3 files changed, 42 insertions, 3 deletions
diff --git a/include/core/SkStream.h b/include/core/SkStream.h
index 10929a8343..8df59afaf3 100644
--- a/include/core/SkStream.h
+++ b/include/core/SkStream.h
@@ -376,6 +376,9 @@ public:
     void copyTo(void* dst) const;
     void writeToStream(SkWStream* dst) const;
 
+    /** Equivalent to copyTo() followed by reset(), but may save memory use. */
+    void copyToAndReset(void* dst);
+
     /** Return the contents as SkData, and then reset the stream. */
     sk_sp<SkData> detachAsData();
 
diff --git a/src/core/SkStream.cpp b/src/core/SkStream.cpp
index 1435dab235..133df83078 100644
--- a/src/core/SkStream.cpp
+++ b/src/core/SkStream.cpp
@@ -570,15 +570,31 @@ void SkDynamicMemoryWStream::padToAlign4() {
     }
 }
 
+
+void SkDynamicMemoryWStream::copyToAndReset(void* ptr) {
+    // By looping through the source and freeing as we copy, we
+    // can reduce real memory use with large streams.
+    char* dst = reinterpret_cast<char*>(ptr);
+    Block* block = fHead;
+    while (block != nullptr) {
+        size_t len = block->written();
+        memcpy(dst, block->start(), len);
+        dst += len;
+        Block* next = block->fNext;
+        sk_free(block);
+        block = next;
+    }
+    fHead = fTail = nullptr;
+    fBytesWrittenBeforeTail = 0;
+}
+
 sk_sp<SkData> SkDynamicMemoryWStream::detachAsData() {
     const size_t size = this->bytesWritten();
     if (0 == size) {
         return SkData::MakeEmpty();
     }
-
     sk_sp<SkData> data = SkData::MakeUninitialized(size);
-    this->copyTo(data->writable_data());
-    this->reset(); // this is the "detach" part
+    this->copyToAndReset(data->writable_data());
     return data;
 }
 
diff --git a/tests/StreamTest.cpp b/tests/StreamTest.cpp
index 2e476a1858..8b5b2ae05a 100644
--- a/tests/StreamTest.cpp
+++ b/tests/StreamTest.cpp
@@ -410,6 +410,26 @@ static void stream_copy_test(skiatest::Reporter* reporter,
     }
 }
 
+DEF_TEST(DynamicMemoryWStream_detachAsData, r) {
+    const char az[] = "abcdefghijklmnopqrstuvwxyz";
+    const unsigned N = 40000;
+    SkDynamicMemoryWStream dmws;
+    for (unsigned i = 0; i < N; ++i) {
+        dmws.writeText(az);
+    }
+    REPORTER_ASSERT(r, dmws.bytesWritten() == N * strlen(az));
+    auto data = dmws.detachAsData();
+    REPORTER_ASSERT(r, data->size() == N * strlen(az));
+    const uint8_t* ptr = data->bytes();
+    for (unsigned i = 0; i < N; ++i) {
+        if (0 != memcmp(ptr, az, strlen(az))) {
+            ERRORF(r, "detachAsData() memcmp failed");
+            return;
+        }
+        ptr += strlen(az);
+    }
+}
+
 DEF_TEST(StreamCopy, reporter) {
     SkRandom random(123456);
     static const int N = 10000;