Bug 1287946 - Improve usage of StringParam in PrefsHelper; r=me

PrefsHelper uses Maybe<> as a replacement for a ternary expression, in
order to work around jni::StringParam's lack of copy constructor.
However, we can add a move constructor to StringParam, which lets us use
it in a ternary expression, and avoid the awkwardness of Maybe<>.

widget/android/PrefsHelper.h

@@ -12,10 +12,8 @@
 #include "nsCOMPtr.h"
 #include "nsVariant.h"
 
-#include "mozilla/Maybe.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/Services.h"
-#include "mozilla/UniquePtr.h"
 
 namespace mozilla {
 
@@ -68,20 +66,17 @@ class PrefsHelper
                 return false;
         }
 
-        Maybe<jni::StringParam> jstrVal;
-        jstrVal.emplace(nullptr);
-        if (type == java::PrefsHelper::PREF_STRING) {
-            jstrVal.reset();
-            jstrVal.emplace(strVal, aPrefName.Env());
-        }
+        jni::StringParam jstrVal(type == java::PrefsHelper::PREF_STRING ?
+                jni::StringParam(strVal, aPrefName.Env()) :
+                jni::StringParam(nullptr));
 
         if (aPrefHandler) {
             java::PrefsHelper::CallPrefHandler(
                     aPrefHandler, type, aPrefName,
-                    boolVal, intVal, jstrVal.ref());
+                    boolVal, intVal, jstrVal);
         } else {
             java::PrefsHelper::OnPrefChange(
-                    aPrefName, type, boolVal, intVal, jstrVal.ref());
+                    aPrefName, type, boolVal, intVal, jstrVal);
         }
         return true;
     }
@@ -194,16 +189,11 @@ public:
                     continue;
             }
 
-            Maybe<jni::StringParam> jstrVal;
-            jstrVal.emplace(nullptr);
-            if (type == java::PrefsHelper::PREF_STRING) {
-                jstrVal.reset();
-                jstrVal.emplace(strVal, aCls.Env());
-            }
-
             java::PrefsHelper::CallPrefHandler(
-                    aPrefHandler, type, nameStr,
-                    boolVal, intVal, jstrVal.ref());
+                    aPrefHandler, type, nameStr, boolVal, intVal,
+                    jni::StringParam(type == java::PrefsHelper::PREF_STRING ?
+                        jni::StringParam(strVal, aCls.Env()) :
+                        jni::StringParam(nullptr)));
         }
 
         java::PrefsHelper::CallPrefHandler(
@@ -323,15 +313,10 @@ public:
                 return;
         }
 
-        Maybe<jni::StringParam> jstrVal;
-        jstrVal.emplace(nullptr);
-        if (type == java::PrefsHelper::PREF_STRING) {
-            jstrVal.reset();
-            jstrVal.emplace(strVal);
-        }
-
         java::PrefsHelper::OnPrefChange(
-                name, type, boolVal, intVal, jstrVal.ref());
+                name, type, boolVal, intVal,
+                jni::StringParam(type == java::PrefsHelper::PREF_STRING ?
+                    jni::StringParam(strVal) : jni::StringParam(nullptr)));
     }
 };
 

widget/android/jni/Refs.h

@@ -680,9 +680,16 @@ public:
         , mEnv(env)
     {}
 
+    StringParam(StringParam&& other)
+        : Ref(other.Get())
+        , mEnv(other.mEnv)
+    {
+        other.mInstance = nullptr;
+    }
+
     ~StringParam()
     {
-        if (mEnv) {
+        if (mEnv && Get()) {
             mEnv->DeleteLocalRef(Get());
         }
     }