Add the Robolectric Architecture page

This is basically a copy of [`ARCHITECTURE.md`](https://github.com/robolectric/robolectric/blob/master/ARCHITECTURE.md)
from [`robolectric/robolectric`](https://github.com/robolectric/robolectric),
with some small formatting changes.

Once this PR is merged, I'll update the original document
to redirect it on the website.

ping @hoisie for information.

docs/architecture.md

@@ -0,0 +1,210 @@
+# Robolectric architecture
+
+Robolectric is a unit testing framework that allows Android code to be tested on
+the JVM without the need for an emulator or device. This allows tests to run
+very quickly in a more hermetic environment. Robolectric has a complex
+architecture and makes use of many advanced features of the JVM, such as bytecode
+instrumentation and custom [`ClassLoader`](https://docs.oracle.com/javase/8/docs/api/java/lang/ClassLoader.html)s.
+This document provides a high level overview of Robolectric's architecture.
+
+## Android framework Jars and instrumentation
+
+At the heart of Robolectric are the Android framework Jars and the bytecode
+instrumentation. The Android framework Jars are a collection of Jar files that
+are built directly from Android platform sources. There is a single Jar file for
+each version of Android. These Jar files can be built by checking out an AOSP
+repo and building the
+[robolectric-host-android\_all](https://cs.android.com/android/platform/superproject/main/+/main:external/robolectric/Android.bp;l=112)
+target. Unlike the `android.jar` (stubs jar) files managed by Android Studio,
+which only contain public method signatures, the Robolectric android-all Jars
+contain the implementation of the Android Java framework. This gives Robolectric
+the ability to use as much real Android code as possible. A new android-all jar
+is uploaded to MavenCentral for each Android release. You can see the current
+android-all jars
+[here](https://repo1.maven.org/maven2/org/robolectric/android-all/).
+
+However, the pristine android-all jars are not the ones used during tests.
+Instead, Robolectric modifies the pristine android-all jars using bytecode
+instrumentation (see
+[`ClassInstrumentor`](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/ClassInstrumentor.java)).
+It performs several modifications:
+
+1. All Android methods, including constructors and static initializers, are
+   modified to support `shadowing`. This allows any method call to the Android
+   framework to be intercepted by Robolectric and delegated to a shadow method.
+   At a high level, this is done by iterating over each Android method and
+   converting it into two methods: the original method (but renamed), and the
+   `invokedynamic delegator` which can optionally invoke shadow methods if they
+   are available.
+
+1. Android constructors are specially modified to create shadow objects if a
+   shadow class is bound to the Android class being instantiated.
+
+1. Because the Android version of Java core classes (libcore) contains subtle
+   differences to the JDKs, certain problematic method calls have to be
+   intercepted and rewritten. See
+   [`AndroidInterceptors`](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/interceptors/AndroidInterceptors.java).
+
+1. Native methods undergo special instrumentation. Currently, native methods are
+   converted to no-op non-native methods that are shadowable by default.
+   However, there is now a native variant of each method also created. There are 
+   more details about native code in a section below.
+
+1. The `final` keyword is stripped from classes and methods.
+
+1. Some bespoke pieces of instrumentation, such as supporting
+   [`SparseArray.set()`](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/ClassInstrumentor.java#L204).
+
+This instrumentation is typically performed when a new release of Robolectric is
+made. These pre-instrumented Android-all jars are published on MavenCentral. See
+the
+[android-all-instrumented](https://repo1.maven.org/maven2/org/robolectric/android-all-instrumented/)
+path. They are lazily downloaded and during tests runtime using
+[`MavenArtifactFetcher`](https://github.com/robolectric/robolectric/blob/master/plugins/maven-dependency-resolver/src/main/java/org/robolectric/internal/dependency/MavenArtifactFetcher.java).
+
+Although Robolectric supports shadowing for Android framework classes, it is
+also possible for users to perform Robolectric instrumentation for any package
+(except built-in Java packages). This enables shadowing of arbitrary third-party code.
+
+## Shadows
+
+By default, when an Android method is invoked during a Robolectric test, the real
+Android framework code is invoked. This is because a lot of Android framework
+classes are pure Java code (e.g., the
+[`Intent`](https://cs.android.com/android/platform/superproject/main/+/main:frameworks/base/core/java/android/content/Intent.java)
+class or the
+[`org.json`](https://cs.android.com/android/platform/superproject/main/+/main:libcore/json/src/main/java/org/json/)
+package) and that code can run on the JVM without any modifications needed.
+
+However, there are cases where Robolectric needs to intercept and replace
+Android method calls. This most commonly occurs when Android system service or
+native methods are invoked. To do this, Robolectric uses a system called Shadow
+classes.
+
+Shadow classes are Java classes that contain the replacement code of Android
+methods when they are invoked. Each shadow class is bound to specific Android
+classes and methods through annotations. There are currently hundreds of shadow
+classes that can be found
+[here](https://github.com/robolectric/robolectric/tree/master/shadows/framework/src/main/java/org/robolectric/shadows).
+
+Shadow classes may optionally contain public APIs that can customize the
+behavior of the methods they are shadowing.
+
+Robolectric allows tests to specify custom shadows as well to provide user
+defined implementation for Android classes.
+
+## Shadow Packages and the Robolectric Annotation Processor 
+
+There are two categories of shadows: Robolectric’s built-in shadows that are
+aggregated using the [Robolectric Annotation Processor
+(RAP)](https://github.com/robolectric/robolectric/blob/master/processor/src/main/java/org/robolectric/annotation/processing/RobolectricProcessor.java),
+and custom shadows that are commonly specified using `@Config(shadows = …)`. RAP
+is configured to process all the shadow files that exist in Robolectric’s
+code. The main shadow package is [framework
+shadows](https://github.com/robolectric/robolectric/tree/master/shadows/framework),
+which contain shadows for the Android framework. There are other shadow packages
+in Robolectric's code, such as [httpclient
+shadows](https://github.com/robolectric/robolectric/tree/master/shadows/httpclient),
+but all of them outside of framework shadows are deprecated. When Robolectric is
+built, each shadow package is processed by RAP and a
+[ShadowProvider](https://github.com/robolectric/robolectric/blob/master/shadowapi/src/main/java/org/robolectric/internal/ShadowProvider.java)
+file is generated. For example, to see the `ShadowProvider` for the framework
+shadows, you can run:
+
+```shell
+./gradlew :shadows:framework:assemble
+cat ./shadows/framework/build/generated/src/apt/main/org/robolectric/Shadows.java
+```
+
+In this file you will see the class `public class Shadows implements
+ShadowProvider`.
+
+During runtime, Robolectric will use `ServiceLoader` to detect all shadow packages
+that implement `ShadowProvider` and the shadow classes contained in them.
+
+## `Sandbox` and `ClassLoader`
+
+Before a Robolectric test is executed, a
+[`Sandbox`](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/Sandbox.java)
+must be initialized. A `Sandbox` consists of some high-level structures that are
+necessary to run a Robolectric test. It primarily contains a
+[`SandboxClassLoader`](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/SandboxClassLoader.java),
+which is a custom `ClassLoader` that is bound to a specific instrumented
+Android-all jar. Sandboxes also contain the `ExecutorService` that serves as the
+main thread (UI thread) as well as high-level instrumentation configuration. The
+`SandboxClassLoader` is installed as the default `ClassLoader` for the test method.
+When any Android class is requested, `SandboxClassLoader` will attempt to load the
+Android class from the instrumented Android-all Jar first. The primary goal of
+`SandboxClassLoader` is to ensure that classes from the `android.jar` stubs jar are
+not inadvertently loaded. When classes from the `android.jar` stubs jar are
+loaded, attempting to invoke any method on them will result in a
+`RuntimeException("Stub!")` error. Typically, the Android stubs jar is on the
+classpath during a Robolectric test, but it is important not to load classes
+from the stubs jar.
+
+## Invokedynamic Delegators and `ShadowWrangler`
+
+This section provides more detail for `invokedynamic delegators` that were
+referenced in the instrumentation section. For an overview of the
+`invokedynamic` JVM instructions, you can search for articles or watch [YouTube
+videos such as this](https://www.youtube.com/watch?v=KhiECfzyVt0).
+
+To reiterate, for any Android method, Robolectric’s instrumentation adds an
+`invokedynamic delegator` that is responsible for determining at runtime to
+either invoke the real Android framework code or a shadow method. The first time
+an Android method is invoked in a `Sandbox`, it will result in a call to one of
+the bootstrap methods in
+[`InvokeDynamicSupport`](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/InvokeDynamicSupport.java).
+This will subsequently invoke the
+[`ShadowWrangler.findShadowMethodHandle()`](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/ShadowWrangler.java#L170)
+to determine if a shadow method exists for the method that is being invoked. If
+a shadow method is available, a `MethodHandle` to it will be returned. Otherwise, a
+`MethodHandle` for the original framework code will be returned.
+
+## Test lifecycle
+
+There is a lot of work done by Robolectric before and after a test is run.
+Besides the `Sandbox` and `ClassLoader` initialization mentioned above, there is
+also extensive Android environment initialization that occurs before each test.
+The high-level class for this is
+[`AndroidTestEnvironment`](https://github.com/robolectric/robolectric/blob/master/robolectric/src/main/java/org/robolectric/android/internal/AndroidTestEnvironment.java).
+This involves:
+
+* Initializing up the `Looper` mode (i.e., the scheduler)
+* Initializing system and app resources
+* Initializing the application context and system context
+* Loading the Android manifest for the test
+* Creating the `Application` object used for the test
+* Initializing the [display configuration](device-configuration.md)
+* Setting up the `ActivityThread`
+* Creating app directories
+
+It is possible for users to extend the test environment setup using
+[`TestEnvironmentLifecyclePlugin`](https://github.com/robolectric/robolectric/blob/master/pluginapi/src/main/java/org/robolectric/pluginapi/TestEnvironmentLifecyclePlugin.java).
+
+Similarly, after each test, many Android classes are reset during
+[`RobolectricTestRunner.finallyAfterTest()`](https://github.com/robolectric/robolectric/blob/master/robolectric/src/main/java/org/robolectric/RobolectricTestRunner.java#L299).
+This will iterate over all shadows and invoke their static `@Resetter` methods.
+
+## Plugin system
+
+Many parts of Robolectric can be customized using a plugin system based on
+Java’s
+[`ServiceLoader`](https://docs.oracle.com/javase/8/docs/api/java/util/ServiceLoader.html).
+This extensibility is useful when running Robolectric in more constrained
+environments. For example, by default, most of the Robolectric classes are
+designed to work in a Gradle/Android Studio environment. However, there are
+companies (such as Google) that use alternate build systems (such as Bazel), and
+it can be helpful to be able to customize the behavior of some core modules.
+
+The
+[`pluginapi`](https://github.com/robolectric/robolectric/tree/master/pluginapi)
+subproject contains many extension points of Robolectric. However, virtually any
+class that is loaded by Robolectric’s
+[`Injector`](https://github.com/robolectric/robolectric/blob/master/utils/src/main/java/org/robolectric/util/inject/Injector.java)
+has the ability to use
+[`PluginFinder`](https://github.com/robolectric/robolectric/blob/master/utils/src/main/java/org/robolectric/util/inject/PluginFinder.java),
+which means it can be extended at runtime.
+
+Typically, `ServiceLoaders` plugins can be easily written using the
+[`AutoService`](https://github.com/google/auto/tree/main/service) project.

docs/building-robolectric.md

@@ -68,5 +68,5 @@ If you're using Windows, it's recommended to use [PowerShell](https://github.com
 
 ## Next step
 
-Once you're up and running, you can have a look at the [architecture](https://github.com/robolectric/robolectric/blob/master/ARCHITECTURE.md) documentation
+Once you're up and running, you can have a look at the [architecture](architecture.md) documentation
 to learn more about Robolectric's components.

mkdocs.yml

@@ -113,6 +113,7 @@ nav:
     - "Customizing the Test Runner": custom-test-runner.md
   - "Contributing":
     - "Building Robolectric": building-robolectric.md
+    - "Robolectric Architecture": architecture.md
     - "Contributor Guidelines": contributing.md
     - "Shadows": extending.md
   - "Resources":