Add SystemUI build flag for Compose code (1/3)

This CL adds a Soong build flag that enables Compose code when building
SystemUI, or anything depending on SystemUI-core. The flag is disabled
by default, and will be first used to measure the impact of Compose on
our apk and system health. It will be turned on for one/many/all targets
once agreed with the Android System Health & Performance team.

To turn this flag on, one simply have to
`export SYSTEMUI_USE_COMPOSE=true` before compiling.

This flag is implemented such that:

 - When it is enabled, SystemUI-core depends on the SystemUIComposeCore
   library and compiles the source files of SystemUIComposeFeatures. In
   an ideal world, SystemUI-core would also just depend on
   SystemUIComposeFeatures instead of recompiling its sources, but given
   that the code in SystemUIComposeFeatures depends on code in
   SystemUI-core, this would lead to a cyclic dependency. Therefore,
   when this flag is enabled, we compile those files together.
   SystemUIComposeFeatures also contains a ComposeFacade object that is
   the bridge between SystemUI-core code and SystemUIComposeFeatures
   code.

 - When it is disabled, we only add another ComposeFacade object with
   the same API as the ComposeFacade object in SystemUIComposeFeatures,
   which throws when its functions are called (except for
   ComposeFacade#isComposeAvailable(), which returns false).

This way, any code in SystemUI-core can check
ComposeFacade#isComposeAvailable() first then call other functions to
indirectly access Compose code. See http://ag/20759151 for an example.

Bug: 242304109
Test: Builds
Change-Id: Ibd895a73041c163816e779273ec0b6588aabef76

packages/SystemUI/Android.bp

@@ -31,6 +31,51 @@ license {
     ],
 }
 
+// Opt-in configuration for code depending on Jetpack Compose.
+soong_config_module_type {
+    name: "systemui_compose_java_defaults",
+    module_type: "java_defaults",
+    config_namespace: "ANDROID",
+    bool_variables: ["SYSTEMUI_USE_COMPOSE"],
+    properties: [
+        "srcs",
+        "static_libs",
+    ],
+}
+
+systemui_compose_java_defaults {
+    name: "SystemUI_compose_defaults",
+    soong_config_variables: {
+        SYSTEMUI_USE_COMPOSE: {
+            // Because files in compose/features/ depend on SystemUI
+            // code, we compile those files when compiling SystemUI-core.
+            // We also compile the ComposeFacade in
+            // compose/facade/enabled/.
+            srcs: [
+                "compose/features/src/**/*.kt",
+                "compose/facade/enabled/src/**/*.kt",
+            ],
+
+            // The dependencies needed by SystemUIComposeFeatures,
+            // except for SystemUI-core.
+            // Copied from compose/features/Android.bp.
+            static_libs: [
+                "SystemUIComposeCore",
+
+                "androidx.compose.runtime_runtime",
+                "androidx.compose.material3_material3",
+            ],
+
+            // By default, Compose is disabled and we compile the ComposeFacade
+            // in compose/facade/disabled/.
+            conditions_default: {
+                srcs: ["compose/facade/disabled/src/**/*.kt"],
+                static_libs: [],
+            },
+        },
+    },
+}
+
 java_library {
     name: "SystemUI-proto",
 
@@ -68,6 +113,9 @@ filegroup {
 
 android_library {
     name: "SystemUI-core",
+    defaults: [
+        "SystemUI_compose_defaults",
+    ],
     srcs: [
         "src/**/*.kt",
         "src/**/*.java",
@@ -227,6 +275,9 @@ android_library {
 
 android_library {
     name: "SystemUI-tests",
+    defaults: [
+        "SystemUI_compose_defaults",
+    ],
     manifest: "tests/AndroidManifest-base.xml",
     additional_manifests: ["tests/AndroidManifest.xml"],
     srcs: [

packages/SystemUI/compose/facade/disabled/src/com/android/systemui/compose/ComposeFacade.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package com.android.systemui.compose
+
+/** The Compose facade, when Compose is *not* available. */
+object ComposeFacade : BaseComposeFacade {
+    override fun isComposeAvailable(): Boolean = false
+
+    private fun throwComposeUnavailableError() {
+        error(
+            "Compose is not available. Make sure to check isComposeAvailable() before calling any" +
+                " other function on ComposeFacade."
+        )
+    }
+}

packages/SystemUI/compose/facade/enabled/src/com/android/systemui/compose/ComposeFacade.kt

@@ -0,0 +1,22 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.systemui.compose
+
+/** The Compose facade, when Compose is available. */
+object ComposeFacade : BaseComposeFacade {
+    override fun isComposeAvailable(): Boolean = true
+}

packages/SystemUI/src/com/android/systemui/compose/BaseComposeFacade.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package com.android.systemui.compose
+
+/**
+ * A facade to interact with Compose, when it is available.
+ *
+ * You should access this facade by calling the static methods on
+ * [com.android.systemui.compose.ComposeFacade] directly.
+ */
+interface BaseComposeFacade {
+    /**
+     * Whether Compose is currently available. This function should be checked before calling any
+     * other functions on this facade.
+     *
+     * This value will never change at runtime.
+     */
+    fun isComposeAvailable(): Boolean
+}