网页在Safari高速滚动和回弹的原理解决Safari不流畅

全文转载整理自互联网，感谢原作者。

其实只需要一行CSS代码

```css
-webkit-overflow-scrolling : touch;
```

实际上Safari真的用了原生控件来实现，对于有`-webkit-overflow-scrolling`的网页，会创建一个UIScrollView提供子layer给渲染模块使用。创建时的堆栈如下：
```objective-c
Thread 1, Queue : com.apple.main-thread
#0	0x00086723 in -[UIScrollView initWithFrame:] ()
#1	0x004ec3bd in -[UIWebOverflowScrollView initWithLayer:node:webDocumentView:] ()
#2	0x001f1769 in -[UIWebDocumentView webView:didCreateOrUpdateScrollingLayer:withContentsLayer:scrollSize:forNode:allowHorizontalScrollbar:allowVerticalScrollbar:] ()
#3	0x01d571bd in __invoking___ ()
#4	0x01d570d6 in -[NSInvocation invoke] ()
#5	0x01d5724a in -[NSInvocation invokeWithTarget:] ()
#6	0x027fb6a1 in -[_WebSafeForwarder forwardInvocation:] ()
#7	0x027fb8ab in __44-[_WebSafeAsyncForwarder forwardInvocation:]_block_invoke_0 ()
#8	0x04ac753f in _dispatch_call_block_and_release ()
#9	0x04ad9014 in _dispatch_client_callout ()
#10	0x04ac97d5 in _dispatch_main_queue_callback_4CF ()
#11	0x01d09af5 in __CFRunLoopRun ()
#12	0x01d08f44 in CFRunLoopRunSpecific ()
#13	0x01d08e1b in CFRunLoopRunInMode ()
#14	0x01cbd7e3 in GSEventRunModal ()
#15	0x01cbd668 in GSEventRun ()
#16	0x00032ffc in UIApplicationMain ()
#17	0x00002ae2 in main at /Users/liuhx/Desktop/UIWebView_Research/WebViewResearch/main.mm:16
```

实际创建的是UIWebOverflowScrollView，它继承自UIScrollView，声明为：
```objective-c
@class DOMNode, UIWebDocumentView, UIWebOverflowContentView, UIWebOverflowScrollListener;

@interface UIWebOverflowScrollView : UIScrollView
{
    UIWebDocumentView *_webDocumentView;
    UIWebOverflowScrollListener *_scrollListener;
    UIWebOverflowContentView *_overflowContentView;
    DOMNode *_node;
    BOOL _beingRemoved;
}

@property(nonatomic, getter=isBeingRemoved) BOOL beingRemoved; // @synthesize beingRemoved=_beingRemoved;
@property(retain, nonatomic) DOMNode *node; // @synthesize node=_node;
@property(retain, nonatomic) UIWebOverflowContentView *overflowContentView; // @synthesize overflowContentView=_overflowContentView;
@property(retain, nonatomic) UIWebOverflowScrollListener *scrollListener; // @synthesize scrollListener=_scrollListener;
@property(nonatomic) UIWebDocumentView *webDocumentView; // @synthesize webDocumentView=_webDocumentView;
- (void)setContentOffset:(struct CGPoint)arg1;
- (void)_replaceLayer:(id)arg1;
- (void)prepareForRemoval;
- (void)fixUpViewAfterInsertion;
- (id)superview;
- (void)dealloc;
- (id)initWithLayer:(id)arg1 node:(id)arg2 webDocumentView:(id)arg3;

@end
```

其还有一个子View作为ContentView，是给WebCore真正用作渲染overflow型内容的layer的容器。

UIWebOverflowContentView的声明为：

```objective-c
@interface UIWebOverflowContentView : UIView
{
}

- (void)_setCachedSubviews:(id)arg1;
- (void)_replaceLayer:(id)arg1;
- (void)fixUpViewAfterInsertion;
- (id)superview;
- (id)initWithLayer:(id)arg1;

@end
```
再往底层跟，都是CALayer的操作。

以上两个类都是UIKit层的实现，需要WebCore有硬件加速的支持才有实际意义，相关的逻辑被包含在ACCELERATED_COMPOSITING这个宏里。从SVN log看，在WebKit 108400版本左右才支持，所以iOS Safari应该是需要5.0，Android只在4.0以上支持。从前端开发的角度讲，只需要知道CSS的属性`-webkit-overflow-scrolling`是真的创建了带有硬件加速的系统级控件，所以效率很高。但是这相对是耗更多内存的，最好在产生了非常大面积的overflow时才应用。

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网页在Safari高速滚动和回弹的原理 解决Safari不流畅

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网页在Safari高速滚动和回弹的原理解决Safari不流畅