Cocoa 在自定义属性更改时重新绘制自定义CALayer子类

我试图建立一个特殊的层，绘制文本。此

TWFlapLayer

具有属性字符串作为属性：

TWFlapLayer.h

：

@interface TWFlapLayer : CALayer
@property(nonatomic, strong) __attribute__((NSObject)) CFAttributedStringRef attrString;
@end

/* Method for subclasses to override. Returning true for a given
 * property causes the layer's contents to be redrawn when the property
 * is changed (including when changed by an animation attached to the
 * layer). The default implementation returns NO. Subclasses should
 * call super for properties defined by the superclass. (For example,
 * do not try to return YES for properties implemented by CALayer,
 * doing will have undefined results.) */

+ (BOOL)needsDisplayForKey:(NSString *)key;

并在

TWFlapLayer.m

中合成：

@implementation TWFlapLayer

@synthesize attrString = _attrString;

/* overwrite method to redraw the layer if the string changed */

+ (BOOL)needsDisplayForKey:(NSString *)key
{
    if ([key isEqualToString:@"attrString"]){
        return YES;
    } else {
        return NO;
    }
}

- (void)drawInContext:(CGContextRef)ctx
{
    NSLog(@"%s: %@",__FUNCTION__,self.attrString);
    if (self.attrString == NULL) return;
    /* my custom drawing code */
}

我的意图是，如果使用合成setter方法更改attrString属性，则使用我的自定义绘制方法自动重新绘制图层。但是，从drawInContext:方法中的NSLog语句中，我看到该层没有被重新绘制

通过在needsDisplayForKey方法中放置断点，我确保在请求attrString键时它返回YES

我现在像这样更改属性字符串

// self.frontString is a NSAttributedString* that is why I need the toll-free bridging
self.frontLayer.attrString = (__bridge CFAttributedStringRef) self.frontString;

//should not be necessary, but without it the drawInContext method is not called
[self.frontLayer setNeedsDisplay]; // <-- why is this still needed?

摘要

---

当自定义属性attrString被更改并由

needsDisplayForKey:

标记时，为什么我的图层没有重新绘制？

CALayer.h

还说：

/* CALayer implements the standard NSKeyValueCoding protocol for all
 * Objective C properties defined by the class and its subclasses. It
 * dynamically implements missing accessor methods for properties
 * declared by subclasses.

显然，

需要displayforkey:

机制依赖于CALayer动态实现的访问器方法。因此，改变这一点：

@synthesize attrString = _attrString;

到

---

太棒了！这就成功了。

CALayer.h

确实是一个很大的文件，我错过了这一点有点尴尬。@kurt revis我也有类似的问题，@dynamic工作正常，谢谢。但是你能解释一下为什么初始渲染不会发生在我身上，即使我在

-（id）init

方法中设置了初始值。needsDisplayForKey不会在

init

中触发。

@dynamic attrString;