iOS:绘图线出现在子视图后面

Question

我有一个简单的UIView,在drawRect中:我添加一个UIImageView作为子视图,然后尝试在该子视图之上绘制一条线.但是,这条线在imageview后面被吸引.

如何使绘图上下文成为子视图,以便我可以在其上绘制？

CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetLineWidth(context, 10.0);
CGContextSetStrokeColorWithColor(context, [UIColor blueColor].CGColor);
CGContextBeginPath(context);
CGContextMoveToPoint(context, 0, 0);
CGContextAddCurveToPoint(context,125,150,175,150,200,100);
CGContextAddCurveToPoint(context,225,50,275,75,300,200);
CGContextStrokePath(context);

Answer 1

制作另一个自定义视图,它只能绘制线条.将其添加为具有与ImageView相同的框架的子视图,并调用bringSubviewToFront以确保它在前面.您可能需要在自定义视图上设置一些属性opaque = NO,并将背景颜色设置为clear([UIColor colorWithWhite:0.0 alpha:0.0]).

顺便说一下,不要在drawRect中添加任何子视图.drawRect应该只绘制,而不是更改视图属性或层次结构.您应该在其他地方添加ImageView和自定义线条图视图,也许在init中.像这样:

@implementation MyView

-(id)initWithFrame:(CGRect)frame {
    if ((self = [super initWithFrame:frame])) {
        imageView = ... // however you create your image view
        [self addSubview:imageView];
        lineView = [[MyLineView alloc] initWithFrame:imageView.frame];
        [self addSubview:lineView];
        [self bringSubviewToFront:lineView];
    }
    return self;
}

...

@end

@implementation MyLineView

-(void)drawRect:(CGRect)rect {
    // your drawing code
    // remember the coordinate system now has (0, 0) at the top left corner of the
    // image view, so adjust your drawing code accordingly
}

@end