如何在iPhone屏幕上绘制连续的线

Question

我刚刚学习核心图形。 所以我创建了一个新项目，包含一个 UIView 类。 现在我的 UIView 实现文件中有以下代码

-(id) initWithCoder:(NSCoder*)sourceCoder
{
    if( ( self = [super initWithCoder:sourceCoder]))
    {
        //place any other initialization  here
    }
    return self;
}

- (void)drawRect:(CGRect)rect {

    CGContextRef    context = UIGraphicsGetCurrentContext();

    CGContextSetLineWidth(context,4);
    CGContextSetStrokeColorWithColor(context, myColor.CGColor);


    CGContextMoveToPoint(context,startPoint.x , startPoint.y);
    CGContextAddLineToPoint(context, endPoint.x, endPoint.y);
    CGContextStrokePath(context);

}
- (void) touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event
{
    UITouch* touchPoint = [touches anyObject]; 
    startPoint = [touchPoint locationInView:self];
    endPoint = [touchPoint locationInView:self];

    [self setNeedsDisplay];
}

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch* touch = [touches anyObject];
    endPoint=[touch locationInView:self];
    [self setNeedsDisplay];
}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch* touch = [touches anyObject];
    endPoint = [touch locationInView:self];
    [self setNeedsDisplay];
}

，但它只是创建线条，如果我再次尝试绘制，则前一行会丢失。

但我希望前一行不应该丢失，我该怎么做？

Answer 1

设置视图的 clearsContextBeforeDrawing 属性 为 NO。

Answer 2

设置 UIImageView 作为背景，保存上下文（行）：

CGContextStrokePath(UIGraphicsGetCurrentContext());
<your imageView>.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();