bug 617008 - avoid creating a rendering context that we may not need. r=roc

gfx/src/nsFontMetrics.cpp

@@ -74,6 +74,14 @@ public:
         NS_ERROR("This shouldn't be called because we never enable hyphens");
         return 0;
     }
+    virtual already_AddRefed<gfxContext> GetContext() {
+        NS_ERROR("This shouldn't be called because we never enable hyphens");
+        return nullptr;
+    }
+    virtual uint32_t GetAppUnitsPerDevUnit() {
+        NS_ERROR("This shouldn't be called because we never enable hyphens");
+        return 60;
+    }
     virtual void GetSpacing(uint32_t aStart, uint32_t aLength,
                             Spacing* aSpacing) {
         NS_ERROR("This shouldn't be called because we never enable spacing");

gfx/thebes/gfxFont.cpp

@@ -4878,13 +4878,17 @@ gfxFontGroup::MakeHyphenTextRun(gfxContext *aCtx, uint32_t aAppUnitsPerDevUnit)
 }
 
 gfxFloat
-gfxFontGroup::GetHyphenWidth(gfxContext *aCtx, uint32_t aAppUnitsPerDevUnit)
+gfxFontGroup::GetHyphenWidth(gfxTextRun::PropertyProvider *aProvider)
 {
     if (mHyphenWidth < 0) {
-        nsAutoPtr<gfxTextRun> hyphRun(MakeHyphenTextRun(aCtx,
-                                                        aAppUnitsPerDevUnit));
-        mHyphenWidth = hyphRun.get() ?
-            hyphRun->GetAdvanceWidth(0, hyphRun->GetLength(), nullptr) : 0;
+        nsRefPtr<gfxContext> ctx(aProvider->GetContext());
+        if (ctx) {
+            nsAutoPtr<gfxTextRun>
+                hyphRun(MakeHyphenTextRun(ctx,
+                                          aProvider->GetAppUnitsPerDevUnit()));
+            mHyphenWidth = hyphRun.get() ?
+                hyphRun->GetAdvanceWidth(0, hyphRun->GetLength(), nullptr) : 0;
+        }
     }
     return mHyphenWidth;
 }

gfx/thebes/gfxFont.h

@@ -2834,6 +2834,14 @@ public:
          */
         virtual void GetSpacing(uint32_t aStart, uint32_t aLength,
                                 Spacing *aSpacing) = 0;
+
+        // Returns a gfxContext that can be used to measure the hyphen glyph.
+        // Only called if the hyphen width is requested.
+        virtual already_AddRefed<gfxContext> GetContext() = 0;
+
+        // Return the appUnitsPerDevUnit value to be used when measuring.
+        // Only called if the hyphen width is requested.
+        virtual uint32_t GetAppUnitsPerDevUnit() = 0;
     };
 
     class ClusterIterator {
@@ -3433,7 +3441,7 @@ public:
      * needed to initialize the cached hyphen width; otherwise they are
      * ignored.
      */
-    gfxFloat GetHyphenWidth(gfxContext *aCtx, uint32_t aAppUnitsPerDevUnit);
+    gfxFloat GetHyphenWidth(gfxTextRun::PropertyProvider* aProvider);
 
     /**
      * Make a text run representing a single hyphen character.

layout/generic/nsTextFrame.cpp

@@ -2839,6 +2839,14 @@ public:
     return mTextStyle->mHyphens;
   }
 
+  virtual already_AddRefed<gfxContext> GetContext() {
+    return GetReferenceRenderingContext(GetFrame(), nullptr);
+  }
+
+  virtual uint32_t GetAppUnitsPerDevUnit() {
+    return mTextRun->GetAppUnitsPerDevUnit();
+  }
+
   void GetSpacingInternal(uint32_t aStart, uint32_t aLength, Spacing* aSpacing,
                           bool aIgnoreTabs);
 
@@ -3194,16 +3202,9 @@ gfxFloat
 PropertyProvider::GetHyphenWidth()
 {
   if (mHyphenWidth < 0) {
-    mHyphenWidth = mLetterSpacing;
-    nsRefPtr<gfxContext> context(GetReferenceRenderingContext(GetFrame(),
-                                                              nullptr));
-    if (context) {
-      mHyphenWidth +=
-        GetFontGroup()->GetHyphenWidth(context,
-                                       mTextRun->GetAppUnitsPerDevUnit());
-    }
+    mHyphenWidth = GetFontGroup()->GetHyphenWidth(this);
   }
-  return mHyphenWidth;
+  return mHyphenWidth + mLetterSpacing;
 }
 
 void