commit to xfermode objects being immutable

src/core/SkXfermode.cpp

Index: src/core/SkXfermode.cpp
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index e50e4f5ce58179cacbea609785d69a27bea8cb4f..6410d18de848b377d750a52cdb29b01184423b10 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -1688,26 +1688,60 @@ void SkDstOutXfermode::toString(SkString* str) const {
 
 ///////////////////////////////////////////////////////////////////////////////
 
+static SkMutex gCachedXfermodesMutex;

[djsollen, 2013/10/04 14:48:28]

use SK_DECLARE_STATIC_MUTEX

+static SkXfermode* gCachedXfermodes[SkXfermode::kLastMode + 1];
+
+void SkXfermode::Term() {
+    SkAutoMutexAcquire ac(gCachedXfermodesMutex);
+    
+    for (size_t i = 0; i < SK_ARRAY_COUNT(gCachedXfermodes); ++i) {
+        SkSafeUnref(gCachedXfermodes[i]);
+        gCachedXfermodes[i] = NULL;
+    }
+}
+
 SkXfermode* SkXfermode::Create(Mode mode) {
     SkASSERT(SK_ARRAY_COUNT(gProcCoeffs) == kModeCount);
-    SkASSERT((unsigned)mode < kModeCount);
+    SkASSERT(SK_ARRAY_COUNT(gCachedXfermodes) == kModeCount);
+    
+    if ((unsigned)mode >= kModeCount) {
+        // report error
+        return NULL;
+    }
 
-    const ProcCoeff& rec = gProcCoeffs[mode];
+    if (kSrcOver_Mode == mode) {
+        return NULL;
+    }
 
-    switch (mode) {
-        case kClear_Mode:
-            return SkNEW_ARGS(SkClearXfermode, (rec));
-        case kSrc_Mode:
-            return SkNEW_ARGS(SkSrcXfermode, (rec));
-        case kSrcOver_Mode:
-            return NULL;
-        case kDstIn_Mode:
-            return SkNEW_ARGS(SkDstInXfermode, (rec));
-        case kDstOut_Mode:
-            return SkNEW_ARGS(SkDstOutXfermode, (rec));
-        default:
-            return SkNEW_ARGS(SkProcCoeffXfermode, (rec, mode));
+    // guard our access to gCachedXfermodes, since we may write into it
+    SkAutoMutexAcquire ac(gCachedXfermodesMutex);
+
+    SkXfermode* xfer = gCachedXfermodes[mode];
+    if (NULL == xfer) {
+        const ProcCoeff& rec = gProcCoeffs[mode];
+        switch (mode) {

[mtklein, 2013/10/04 15:07:01]

As someone who's not too familiar with this code, some things are tripping me
up:
  - why is kSrcOver special?
  - why do all the other modes get lumped into ProcCoeffXfermode?

I realize this isn't a logical change.  The original code confuses me too. :)

+            case kClear_Mode:
+                xfer = SkNEW_ARGS(SkClearXfermode, (rec));
+                break;
+            case kSrc_Mode:
+                xfer = SkNEW_ARGS(SkSrcXfermode, (rec));
+                break;
+            case kSrcOver_Mode:
+                SkASSERT(false);    // should not land here
+                break;
+            case kDstIn_Mode:
+                xfer = SkNEW_ARGS(SkDstInXfermode, (rec));
+                break;
+            case kDstOut_Mode:
+                xfer = SkNEW_ARGS(SkDstOutXfermode, (rec));
+                break;
+            default:
+                xfer = SkNEW_ARGS(SkProcCoeffXfermode, (rec, mode));
+                break;
+        }
+        gCachedXfermodes[mode] = xfer;
     }
+    return SkSafeRef(xfer);
 }
 
 SkXfermodeProc SkXfermode::GetProc(Mode mode) {