在 UIImageView 中画一条线的问题

Question

在我的应用程序中，我可以通过下面的代码在 UIImageView 中画一条线，当我调用该函数时，我想重画这条线更长的时间，但是，输出的结果与预期的不一样，它只会画一个新的行并删除旧的，长度保持不变只是 y 位置不同，我不知道我的代码的哪一行是错误的，或者我没有以正确的方式理解 CGContext 类，请帮助我整天挠头并且找不到问题

- (void) drawLine {
    lastPoint = currentPoint;
    lastPoint.y -= 40;

    //drawImage is a UIImageView declared at header
    UIGraphicsBeginImageContext(drawImage.frame.size);
    [drawImage.image drawInRect:CGRectMake(0, 0, drawImage.frame.size.width, drawImage.frame.size.height)];

    //sets the style for the endpoints of lines drawn in a graphics context
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGContextSetLineCap(ctx, kCGLineCapButt);
    //sets the line width for a graphic context
    CGContextSetLineWidth(ctx,2.0);
    //set the line colour
    CGContextSetRGBStrokeColor(ctx, 1.0, 0.0, 0.0, 1.0);
    //creates a new empty path in a graphics context
    CGContextBeginPath(ctx);
    //begin a new path at the point you specify
    CGContextMoveToPoint(ctx, currentPoint.x, currentPoint.y);
    //Appends a straight line segment from the current point to the provided point 
    CGContextAddLineToPoint(ctx, currentPoint.x,lastPoint.y);
    //paints a line along the current path
    CGContextStrokePath(ctx);
    drawImage.image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    currentPoint = lastPoint;
}

Answer 1

我确定你已经完成了这个，但你有一个错字。

CGContextAddLineToPoint(ctx, currentPoint.x,lastPoint.y);

应该是：

CGContextAddLineToPoint(ctx, lastPoint.x,lastPoint.y);