title: Don't clear the context before drawing
body: <p>I'm trying to find a way to draw lines on a view without redrawing all the context.</p>
  
  <p>Here is my drawing methods :</p>
  
  <pre><code>-(void)drawInContext:(CGContextRef)context {
      for (int i = 0; i &lt; self.drawings.count; ++i) {
          Drawing* drawing = [self.drawings objectAtIndex:i];
  
          CGContextSetStrokeColorWithColor(context, drawing.colorTrait.CGColor);
          CGContextSetLineWidth(context, 1.0);
  
          CGContextMoveToPoint(context, [[drawing.points objectAtIndex:0] CGPointValue].x * self.zoomScale, [[drawing.points objectAtIndex:] CGPointValue].y * self.zoomScale);
  
          for (int i = 1; i &lt; drawing.points.count; i++) {
              CGContextAddLineToPoint(context, [[drawing.points objectAtIndex:i] CGPointValue].x * self.zoomScale, [[drawing.points objectAtIndex:i] CGPointValue].y * self.zoomScale);
          }
  
          CGContextStrokePath(context);
      }
  }
  
  -(void)drawRect:(CGRect)rect {
      if (isRedrawing) {
          [self drawInContext:UIGraphicsGetCurrentContext()];
          isRedrawing = NO;
      }
  
      [[UIColor redColor] set];
      [currentPath stroke];
  }
  </code></pre>
  
  <p>But when I call setNeedsDisplay in my touches methods, the view is entirely cleared.
  Is there a way to make my method work ?</p>