Merge pull request #578 from gmilou/encode-start

Add a start method to the PngEncoder.

lib/src/formats/png_encoder.dart

@@ -92,6 +92,17 @@ class PngEncoder extends Encoder {
     }
   }
 
+  /// Start encoding a PNG.
+  ///
+  /// Call this method once before calling addFrame.
+  void start(int frameCount) {
+      _frames = frameCount;
+      isAnimated = frameCount > 1;
+  }
+
+  /// Finish encoding a PNG, and return the resulting bytes.
+  ///
+  /// Call this method to finalize the encoding, after all addFrame calls.
   Uint8List? finish() {
     Uint8List? bytes;
 
@@ -116,11 +127,10 @@ class PngEncoder extends Encoder {
   @override
   Uint8List encode(Image image, {bool singleFrame = false}) {
     if (!image.hasAnimation || singleFrame) {
-      isAnimated = false;
+      start(1);
       addFrame(image);
     } else {
-      isAnimated = true;
-      _frames = image.frames.length;
+      start(image.frames.length);
       repeat = image.loopCount;
 
       if (image.hasPalette) {

test/formats/png_test.dart

@@ -410,6 +410,20 @@ void main() {
           ..writeAsBytesSync(png);
       });
 
+      test('encodeAnimation with mulitple single frame Images', () {
+        final encoder = PngEncoder()..start(10);
+        for (var i = 0; i < 10; i++) {
+          final frame = Image(width: 480, height: 120)..loopCount = 10;
+          drawString(frame, i.toString(), font: arial48, x: 100, y: 60);
+          encoder.addFrame(frame);
+        }
+
+        final png = encoder.finish()!;
+        File('$testOutputPath/png/encodeAnimation.png')
+          ..createSync(recursive: true)
+          ..writeAsBytesSync(png);
+      });
+
       test('textData', () {
         final img = Image(width: 16, height: 16, textData: {"foo": "bar"});
         final png = PngEncoder().encode(img);