Support `@RpcOptional`

With this annotation, arguments which are not present when calling will be assign `null` value by default.

Usage:

```java
@Rpc(description = "Returns true if value is null, false otherwise.")
public boolean isValueNull(@RpcOptional Integer value) {
    return value == null;
}
```

```python
assert mbs.isValueNull()==True
assert mbs.isValueNull(0)==False
```

2 files changed, 73 insertions, 0 deletions

diff --git a/third_party/sl4a/src/main/java/com/google/android/mobly/snippet/rpc/MethodDescriptor.java b/third_party/sl4a/src/main/java/com/google/android/mobly/snippet/rpc/MethodDescriptor.java
index b9c8a7a..506fae9 100644
--- a/third_party/sl4a/src/main/java/com/google/android/mobly/snippet/rpc/MethodDescriptor.java
+++ b/third_party/sl4a/src/main/java/com/google/android/mobly/snippet/rpc/MethodDescriptor.java
@@ -22,6 +22,7 @@ import com.google.android.mobly.snippet.Snippet;
 import com.google.android.mobly.snippet.manager.SnippetManager;
 import com.google.android.mobly.snippet.manager.SnippetObjectConverterManager;
 import com.google.android.mobly.snippet.util.AndroidUtil;
+import java.lang.annotation.Annotation;
 import java.lang.reflect.Method;
 import java.lang.reflect.Type;
 import java.util.ArrayList;
@@ -68,6 +69,7 @@ public final class MethodDescriptor {
      * @throws Throwable the exception raised from executing the RPC method.
      */
     public Object invoke(SnippetManager manager, final JSONArray parameters) throws Throwable {
+        final Annotation annotations[][] = getParameterAnnotations();
         final Type[] parameterTypes = getGenericParameterTypes();
         final Object[] args = new Object[parameterTypes.length];
 
@@ -79,6 +81,8 @@ public final class MethodDescriptor {
             final Type parameterType = parameterTypes[i];
             if (i < parameters.length()) {
                 args[i] = convertParameter(parameters, i, parameterType);
+            } else if (MethodDescriptor.hasDefaultValue(annotations[i])) {
+                args[i] = MethodDescriptor.getDefaultValue(parameterType, annotations[i]);
             } else {
                 throw new RpcError("Argument " + (i + 1) + " is not present");
             }
@@ -218,6 +222,10 @@ public final class MethodDescriptor {
         return mClass;
     }
 
+    public Annotation[][] getParameterAnnotations() {
+        return mMethod.getParameterAnnotations();
+    }
+
     private String getAnnotationDescription() {
         if (isAsync()) {
             AsyncRpc annotation = mMethod.getAnnotation(AsyncRpc.class);
@@ -226,6 +234,7 @@ public final class MethodDescriptor {
         Rpc annotation = mMethod.getAnnotation(Rpc.class);
         return annotation.description();
     }
+
     /**
      * Returns a human-readable help text for this RPC, based on annotations in the source code.
      *
@@ -249,4 +258,34 @@ public final class MethodDescriptor {
                 mMethod.getReturnType().getSimpleName(),
                 getAnnotationDescription());
     }
+
+    /**
+     * Returns the default value for a specific parameter.
+     *
+     * @param parameterType parameterType
+     * @param annotations   annotations of the parameter
+     */
+    public static Object getDefaultValue(Type parameterType, Annotation[] annotations) {
+        for (Annotation a : annotations) {
+            if (a instanceof RpcOptional) {
+                return null;
+            }
+        }
+        throw new IllegalStateException("No default value for " + parameterType);
+    }
+
+    /**
+     * Determines whether or not this parameter has default value.
+     *
+     * @param annotations annotations of the parameter
+     */
+    public static boolean hasDefaultValue(Annotation[] annotations) {
+        for (Annotation a : annotations) {
+            if (a instanceof RpcOptional) {
+                return true;
+            }
+        }
+        return false;
+    }
+
 }
diff --git a/third_party/sl4a/src/main/java/com/google/android/mobly/snippet/rpc/RpcOptional.java b/third_party/sl4a/src/main/java/com/google/android/mobly/snippet/rpc/RpcOptional.java
new file mode 100644
index 0000000..b4b43aa
--- /dev/null
+++ b/third_party/sl4a/src/main/java/com/google/android/mobly/snippet/rpc/RpcOptional.java
@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2016 Google Inc.
+ *
+ * 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.android.mobly.snippet.rpc;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Use this annotation to mark RPC parameter as optional.
+ *
+ * <p>The parameter marked as optional has no explicit default value. {@code null} is used as
+ * default value.
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.PARAMETER)
+@Documented
+public @interface RpcOptional {}