如何完成子类CALayer并使用自定义属性？ [英] How exactly to subclass CALayer and use a custom property?

问题描述

我正在尝试使用自定义索引属性创建 CALayer 的子类，我可以设置其动画和更改直接为了根据索引显示不同的图片。

I am trying to create a subclass of CALayer with a custom index property that I can both animate and change directly in order to display a different picture based on the index.

在标题中，我声明：

@property NSUInteger index;

在实施中，我覆盖 needDisplayForKey ：

+ (BOOL)needsDisplayForKey:(NSString *)key
{
    if ([key isEqualToString:@"index"])
        return YES;
    else
        return [super needsDisplayForKey:key];
}

现在，在显示方法，我想根据索引显示图片。但是，在动画期间， self.index 的值永远不会改变，所以我必须查询表示层的值，这与提供CALayer内容通过子类化：

Now, in the display method, I want to display a picture based on the index. However, during an animation, the value of self.index never changes, so I have to query the value of the presentation layer, contrary to the example on Providing CALayer Content by Subclassing:

- (void)display
{
    NSUInteger presentationIndex = [(CustomLayer *)[self presentationLayer] index];
    self.contents = (id)[[images objectAtIndex:presentationIndex] CGImage];
}

问题是，如果我这样做，我就不能设置 index 直接在动画之外的值，因为它只会更改模型层，而 display 方法会显式查询表示层。

The problem is, if I do that, I cannot set the index value directly outside of an animation, because it will only change the model layer, and the display method explicitly queries the presentation layer.

如果我添加一个复制 index 值的初始化方法，它可以工作：

If I add an initialization method that copies the value of index, it works:

- (id)initWithLayer:(id)layer
{
    self = [super initWithLayer:layer];
    if (self) {
        if ([layer isKindOfClass:[CustomLayer class]])
            self.index = [(CustomLayer *)layer index];
    }
    return self;
}

然而，在动画之前或之后，总会出现1个图像故障，因为演示文稿或模型值不匹配（取决于我是否将 index 设置为目标值）。

However, after or before an animation, there is always a 1 image glitch because the presentation or the model value don't match (depending if I set index to the destination value or not).

• 令人惊讶的是，似乎 drawInContext：方法总是有
  的正确值[self index ] ，但这不是我想要使用的方法，因为我只是用图像设置 content 属性。


• 根据我实现 index 属性的方式，我会得到不同的行为。如果我使用 @dynamic index （即使文档没有说动态实现自定义属性getter / setter），显示每次更改 index 的值时，都会调用。如果我使用 @synthesize 或实现一个setter，则不会调用 display ，所以我需要更改<$ c在setter中也是$ c> content 。


• 我应该使用实例变量吗？我应该使用动态实现吗？或者我应该使用 setValue：forKey：而不是？

• Surprisingly, it seems like the drawInContext: method always has the right value for [self index], but it is not the method I want to use since I just set the content property with an image.
• I get different behaviors depending on the way I implement the index property. If I use @dynamic index (which works, even though the documentation doesn't say that custom property getters/setters would be dynamically implemented), display is called every time the value of index is changed. If I use @synthesize or implement a setter, display is not called, so I would need to change content in the setter too.
• Should I use an instance variable? Should I use the dynamic implementation? Or should I use setValue:forKey: instead?

正如你所看到的，我对如何获得我想要的结果以及如何使用自定义属性正确实现CALayer的子类感到困惑。任何帮助和解释，将不胜感激！

As you can see, I am a bit confused about how to get the result I want, and how to correctly implement a subclass of CALayer with a custom property. Any help, and explanations, would be appreciated!

推荐答案

网上有一个很好的教程，解释了如何创建自定义具有可动画属性的CALayer子类。我没有机会尝试它，但我给它添加了书签。这是链接：

There is an excellent tutorial on the web that explains how to create a custom CALayer subclass with animatable properties. I haven't had occasion to try it yet, but I bookmarked it. Here is the link:

使用自定义CALayer动画饼图片

看起来主要技巧是：

使用@dynamic，而不是@synthesize来获取自定义CALayer的可动画属性。

Use @dynamic, not @synthesize for the animatable properties of your custom CALayer.

覆盖actionForKey：，initWithLayer：和needsDisplayForKey：，也许还有drawInContext：

Override actionForKey:, initWithLayer:, and needsDisplayForKey:, and perhaps drawInContext:

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