[ios] Restore line smoothing

ios/RNSketchCanvas/RNSketchCanvas/RNSketchData.m

@@ -43,15 +43,32 @@ - (instancetype)initWithId:(int) pathId strokeColor:(UIColor*) strokeColor strok
 }
 
 - (CGRect)addPoint:(CGPoint) point {
-    NSValue *lastPointValue = _points.lastObject;
     [_points addObject: [NSValue valueWithCGPoint: point]];
 
-    CGRect updateRect = CGRectMake(point.x, point.y, 0, 0);
+    CGRect updateRect;
 
-    if (lastPointValue) {
-        CGPoint lastPoint = lastPointValue.CGPointValue;
-        updateRect = CGRectUnion(updateRect, CGRectMake(lastPoint.x, lastPoint.y, 0, 0));
+    NSUInteger pointsCount = _points.count;
+    if (pointsCount >= 3) {
+        CGPoint a = _points[pointsCount - 3].CGPointValue;
+        CGPoint b = _points[pointsCount - 2].CGPointValue;
+        CGPoint c = point;
+        CGPoint prevMid = midPoint(a, b);
+        CGPoint currentMid = midPoint(b, c);
+
+        updateRect = CGRectMake(prevMid.x, prevMid.y, 0, 0);
+        updateRect = CGRectUnion(updateRect, CGRectMake(b.x, b.y, 0, 0));
+        updateRect = CGRectUnion(updateRect, CGRectMake(currentMid.x, currentMid.y, 0, 0));
+    } else if (pointsCount >= 2) {
+        CGPoint a = _points[pointsCount - 2].CGPointValue;
+        CGPoint b = point;
+        CGPoint mid = midPoint(a, b);
+
+        updateRect = CGRectMake(a.x, a.y, 0, 0);
+        updateRect = CGRectUnion(updateRect, CGRectMake(mid.x, mid.y, 0, 0));
+    } else {
+        updateRect = CGRectMake(point.x, point.y, 0, 0);
     }
+
     updateRect = CGRectInset(updateRect, -_strokeWidth * 2, -_strokeWidth * 2);
 
     return updateRect;
@@ -63,25 +80,21 @@ - (void)drawLastPointInContext:(CGContextRef)context {
         return;
     };
 
-    CGPoint fromPoint = _points[pointsCount - (pointsCount > 1 ? 2 : 1)].CGPointValue;
-    CGPoint toPoint = _points[pointsCount - 1].CGPointValue;
-
-    [self drawSegmentInContext:context from:fromPoint to:toPoint];
+    [self drawInContext:context pointIndex:pointsCount - 1];
 }
 - (void)drawInContext:(CGContextRef)context {
-    NSValue *prevPointValue = _points.count == 1 ? _points[0] : nil;
-    for (NSValue *pointValue in _points) {
-        if (!prevPointValue) {
-            prevPointValue = pointValue;
-            continue;
-        }
-
-        [self drawSegmentInContext:context from:prevPointValue.CGPointValue to:pointValue.CGPointValue];
-        prevPointValue = pointValue;
+    NSUInteger pointsCount = _points.count;
+    for (NSUInteger i = 0; i < pointsCount; i++) {
+        [self drawInContext:context pointIndex:i];
     }
 }
 
-- (void)drawSegmentInContext:(CGContextRef)context from:(CGPoint)fromPoint to:(CGPoint)toPoint {
+- (void)drawInContext:(CGContextRef)context pointIndex:(NSUInteger)pointIndex {
+    NSUInteger pointsCount = _points.count;
+    if (pointIndex >= pointsCount) {
+        return;
+    };
+
     BOOL isErase = [Utility isSameColor:_strokeColor color:[UIColor clearColor]];
 
     CGContextSetStrokeColorWithColor(context, _strokeColor.CGColor);
@@ -90,8 +103,34 @@ - (void)drawSegmentInContext:(CGContextRef)context from:(CGPoint)fromPoint to:(C
     CGContextSetLineJoin(context, kCGLineJoinRound);
     CGContextSetBlendMode(context, isErase ? kCGBlendModeClear : kCGBlendModeNormal);
     CGContextBeginPath(context);
-    CGContextMoveToPoint(context, fromPoint.x, fromPoint.y);
-    CGContextAddLineToPoint(context, toPoint.x, toPoint.y);
+
+    if (pointsCount >= 3 && pointIndex >= 2) {
+        CGPoint a = _points[pointIndex - 2].CGPointValue;
+        CGPoint b = _points[pointIndex - 1].CGPointValue;
+        CGPoint c = _points[pointIndex].CGPointValue;
+        CGPoint prevMid = midPoint(a, b);
+        CGPoint currentMid = midPoint(b, c);
+
+        // Draw a curve
+        CGContextMoveToPoint(context, prevMid.x, prevMid.y);
+        CGContextAddQuadCurveToPoint(context, b.x, b.y, currentMid.x, currentMid.y);
+    } else if (pointsCount >= 2 && pointIndex >= 1) {
+        CGPoint a = _points[pointIndex - 1].CGPointValue;
+        CGPoint b = _points[pointIndex].CGPointValue;
+        CGPoint mid = midPoint(a, b);
+
+        // Draw a line to the middle of points a and b
+        // This is so the next draw which uses a curve looks correct and continues from there
+        CGContextMoveToPoint(context, a.x, a.y);
+        CGContextAddLineToPoint(context, mid.x, mid.y);
+    } else if (pointsCount >= 1) {
+        CGPoint a = _points[pointIndex].CGPointValue;
+
+        // Draw a single point
+        CGContextMoveToPoint(context, a.x, a.y);
+        CGContextAddLineToPoint(context, a.x, a.y);
+    }
+
     CGContextStrokePath(context);
 }