A new Client-Side FeatureFlags library.

This is just a stub at the moment. Follow-on cls contain more
implementation.

This cl describes the basic shape for usage:

  BooleanFlag FOOBAR = FeatureFlags.booleanFlag("foo", "bar", false);
  FeatureFlags.getInstance().isEnabled(FOOBAR);

Defined are 4 flags types:
  - BooleanFlag: can be either true or false. Held constant until
        device restart.
  - DynamicBooleanFlag: can be either true or false. Can change while
        a process is running.
  - FusedOnFlag: Always true.
  - FusedOffFlag: Always false.

More flag types are intended (Resource & SystemProperty backed,
Strings, Floats, Ints, etc). but this cl represents the MVP.

More about the details of this client can be read at:
http://go/android-flagging-dd

Test: m Android-Flags
Bug: 279054964
Change-Id: I0e455bf69f01dc675ec8bf64e584e2beff4e72ce

core/java/android/flags/BooleanFlag.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2023 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 android.flags;
+
+import android.annotation.NonNull;
+
+/**
+ * A flag representing a true or false value.
+ *
+ * The value will always be the same during the lifetime of the process it is read in.
+ *
+ * @hide
+ */
+public class BooleanFlag implements Flag<Boolean> {
+    private final String mNamespace;
+    private final String mName;
+    private final boolean mDefault;
+
+    /**
+     * @param namespace A namespace for this flag. See {@link android.provider.DeviceConfig}.
+     * @param name A name for this flag.
+     * @param defaultValue The value of this flag if no other override is present.
+     */
+    BooleanFlag(String namespace, String name, boolean defaultValue) {
+        mNamespace = namespace;
+        mName = name;
+        mDefault = defaultValue;
+    }
+
+    @Override
+    @NonNull
+    public Boolean getDefault() {
+        return mDefault;
+    }
+
+    @Override
+    @NonNull
+    public String getNamespace() {
+        return mNamespace;
+    }
+
+    @Override
+    @NonNull
+    public String getName() {
+        return mName;
+    }
+
+    @Override
+    @NonNull
+    public String toString() {
+        return getNamespace() + "." + getName() + "[" + getDefault() + "]";
+    }
+}

core/java/android/flags/DynamicBooleanFlag.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2023 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 android.flags;
+
+/**
+ * A flag representing a true or false value.
+ *
+ * The value may be different from one read to the next.
+ *
+ * @hide
+ */
+public class DynamicBooleanFlag implements DynamicFlag<Boolean> {
+
+    private final String mNamespace;
+    private final String mName;
+    private final boolean mDefault;
+
+    /**
+     * @param namespace A namespace for this flag. See {@link android.provider.DeviceConfig}.
+     * @param name A name for this flag.
+     * @param defaultValue The value of this flag if no other override is present.
+     */
+    DynamicBooleanFlag(String namespace, String name, boolean defaultValue) {
+        mNamespace = namespace;
+        mName = name;
+        mDefault = defaultValue;
+    }
+
+    @Override
+    public String getNamespace() {
+        return mNamespace;
+    }
+
+    @Override
+    public String getName() {
+        return mName;
+    }
+
+    @Override
+    public Boolean getDefault() {
+        return mDefault;
+    }
+
+    @Override
+    public String toString() {
+        return getNamespace() + "." + getName() + "[" + getDefault() + "]";
+    }
+}

core/java/android/flags/DynamicFlag.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2023 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 android.flags;
+
+/**
+ * A flag for which the value may be different from one read to the next.
+ *
+ * @param <T> The type of value that this flag stores. E.g. Boolean or String.
+ *
+ * @hide
+ */
+public interface DynamicFlag<T> extends Flag<T> {
+}

core/java/android/flags/FeatureFlags.java

@@ -0,0 +1,131 @@
+/*
+ * Copyright (C) 2023 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 android.flags;
+
+import android.annotation.NonNull;
+
+import java.util.HashSet;
+import java.util.Set;
+
+/**
+ * A class for querying constants from the system - primarily booleans.
+ *
+ * Clients using this class can define their flags and their default values in one place,
+ * can override those values on running devices for debugging and testing purposes, and can control
+ * what flags are available to be used on release builds.
+ *
+ * TODO(b/279054964): A lot. This is skeleton code right now.
+ * @hide
+ */
+public class FeatureFlags {
+    private static FeatureFlags sInstance;
+    private static final Object sInstanceLock = new Object();
+
+    private final Set<ChangeListener> mListeners = new HashSet<>();
+
+    /**
+     * Obtain a per-process instance of FeatureFlags.
+     * @return
+     */
+    @NonNull
+    public static FeatureFlags getInstance() {
+        synchronized (sInstanceLock) {
+            if (sInstance == null) {
+                sInstance = new FeatureFlags();
+            }
+        }
+
+        return sInstance;
+    }
+
+    FeatureFlags() {
+    }
+
+    /**
+     * Returns whether the supplied flag is true or not.
+     *
+     * {@link BooleanFlag} should only be used in debug builds. They do not get optimized out.
+     *
+     * The first time a flag is read, its value is cached for the lifetime of the process.
+     */
+    public boolean isEnabled(@NonNull BooleanFlag flag) {
+        return flag.getDefault();
+    }
+
+    /**
+     * Returns whether the supplied flag is true or not.
+     *
+     * Always returns false.
+     */
+    public boolean isEnabled(@NonNull FusedOffFlag flag) {
+        return false;
+    }
+
+    /**
+     * Returns whether the supplied flag is true or not.
+     *
+     * Always returns true;
+     */
+    public boolean isEnabled(@NonNull FusedOnFlag flag) {
+        return true;
+    }
+
+    /**
+     * Returns whether the supplied flag is true or not.
+     *
+     * Can return a different value for the flag each time it is called if an override comes in.
+     */
+    public boolean isCurrentlyEnabled(@NonNull DynamicBooleanFlag flag) {
+        return flag.getDefault();
+    }
+
+    /**
+     * Add a listener to be alerted when a {@link DynamicFlag} changes.
+     *
+     * See also {@link #removeChangeListener(ChangeListener)}.
+     *
+     * @param listener The listener to add.
+     */
+    public void addChangeListener(@NonNull ChangeListener listener) {
+        mListeners.add(listener);
+    }
+
+    /**
+     * Remove a listener that was added earlier.
+     *
+     * See also {@link #addChangeListener(ChangeListener)}.
+     *
+     * @param listener The listener to remove.
+     */
+    public void removeChangeListener(@NonNull ChangeListener listener) {
+        mListeners.remove(listener);
+    }
+
+    /**
+     * A simpler listener that is alerted when a {@link DynamicFlag} changes.
+     *
+     * See {@link #addChangeListener(ChangeListener)}
+     */
+    public interface ChangeListener {
+        /**
+         * Called when a {@link DynamicFlag} changes.
+         *
+         * @param flag The flag that has changed.
+         */
+        void onFlagChanged(DynamicFlag<?> flag);
+    }
+}

core/java/android/flags/FeatureFlagsFake.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) 2023 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 android.flags;
+
+import android.annotation.NonNull;
+
+/**
+ * An implementation of {@link FeatureFlags} for testing.
+ * @hide
+ */
+public class FeatureFlagsFake extends FeatureFlags {
+    public FeatureFlagsFake() {
+        super();
+    }
+
+    @Override
+    public boolean isEnabled(@NonNull BooleanFlag flag) {
+        return flag.getDefault();
+    }
+
+    @Override
+    public boolean isEnabled(@NonNull FusedOffFlag flag) {
+        return false;
+    }
+
+    @Override
+    public boolean isEnabled(@NonNull FusedOnFlag flag) {
+        return true;
+    }
+
+    @Override
+    public boolean isCurrentlyEnabled(@NonNull DynamicBooleanFlag flag) {
+        return flag.getDefault();
+    }
+}

core/java/android/flags/Flag.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2023 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 android.flags;
+
+import android.annotation.NonNull;
+
+/**
+ * Base class for constants read via {@link android.flags.FeatureFlags}.
+ *
+ * @param <T> The type of value that this flag stores. E.g. Boolean or String.
+ *
+ * @hide
+ */
+public interface Flag<T> {
+    /** The namespace for a flag. Should combine uniquely with its name. */
+    @NonNull
+    String getNamespace();
+
+    /** The name of the flag. Should combine uniquely with its namespace. */
+    @NonNull
+    String getName();
+
+    /** The value of this flag if no override has been set. Null values are not supported. */
+    @NonNull
+    T getDefault();
+}

core/java/android/flags/FusedOffFlag.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2023 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 android.flags;
+
+import android.annotation.NonNull;
+
+/**
+ * A flag representing a false value.
+ *
+ * The flag can never be changed or overridden. It is false at compile time.
+ *
+ * @hide
+ */
+public final class FusedOffFlag implements Flag<Boolean> {
+    private final String mNamespace;
+    private final String mName;
+
+    FusedOffFlag(String namespace, String name) {
+        mNamespace = namespace;
+        mName = name;
+    }
+
+    @Override
+    @NonNull
+    public Boolean getDefault() {
+        return false;
+    }
+
+    @Override
+    @NonNull
+    public String getNamespace() {
+        return mNamespace;
+    }
+
+    @Override
+    @NonNull
+    public String getName() {
+        return mName;
+    }
+
+    @Override
+    @NonNull
+    public String toString() {
+        return getNamespace() + "." + getName() + "[false]";
+    }
+}

core/java/android/flags/FusedOnFlag.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2023 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 android.flags;
+
+import android.annotation.NonNull;
+
+/**
+ * A flag representing a true value.
+ *
+ * The flag can never be changed or overridden. It is true at compile time.
+ *
+ * @hide
+ */
+public final class FusedOnFlag implements Flag<Boolean> {
+    private final String mNamespace;
+    private final String mName;
+
+    FusedOnFlag(String namespace, String name) {
+        mNamespace = namespace;
+        mName = name;
+    }
+
+    @Override
+    @NonNull
+    public Boolean getDefault() {
+        return true;
+    }
+
+    @Override
+    @NonNull
+    public String getNamespace() {
+        return mNamespace;
+    }
+
+    @Override
+    @NonNull
+    public String getName() {
+        return mName;
+    }
+
+    @Override
+    @NonNull
+    public String toString() {
+        return getNamespace() + "." + getName() + "[true]";
+    }
+}

core/java/android/flags/OWNERS

@@ -0,0 +1,7 @@
+# Bug component: 1306523
+
+mankoff@google.com
+pixel@google.com
+
+dsandler@android.com
+

core/tests/coretests/src/android/flags/FeatureFlagsTest.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2020 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 android.flags;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import android.platform.test.annotations.Presubmit;
+
+import androidx.test.filters.SmallTest;
+
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Ignore;
+import org.junit.Test;
+import org.mockito.MockitoAnnotations;
+
+@SmallTest
+@Presubmit
+public final class FeatureFlagsTest {
+
+    @Before
+    public void setup() {
+        MockitoAnnotations.initMocks(this);
+    }
+
+    @After
+    public void tearDown() {
+    }
+
+    @Test
+    public void testPass() {
+        assertThat(true).isTrue();
+    }
+
+    @Ignore
+    @Test
+    public void testFail() {
+        assertThat(false).isTrue();
+    }
+}