在 UIView 子类中调用drawRect

我已经对 UIView 进行了子类化，并且实现了一个 drawRect 。我基本上希望在设置 CGPoint（这是此 UIView 的一个属性）之后调用此 drawRect。我该怎么做呢？这是我的代码：

-(id) initWithCoder:(NSCoder *)aDecoder
{
    if(self = [super initWithCoder:aDecoder]) {
        point_.x = 0;
        self.page_ = [UIImage imageNamed:@"pages"];
    }
    return self;
}

// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
    if (point_.x != 0){
        [self.page_ drawInRect:rect];
        CGContextRef context = UIGraphicsGetCurrentContext();
        CGContextSetStrokeColorWithColor(context, [UIColor blackColor].CGColor);
        CGContextSetLineWidth(context, 1.0f);
        CGContextMoveToPoint(context, 40, point_.y); 
        CGContextAddLineToPoint(context, 420, point_.y);
        CGContextStrokePath(context);
    }
}

当我设置了这个 UIView 的点并在另一个 UIViewController 中调用 drawRect 时，如下所示：

 self.page_.point_ = CGPointMake(-100, self.title_.frameHeight + self.title_.frameX + 40);
    [self.page_ drawRect:self.page_.frame];

它给了我所有这些错误：

  <Error>: CGContextSaveGState: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextSetBlendMode: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextSetAlpha: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextTranslateCTM: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextScaleCTM: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextDrawImage: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextRestoreGState: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextSetStrokeColorWithColor: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextSetLineWidth: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextMoveToPoint: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextAddLineToPoint: invalid context 0x0
    Feb 22 20:24:38 <Error>: CGContextDrawPath: invalid context 0x0

为什么是这样？

你永远不应该打电话-drawRect:直接地。

Use -setNeedsDisplay反而：

self.page_.point_ = CGPointMake(-100, self.title_.frameHeight + self.title_.frameX + 40);
[self.page_ setNeedsDisplay];

如果您需要自定义绘图区域（又名CGRect这将被传递到-drawRect:）， 您可以使用-setNeedsDisplayInRect:

As @罗布·梅奥夫 https://stackoverflow.com/users/77567/rob-mayoff已建议，您可以覆盖设置器point_财产：

- (void)setPoint_:(CGPoint)point {
   if(!CGPointEqualToPoint(point_, point)) {
      point_ = point;
      [self setNeedsDisplay];
   }
}