same way as any other extension.
+
+## Framework capabilities
+
+The platform exposes a specialized interface named `ITestFrameworkCapability` that is the base of all capabilities exposed for test frameworks. These capabilities are provided when [registering the test framework to the platform](./unit-testing-platform-architecture-extensions.md#register-a-testing-framework).
+
+### `IBannerMessageOwnerCapability`
+
+An optional [test framework capability](#framework-capabilities) that allows the test framework to provide the banner message to the platform. If the message is `null` or if the capability is n't present, the platform will use its default banner message.
+
+This capability implementation allows you to abstract away the various conditions that the test framework may need to consider when deciding whether or not the banner message should be displayed.
+
+The platform exposes the [`IPlatformInformation` service](./unit-testing-platform-architecture-services.md#the-iplatforminformation-service) to provide some information about the platform that could be useful when building your custom banner message.