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像 UICollectionView 一样重写 UIScrollView 的代理属性

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UIScrollView 有包含一个 UIScrollViewDelegate 的代理属性:

public protocol UIScrollViewDelegate : NSObjectProtocol {
    //...
}
open class UIScrollView : UIView, NSCoding {
    weak open var delegate: UIScrollViewDelegate?
    //...
}

而 UICollectionView 重写了这个属性,但是用的是 UICollectionViewDelegate:

public protocol UICollectionViewDelegate : UIScrollViewDelegate {
   //...
}

open class UICollectionView : UIScrollView, UIDataSourceTranslating {
    weak open var delegate: UICollectionViewDelegate?
    //...
}

但是当尝试用自定义协议覆盖 UIScrollViews 代理,如下所示:

protocol MyScrollViewDelegate : UIScrollViewDelegate {
    //...
}

class MyScrollView: UIScrollView {
    weak open var delegate: MyScrollViewDelegate?
    //...
}

出现了错误:

Property 'delegate' with type 'MyScrollViewDelegate?' cannot override a property with type 'UIScrollViewDelegate?'

自定义的 UIScrollView 子类如何重写代理属性呢?

答案就是 didSet

private weak var myDelegate: MyScrollViewDelegate? = nil

override var delegate: UIScrollViewDelegate? {
    didSet {
        myDelegate = delegate as? MyScrollViewDelegate
    }
}

补充:

OC 下的重写 delegate 的做法,思路:

  1. 将 delegate 赋值给 myDelegate;

  2. 将父类的代理设置成子类自己。

代理方法调用时,会通过 methodSignatureForSelector 寻找自己有没有方法签名,自己若没有,则查看 myDelegate 有没有实现。将签名返回给系统,系统会将 methodSignatureForSelector 获取到的方法签名包装成 Invocation 传入 forwardInvocation,如果 myDelegate 实现了代理方法,则将调用。

调用父类的代理方法时,会先走 respondsToSelector 方法,判断是否实现了传进来的 SEL。如果实现了,则会再走 methodSignatureForSelectorforwardInvocation

@interface MyOCScrollView () <MyOCScrollViewDelegate, UIScrollViewDelegate>

@property(nonatomic, weak)  id <MyOCScrollViewDelegate> myDelegate;

//...

@end

@implementation MyOCScrollView

@dynamic delegate;

//...

- (void)setDelegate:(id<MyOCScrollViewDelegate>)delegate {
    self.myDelegate = delegate != self ? delegate : nil;
    [super setDelegate:delegate ? self : nil];
}

/**
 尝试获得一个方法签名
 */
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    NSMethodSignature *a = [super methodSignatureForSelector:aSelector];
    NSMethodSignature *b = [(id)self.myDelegate methodSignatureForSelector:aSelector];
    NSMethodSignature *result = a ? a : b;
    return result;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation {
    if ([(id)self.myDelegate respondsToSelector:anInvocation.selector]) {
        [anInvocation invokeWithTarget:(id)self.myDelegate];
    }
}

/**
 只要自己或代理实现了代理方法,就返回 YES
 */
- (BOOL)respondsToSelector:(SEL)aSelector {
    BOOL a = [super respondsToSelector:aSelector];
    BOOL c = [self.myDelegate respondsToSelector:aSelector];
    BOOL result = a || c;
    return result;
}

@end

注:MyOCScrollView 也遵守了 <MyOCScrollViewDelegate> 协议,我感觉是为了避免 self.myDelegate = delegate != self ? delegate : nil; 的警告,但是这样一来,NSMethodSignature *a = [super methodSignatureForSelector:aSelector];永远都会返回非 nil 的对象。如此的话,直接在 forwardInvocation 中判断调用即可。所以可以将 <MyOCScrollViewDelegate> 去掉,然后改为 self.myDelegate = (id)delegate != self ? delegate : nil;,经测试也没有问题。这样流程走起来更容易理解。