_UIWebViewScrollView的继承链为：_UIWebViewScrollView->UIWebScrollView->UIScrollView。

通过反编译可以得到实现。_UIWebViewScrollView和UIWebScrollView的实现为：（代码好长，点此跳过）

@interface UIWebScrollView : UIScrollView

- (void)didMoveToWindow;
- (BOOL)touchesShouldCancelInContentView:(id)arg1;

@end

@implementation UIWebScrollView

- (void)didMoveToWindow
{
    [super didMoveToWindow];
    if (!self.window)
    {
        SEL sel = @selector(scrollViewWasRemoved:);
        if ([super.delegate respondsToSelector:sel])
            objc_msgSend(super.delegate, sel, self);
    }
}

- (BOOL)touchesShouldCancelInContentView:(id)arg1
{
    return YES;
}

@end

@interface _UIWebViewScrollView : UIWebScrollView
{
    _UIWebViewScrollViewDelegateForwarder *_forwarder;
    BOOL _bouncesSetExplicitly;
    UIWebBrowserView *_browserView;
}

- (void)_setWebView:(id)arg1;
- (id)delegate;
- (void)setDelegate:(id)arg1;
- (void)setBouncesHorizontally:(BOOL)arg1;
- (void)setBouncesVertically:(BOOL)arg1;
- (void)setBounces:(BOOL)arg1;
- (void)_weaklySetBouncesHorizontally:(BOOL)arg1;
- (void)dealloc;
- (id)initWithFrame:(struct CGRect)arg1;

@end

@implementation _UIWebViewScrollView

- (void)_setWebView:(id)arg1
{
    [_forwarder setWebView:arg1];
    super.delegate = nil;
    super.delegate = _forwarder;
}

- (id)delegate
{
    return _forwarder.delegate;
}

- (void)setDelegate:(id)arg1
{
    _forwarder.delegate = arg1;
    super.delegate = nil;
    super.delegate = _forwarder;
}

- (void)setBouncesHorizontally:(BOOL)arg1
{
    [super setBouncesHorizontally:arg1];
    _bouncesSetExplicitly = YES;
}

- (void)setBouncesVertically:(BOOL)arg1
{
    [super setBouncesVertically:arg1];
    _bouncesSetExplicitly = YES;
}

- (void)setBounces:(BOOL)arg1
{
    [super setBounces:arg1];
    _bouncesSetExplicitly = YES;
}

- (void)_weaklySetBouncesHorizontally:(BOOL)arg1
{
    if (!_bouncesSetExplicitly)
        [super setBouncesHorizontally:arg1];
}

- (void)dealloc
{
    [_forwarder release];
    _forwarder = nil;
    [super dealloc];
}

- (id)initWithFrame:(struct CGRect)arg1
{
    self = [super initWithFrame:arg1];
    if (self)
    {
        _forwarder = [[_UIWebViewScrollViewDelegateForwarder alloc] init];
    }
    return self;
}

@end

UIWebScrollView比UIScrollView多做了两件事：

• 永远对contentView cancel touch
• 通知delegate自己被移出window的view hierarchy

_UIWebViewScrollView比UIWebScrollView多做了一件事：记录是否被显式设置了Bounces属性。

(UIWebScrollView有其它子类）

_UIWebViewScrollViewDelegateForwarder的实现如下：（代码好长，点此跳过）

@interface _UIWebViewScrollViewDelegateForwarder : NSObject <UIScrollViewDelegate>
{
    id <UIScrollViewDelegate> _delegate;
    UIWebView *_webView;
}

@property(nonatomic) UIWebView *webView; // @synthesize webView=_webView;
@property(nonatomic) id <UIScrollViewDelegate> delegate; // @synthesize delegate=_delegate;
- (void)forwardInvocation:(id)arg1;
- (BOOL)respondsToSelector:(SEL)arg1;
- (id)methodSignatureForSelector:(SEL)arg1;

@end

@implementation _UIWebViewScrollViewDelegateForwarder

@synthesize webView=_webView;
@synthesize delegate=_delegate;

- (void)forwardInvocation:(NSInvocation*)arg1
{
    SEL sel = [arg1 selector];
    bool hasWebViewResponded = false;
    if ([_webView respondsToSelector:sel])
    {
        [arg1 invokeWithTarget:_webView];
        hasWebViewResponded = true;
    }
    if([_delegate respondsToSelector:sel])
    {
        [arg1 invokeWithTarget:_delegate];
    }
    else
    {
        if (!hasWebViewResponded)
        {
            [super forwardInvocation:arg1];
        }
    }
}

- (BOOL)respondsToSelector:(SEL)arg1
{
    bool result = [super respondsToSelector:arg1];
    if (!result)
    {
        result = [_webView respondsToSelector:arg1];
        if (!result)
        {
            result = [_delegate respondsToSelector:arg1];
        }
    }
    return result;
}

- (id)methodSignatureForSelector:(SEL)arg1
{
    id result = [super methodSignatureForSelector:arg1];
    if (!result)
    {
        result = [_webView methodSignatureForSelector:arg1];
        if (!result)
        {
            result = [(NSObject*)_delegate methodSignatureForSelector:arg1];
        }
    }
    return result;
}

@end

_UIWebViewScrollViewDelegateForwarder所做的就是把传向_UIWebViewScrollView的消息转发给delegate。这个forwarder的设计可以借鉴到别处，是代理模式的应用吧。