Add Deferred<T> api definitions.

This is needed to start preparing interop interfaces before we enable
deferred lint enforcement.

Author: vkryachko

firebase-components/src/main/java/com/google/firebase/components/annotations/DeferredApi.java

@@ -0,0 +1,42 @@
+// Copyright 2020 Google LLC
+//
+// 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.google.firebase.components.annotations;
+
+import com.google.firebase.inject.Deferred.DeferredHandler;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Inherited;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Indicates that the annotated symbol should be called from a {@link
+ * com.google.firebase.inject.Deferred} dependency.
+ *
+ * <p>This is particularly important for callback-style APIs in the context of <a
+ * href="https://developer.android.com/guide/app-bundle/play-feature-delivery">dynamically loaded
+ * modules</a>.
+ *
+ * <p>Calling {@link DeferredApi }-annotated methods is allowed only from:
+ *
+ * <ol>
+ *   <li>{@link com.google.firebase.inject.Deferred#whenAvailable(DeferredHandler)} handlers.
+ *   <li>Any method that is itself annotated with {@link DeferredApi}(Useful to be able to call such
+ *       methods from {@link com.google.firebase.inject.Deferred#whenAvailable(DeferredHandler)}).
+ */
+@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
+@Retention(RetentionPolicy.CLASS)
+@Inherited
+public @interface DeferredApi {}

firebase-components/src/main/java/com/google/firebase/components/annotations/package-info.java

@@ -0,0 +1,16 @@
+// Copyright 2018 Google LLC
+//
+// 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.
+
+/** @hide */
+package com.google.firebase.components.annotations;

firebase-components/src/main/java/com/google/firebase/inject/Deferred.java

@@ -0,0 +1,49 @@
+// Copyright 2020 Google LLC
+//
+// 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.google.firebase.inject;
+
+import androidx.annotation.NonNull;
+import com.google.firebase.components.annotations.DeferredApi;
+
+/**
+ * Represents a continuation-style dependency.
+ *
+ * <p>The motivation for it is to model optional dependencies that may become available in the
+ * future and once they do, the depender will get notified automatically via the registered {@link
+ * DeferredHandler}.
+ *
+ * <p>Example:
+ *
+ * <pre>{@code
+ * class Foo {
+ *   Foo(Deferred<Bar> bar) {
+ *     bar.whenAvailable(barProvider -> {
+ *       // automatically called when Bar becomes available
+ *       use(barProvider.get());
+ *     });
+ *   }
+ * }
+ * }</pre>
+ */
+public interface Deferred<T> {
+  /** Used by dependers to register their callbacks. */
+  interface DeferredHandler<T> {
+    @DeferredApi
+    void handle(Provider<T> provider);
+  }
+
+  /** Register a callback that is executed once {@link T} becomes available */
+  void whenAvailable(@NonNull DeferredHandler<T> handler);
+}

firebase-components/src/main/java/com/google/firebase/inject/Provider.java

@@ -14,11 +14,7 @@
 
 package com.google.firebase.inject;
 
-/**
- * Provides instances of T.
- *
- * @hide
- */
+/** Provides instances of T. */
 public interface Provider<T> {
   /** Provides a fully constructed instance of T. */
   T get();

firebase-components/src/main/java/com/google/firebase/inject/package-info.java

@@ -0,0 +1,16 @@
+// Copyright 2018 Google LLC
+//
+// 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.
+
+/** @hide */
+package com.google.firebase.inject;