dart Builder - expose finished buffer size (#6403)

2 files changed, 9 insertions, 6 deletions

diff --git a/dart/lib/flat_buffers.dart b/dart/lib/flat_buffers.dart
index 99fd6a2d..c77a835c 100644
--- a/dart/lib/flat_buffers.dart
+++ b/dart/lib/flat_buffers.dart
@@ -149,6 +149,9 @@ class Builder {
     reset();
   }
 
+  /// Calculate the finished buffer size (aligned).
+  int size() => _tail + ((-_tail) % _maxAlign);
+
   /// Add the [field] with the given boolean [value].  The field is not added if
   /// the [value] is equal to [def].  Booleans are stored as 8-bit fields with
   /// `0` for `false` and `1` for `true`.
@@ -335,8 +338,7 @@ class Builder {
   ///
   /// Most clients should prefer calling [finish].
   Uint8List lowFinish() {
-    int alignedTail = _tail + ((-_tail) % _maxAlign);
-    return _buf.buffer.asUint8List(_buf.lengthInBytes - alignedTail);
+    return _buf.buffer.asUint8List(_buf.lengthInBytes - size());
   }
 
   /// Finish off the creation of the buffer.  The given [offset] is used as the
@@ -346,15 +348,15 @@ class Builder {
   /// bytes 4-7 of the file.
   Uint8List finish(int offset, [String fileIdentifier]) {
     _prepare(max(_sizeofUint32, _maxAlign), fileIdentifier == null ? 1 : 2);
-    int alignedTail = _tail + ((-_tail) % _maxAlign);
-    _setUint32AtTail(_buf, alignedTail, alignedTail - offset);
+    final finishedSize = size();
+    _setUint32AtTail(_buf, finishedSize, finishedSize - offset);
     if (fileIdentifier != null) {
       for (int i = 0; i < 4; i++) {
-        _setUint8AtTail(_buf, alignedTail - _sizeofUint32 - i,
+        _setUint8AtTail(_buf, finishedSize - _sizeofUint32 - i,
             fileIdentifier.codeUnitAt(i));
       }
     }
-    return _buf.buffer.asUint8List(_buf.lengthInBytes - alignedTail);
+    return _buf.buffer.asUint8List(_buf.lengthInBytes - finishedSize);
   }
 
   /// Writes a Float64 to the tail of the buffer after preparing space for it.
diff --git a/dart/test/flat_buffers_test.dart b/dart/test/flat_buffers_test.dart
index 3f854d87..999a837a 100644
--- a/dart/test/flat_buffers_test.dart
+++ b/dart/test/flat_buffers_test.dart
@@ -265,6 +265,7 @@ class BuilderTest {
       builder.addInt32(1, 20, 10);
       int offset = builder.endTable();
       byteList = builder.finish(offset);
+      expect(builder.size(), byteList.length);
     }
     // read and verify
     BufferContext buffer = new BufferContext.fromBytes(byteList);