网页在Safari快速滚动和回弹的原理: -webkit-overflow-scrolling : touch;的实现
现在很多for Mobile的HTML5网页内都有快速滚动和回弹的效果,看上去和原生app的效率都有得一拼。
要实现这个效果很简单,只需要加一行css代码即可:
- -webkit-overflow-scrolling : touch;
可用以下网页测试:
- <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
- <html>
- <head>
- <meta charset="utf-8" />
- <title>scroll</title>
- <style type="text/css">
- .container {
- width : 300px;
- height : 50%;
- -webkit-box-sizing : border-box;
- position : relative;
- overflow-y : auto;
- background-color : cyan;
- -webkit-overflow-scrolling : touch; /* liuhx:可以把这整行注释掉对比差别 */
- }
- ul {
- height: 50px;
- }
- </style>
- </head>
- <body>
- <div align="center">
- <nav class="container">
- <ul>1</ul>
- <ul>2</ul>
- <ul>3</ul>
- <ul>4</ul>
- <ul>5</ul>
- <ul>6</ul>
- <ul>7</ul>
- <ul>8</ul>
- <ul>9</ul>
- <ul>10</ul>
- <ul>11</ul>
- <ul>12</ul>
- <ul>13</ul>
- <ul>14</ul>
- <ul>15</ul>
- <ul>16</ul>
- <ul>17</ul>
- <ul>18</ul>
- <ul>19</ul>
- <ul>20</ul>
- </nav>
- </div>
- </body>
- </html>
可以用手指滑动中间的蓝色区域,会发现回弹效果以及滚动得很快:
(点击图片查看大图)
如果把-webkit-overflow-scrolling那行注释掉,就会发现滚动得很慢。
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:
- 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:
- @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:
- @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。