ios - CALayers 从不称其为委托(delegate)的 drawLayer :inContext: even after [layer setNeedsDisplay] and [layer display]

Question

我不明白为什么我创建的 CALayer 没有调用它们的 drawLayer 方法。我已经为他们创建了一个 drawLayer 委托(delegate)对象，但它从未被调用过。来自 UIView 子类:

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {

       UIView *bg = [[UIView alloc] initWithFrame:self.bounds];
       // I have a property called 'bg' to reference the child UIView
       self.bg = bg;
       [self addSubview:self.bg];

       // an NSObject subclass I use as a CALayer delegate
       MyLayerDelegate *drawer = [MyLayerDelegate new];
       self.drawer = drawer;


       CAGradientLayer *layer = [CAGradientLayer layer];;
       layer.delegate = self.drawer;
       layer.name = @"bg";

       [self.bg.layer addSublayer:layer];

       [layer setNeedsDisplay];
       [layer display];

    }
    return self;
}

在 self.drawer 我有:

- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)context
{
    NSLog(@"layer called drawLayer");
    // Some drawing goes here
}

我尝试过很多不同的东西。我试过在 self.bg.layer 上设置 setNeedsDisplay，我试过在 self.bg 上设置 setNeedsDisplay，我在 addSublayer 之前和之后尝试过 [layer setNeedsDisplay]，我在 addSublayer 之前和之后尝试过 [layer display]。我试过使用和不使用 [layer display] 和 [layer displayIfNeeded]。

为什么图层拒绝绘制？

Answer 1

层框架需要是非零的。这解决了问题:

layer.frame = self.bg.bounds;
[layer setNeedsDisplay];
[self.bg.layer addSublayer:layer];