iOS开源:LazyScrollView - 阿里开源iOS高性能异构滚动视图构建方案

OPEN编辑 4年前
   <h2>LazyScroll是什么</h2>    <p>LazyScrollView 继承自ScrollView,目标是解决异构(与TableView的同构对比)滚动视图的复用回收问题。它可以支持跨View层的复用,用易用方式来生成一个高性能的滚动视图。此方案最先在天猫iOS客户端的首页落地。</p>    <h2>为什么要用LazyScrollView</h2>    <p>猫客首页之前首页的View比较少,不需要复用和回收也有很优秀的性能,但是之后首页的View数量逐渐膨胀,没有一套复用回收机制的ScrollView已经影响到性能了,迫切需要处理对ScrollView中View的复用和回收。</p>    <p>使用TableView只能用来解决同类Cell的展示,而在猫客首页这个ScrollView里面,View的种类太多了。不适合我们的场景。</p>    <p>而UICollectionView本身的布局和复用回收机制不够灵活,用起来也较为繁琐。并且本来猫客的首页就有一套相对成熟的卡片布局方案。所以诞生了LazyScrollView去解决这个问题。</p>    <h2>LazyScroll如何用</h2>    <p>LazyScrollView的使用和TableView很像,不过多了一个需要实现的方法:返回对应index的View 相对LazyScrollView的绝对坐标。</p>    <h3>实现LazyScrollViewDatasource</h3>    <p>类似TableView的用法,我们需要使用方实现LazyScrollViewDatasource这个Delegate</p>    <pre>  <code class="language-objectivec">@protocol TMMuiLazyScrollViewDataSource <NSObject>  @required  //ScrollView一共展示多少个item  - (NSUInteger)numberOfItemInScrollView:(TMMuiLazyScrollView *)scrollView;  //要求根据index直接返回RectModel  - (TMMuiRectModel *)scrollView:(TMMuiLazyScrollView *)scrollView rectModelAtIndex:(NSUInteger)index;   //返回下标所对应的view  - (UIView *)scrollView:(TMMuiLazyScrollView *)scrollView itemByMuiID:(NSString *)muiID;  @end  </code></pre>    <p>LazyScrollView的核心是在初始状态就得知所有View应该显示的位置。这个Protocol可以让LazyScrollView获取到这些信息。</p>    <p>第一个方法很简单,获取LazyScrollView中item的个数。</p>    <p>第二个方法需要按照Index返回<code>TMMuiRectModel</code> ,它会携带对应index的View 相对LazyScrollView的绝对坐标。TMMuiRectModel是这么个东西:</p>    <pre>  <code class="language-objectivec">@interface TMMuiRectModel:NSObject  //转换后的绝对值rect  @property (nonatomic,assign) CGRect absRect;  //业务下标  @property (nonatomic,copy) NSString *muiID;    @end  </code></pre>    <p><code>absRect</code>是LazyScroll中的View相对LazyScrollView的绝对坐标,<code>muiID</code>是这个View在LazyScrollView中唯一的标识符,可赋值也可不赋值,不赋值的话LazyScroll会处理成转换为字符串的下标。如果这个标识符在Protocol的第三个方法中会用到。</p>    <p>第三个方法,返回View。首先,我们在UIView之外加了一个Category:</p>    <pre>  <code class="language-objectivec">@interface UIView(TMMui)    //索引过的标识,在LazyScrollView范围内唯一  @property (nonatomic, copy) NSString  *muiID;  //重用的ID  @property (nonatomic, copy) NSString *reuseIdentifier;  </code></pre>    <p>这个category可以让View携带muiID和reuseIdentifier,对于返回的View来说,只需要在乎对View的reuseIdentifier赋值,muiID的赋值会在lazyScrollView中处理掉。reuseIdentifier相同的View会被复用,如果这个View的reuseIdentifier是nil或者空字符串,则不会被复用。</p>    <h3>调用核心API</h3>    <p><code>- (void)reloadData;</code></p>    <p>重新走一遍DataSource的这些方法,等同于TableView中的reloadData</p>    <p><code>- (UIView *)dequeueReusableItemWithIdentifier:(NSString *)identifier</code></p>    <p>根据identifier获取可以复用的View。和TableView的<code>dequeueReusableCellWithIdentifier:(NSString *)identifier</code>方法意义相同。通常是在<code>LazyScrollViewDatasource</code>第三个方法,返回View的时候使用。先尝试获取复用池中的View,如果没有再去新建。</p>    <h2>LazyScrollView的内部实现</h2>    <p>这是一个Demo, 被复用的View,标记的backgroundColor会和之前生成的时候有所不同。</p>    <p><img alt="" src="https://simg.open-open.com/show/c83f70fa81bf9a9545be42d8eb5bce35.gif"></p>    <h3>STEP 1 根据DataSource获取所有的TMMuiRectModel</h3>    <p>根据DataSource的Delegate,拿到所有的View应该被显示的位置。这一步,核心是拿到的位置是确定的。</p>    <p>根据Demo,我们观察从 0/1 - 2/3 之间这些View</p>    <p>这个时候LazyScrollView拿到的Rect如下:</p>    <table>     <thead>      <tr>       <th>Index</th>       <th>标号(MUIID)</th>       <th>Rect</th>      </tr>     </thead>     <tbody>      <tr>       <td>0</td>       <td>0/0</td>       <td>origin = (x = 25, y = 15), size = (width = 156, height = 150</td>      </tr>      <tr>       <td>1</td>       <td>0/1</td>       <td>origin = (x = 194, y = 15), size = (width = 156, height = 150)</td>      </tr>      <tr>       <td>2</td>       <td>0/2</td>       <td>origin = (x = 25, y = 180), size = (width = 156, height = 150)</td>      </tr>      <tr>       <td>3</td>       <td>0/3</td>       <td>origin = (x = 194, y = 180), size = (width = 156, height = 150</td>      </tr>      <tr>       <td>4</td>       <td>1/0</td>       <td>origin = (x = 5, y = 360), size = (width = 177.5, height = 150)</td>      </tr>      <tr>       <td>5</td>       <td>1/1</td>       <td>origin = (x = 192.5, y = 426), size = (width = 84, height = 84)</td>      </tr>      <tr>       <td>6</td>       <td>1/2</td>       <td>origin = (x = 192.5, y = 360), size = (width = 177.5, height = 56)</td>      </tr>      <tr>       <td>7</td>       <td>1/3</td>       <td>origin = (x = 286.5, y = 426), size = (width = 83.5, height = 84)</td>      </tr>      <tr>       <td>8</td>       <td>2/0</td>       <td>origin = (x = 25, y = 530), size = (width = 325, height = 150)</td>      </tr>      <tr>       <td>9</td>       <td>2/1</td>       <td>origin = (x = 25, y = 695), size = (width = 325, height = 150)</td>      </tr>      <tr>       <td>10</td>       <td>2/2</td>       <td>origin = (x = 25, y = 860), size = (width = 325, height = 150)</td>      </tr>     </tbody>    </table>    <h3>STEP 2 排序</h3>    <p>拿到了这些位置之后,接下来做的事情就是排序。排序生成的索引会有两个:根据顶边(y)升序排序的索引和根据底边(y+height)降序排序的索引。</p>    <p><strong>根据顶边(y)升序排序的索引</strong></p>    <table>     <thead>      <tr>       <th>RANK</th>       <th>标号(MUIID)</th>       <th>Rect</th>      </tr>     </thead>     <tbody>      <tr>       <td>0</td>       <td>0/0</td>       <td>origin = (x = 25, y = 15), size = (width = 156, height = 150</td>      </tr>      <tr>       <td>1</td>       <td>0/1</td>       <td>origin = (x = 194, y = 15), size = (width = 156, height = 150)</td>      </tr>      <tr>       <td>2</td>       <td>0/2</td>       <td>origin = (x = 25, y = 180), size = (width = 156, height = 150)</td>      </tr>      <tr>       <td>3</td>       <td>0/3</td>       <td>origin = (x = 194, y = 180), size = (width = 156, height = 150</td>      </tr>      <tr>       <td>4</td>       <td>1/0</td>       <td>origin = (x = 5, y = 360), size = (width = 177.5, height = 150)</td>      </tr>      <tr>       <td>5</td>       <td>1/2</td>       <td>origin = (x = 192.5, y = 360), size = (width = 177.5, height = 56)</td>      </tr>      <tr>       <td>6</td>       <td>1/1</td>       <td>origin = (x = 192.5, y = 426), size = (width = 84, height = 84)</td>      </tr>      <tr>       <td>7</td>       <td>1/3</td>       <td>origin = (x = 286.5, y = 426), size = (width = 83.5, height = 84)</td>      </tr>      <tr>       <td>8</td>       <td>2/0</td>       <td>origin = (x = 25, y = 530), size = (width = 325, height = 150)</td>      </tr>      <tr>       <td>9</td>       <td>2/1</td>       <td>origin = (x = 25, y = 695), size = (width = 325, height = 150)</td>      </tr>      <tr>       <td>10</td>       <td>2/2</td>       <td>origin = (x = 25, y = 860), size = (width = 325, height = 150)</td>      </tr>     </tbody>    </table>    <p><strong>根据底边(y+height)降序排序的索引</strong></p>    <table>     <thead>      <tr>       <th>RANK</th>       <th>标号(MUIID)</th>       <th>Rect</th>      </tr>     </thead>     <tbody>      <tr>       <td>0</td>       <td>2/2</td>       <td>origin = (x = 25, y = 860), size = (width = 325, height = 150)</td>      </tr>      <tr>       <td>1</td>       <td>2/1</td>       <td>origin = (x = 25, y = 695), size = (width = 325, height = 150)</td>      </tr>      <tr>       <td>2</td>       <td>2/0</td>       <td>origin = (x = 25, y = 530), size = (width = 325, height = 150)</td>      </tr>      <tr>       <td>3</td>       <td>1/0</td>       <td>origin = (x = 5, y = 360), size = (width = 177.5, height = 150)</td>      </tr>      <tr>       <td>4</td>       <td>1/2</td>       <td>origin = (x = 192.5, y = 360), size = (width = 177.5, height = 56)</td>      </tr>      <tr>       <td>5</td>       <td>1/3</td>       <td>origin = (x = 286.5, y = 426), size = (width = 83.5, height = 84)</td>      </tr>      <tr>       <td>6</td>       <td>1/1</td>       <td>origin = (x = 192.5, y = 426), size = (width = 84, height = 84)</td>      </tr>      <tr>       <td>7</td>       <td>0/2</td>       <td>origin = (x = 25, y = 180), size = (width = 156, height = 150)</td>      </tr>      <tr>       <td>8</td>       <td>0/3</td>       <td>origin = (x = 194, y = 180), size = (width = 156, height = 150</td>      </tr>      <tr>       <td>9</td>       <td>0/0</td>       <td>origin = (x = 25, y = 15), size = (width = 156, height = 150</td>      </tr>      <tr>       <td>10</td>       <td>0/1</td>       <td>origin = (x = 194, y = 15), size = (width = 156, height = 150)</td>      </tr>     </tbody>    </table>    <h3>STEP 3 查找</h3>    <p>前两步是在执行完reload,在视图还没有生成的时候就开始做了,而接下来的步骤在要生成视图(初始化或滚动的时候)才会去做。</p>    <p>我们设定了Buffer为上下各20,滚动超过20个像素后才会指定查找视图并显示的动作。</p>    <p>接下来就是找哪些View应该被显示了。举个例子,如下图,红圈是应该显示的区域。</p>    <p><img alt="iOS开源:LazyScrollView - 阿里开源iOS高性能异构滚动视图构建方案" src="https://simg.open-open.com/show/e14f040a7075b0ef07bb530f76fe3202.png"></p>    <p>现在已知的是红圈顶边y是<strong>242</strong>,底边y是<strong>949</strong>,加上缓冲区Buffer,应该是找<strong>222 - 969</strong> 之间的View。我们要做的是,找到<strong>底边y小于969的Model</strong>和<strong>顶边y大于222的Model</strong>,取交集,就是我们要显示的View</p>    <p>采用的方法为二分查找,在根据顶边升序排序的索引中找949,找到的index为0(MUIID为2/2),我们使用一个Set,把根据顶边排序中<strong>index >= 0</strong> 的元素先放在这里。获取的Set中包含的muiID为<strong>0/0,0/1,0/2,0/3,1/0,1/1,1/2,1/3,2/0,2/1,2/2</strong>。</p>    <p>根据底边排序的索引中找222,找到的index为2,我们把<strong>index >= 2</strong>的元素放在另一个Set,获取的Set中包含的muiID为<strong>0/2,0/3,1/0,1/1,1/2,1/3,2/0,2/1,2/2</strong></p>    <p>两个Set取交集,得到的就是我们的ResultSet,这里面都是我们要显示View的Model,它们的muiID是<strong>0/2,0/3,1/0,1/1,1/2,1/3,2/0,2/1,2/2</strong>。</p>    <h3>STEP 4 回收、复用、生成</h3>    <p>我们知道了应该显示哪些View,但是我们之后做的第一步是把不需要显示的View加入到复用池中。</p>    <p>LazyScroll可以取到当前显示了的View,拿当前显示的View的muiID和将要显示view的Model的muiID做对比,可以知道当前显示的View哪些应该被回收。</p>    <p>LazyScrollView中有一个Dictionary,key是reuseIdentifier,Value是对应reuseIdentifier被回收的View,当LazyScrollView得知这个View不该再出现了,会把View放在这里,并且把这个View hidden掉。</p>    <p>接下来,LazyScrollView会去调用datasource的<code>- (UIView *)scrollView:(TMMuiLazyScrollView *)scrollView itemByMuiID:(NSString *)muiID;</code> 复用还是不复用,是由datasource决定的。如果要复用,需要datasource方法内调用<code>- (UIView *)dequeueReusableItemWithIdentifier:(NSString *)identifier</code>获取复用的View,这个方法取出来的View就是在上一段所说的Dictionary中拿的。</p>    <p>这样,我们就完成了一次完整的循环 : 找到所有View将要显示的位置 – 排序 – 查找应该显示的View – 回收 – 创建/复用。</p>    <p>##最后</p>    <p>LazyScroll的复用和回收能力是比较强大的,在猫客首页使用之后,因为View数量而导致内存过多的问题得到了解决。</p>    <p>在这套复用和回收机制的加持之下,我们将LazyScrollView继续延伸,构造出一套完整的布局解决方案<code>Tangram</code>,它将Native中View的布局方式变得更动态化,敬请期待。</p>