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<article id="post-Designated-Initializers" class="article article-type-post" itemscope itemprop="blogPost">
<div class="article-meta">
<a href="/2016/04/26/Designated-Initializers/" class="article-date">
<time datetime="2016-04-26T10:18:50.000Z" itemprop="datePublished">2016-04-26</time>
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<a class="article-title" href="/2016/04/26/Designated-Initializers/">Designated Initializers</a>
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<p>Objective-C<br>一个对象可以有多个构造方法</p>
<p>指定构造器(Designated Initializers)<br>必须调用父类的指定构造器</p>
<p>便利构造器(Convenience Initializers)<br>必须调用指定构造器</p>
<p>iOS8后在指定构造器后添加宏｀NS_DESIGNATED_INITIALIZER｀，跟编译器指明哪个是指定构造器</p>
<p>Profile.h</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">#import <Foundation/Foundation.h></span><br><span class="line"></span><br><span class="line">@interface Profile : NSObject</span><br><span class="line"></span><br><span class="line">@property (nonatomic, copy) NSString *name;</span><br><span class="line"></span><br><span class="line">- (instancetype)init;</span><br><span class="line"></span><br><span class="line">- (instancetype)initWithName:(NSString *)name NS_DESIGNATED_INITIALIZER;</span><br><span class="line"></span><br><span class="line">@end</span><br></pre></td></tr></table></figure>
<p>Profile.m</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line">#import "Profile.h"</span><br><span class="line"></span><br><span class="line">@implementation Profile</span><br><span class="line"></span><br><span class="line">- (instancetype)init {</span><br><span class="line"> // 必须调用指定构造器</span><br><span class="line"> return [self initWithName:@"Unknown"];</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">- (instancetype)initWithName:(NSString *)name {</span><br><span class="line"> // 必须调用父类的指定构造器</span><br><span class="line"> self = [super init];</span><br><span class="line"> if (self) {</span><br><span class="line"> _name = [name copy];</span><br><span class="line"> }</span><br><span class="line"> return self;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">@end</span><br></pre></td></tr></table></figure>
<p>参考：<br><a href="http://useyourloaf.com/blog/xcode-6-objective-c-modernization/" target="_blank" rel="external">http://useyourloaf.com/blog/xcode-6-objective-c-modernization/</a><br><a href="http://stackoverflow.com/questions/26185239/ios-designated-initializers-using-ns-designated-initializer" target="_blank" rel="external">http://stackoverflow.com/questions/26185239/ios-designated-initializers-using-ns-designated-initializer</a><br><a href="http://honglu.me/2015/09/16/Objective-C%E5%BE%88%E6%9C%89%E7%94%A8%E7%9A%84%E7%89%B9%E6%80%A7/" target="_blank" rel="external">http://honglu.me/2015/09/16/Objective-C%E5%BE%88%E6%9C%89%E7%94%A8%E7%9A%84%E7%89%B9%E6%80%A7/</a><br><a href="http://www.starfelix.com/blog/2014/04/13/zheng-que-bian-xie-designated-initializerde-ji-ge-yuan-ze/" target="_blank" rel="external">http://www.starfelix.com/blog/2014/04/13/zheng-que-bian-xie-designated-initializerde-ji-ge-yuan-ze/</a></p>
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<article id="post-CollectionView动态居中显示解析" class="article article-type-post" itemscope itemprop="blogPost">
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<a href="/2016/04/15/CollectionView动态居中显示解析/" class="article-date">
<time datetime="2016-04-15T08:25:50.000Z" itemprop="datePublished">2016-04-15</time>
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<a class="article-title" href="/2016/04/15/CollectionView动态居中显示解析/">CollectionView动态居中显示解析</a>
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<p><img src="https://raw.githubusercontent.com/KakaMic/BlogResource/master/collectionviewcell1.png" alt=""></p>
<p>控制最后滚动的位置<br>-(CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity</p>
<p>参数<br>proposedContentOffset：停止滚动时的偏移量(正数)<br>velocity：滚动速度</p>
<p>手停止滚动离开控件时系统调用targetContentOffsetForProposedContentOffset</p>
<h1 id="关键方法："><a href="#关键方法：" class="headerlink" title="关键方法："></a>关键方法：</h1><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br></pre></td><td class="code"><pre><span class="line">/**</span><br><span class="line"> * 初始化</span><br><span class="line"> */</span><br><span class="line">- (void)prepareLayout {</span><br><span class="line"> [super prepareLayout];</span><br><span class="line"> self.itemSize = CGSizeMake(itemWidth, CGRectGetHeight(self.collectionView.bounds));</span><br><span class="line"> self.scrollDirection = UICollectionViewScrollDirectionHorizontal;</span><br><span class="line"> self.minimumLineSpacing = kCellInterval;</span><br><span class="line"> CGFloat inset = 100;</span><br><span class="line"> self.sectionInset = UIEdgeInsetsMake(0, inset, 0, inset);// 设置inset左右留白</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">/**</span><br><span class="line"> * 控件滚动 重新布局</span><br><span class="line"> */</span><br><span class="line">- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {</span><br><span class="line"> return YES;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">/**</span><br><span class="line"> * 控制最后滚动的位置</span><br><span class="line"> *</span><br><span class="line"> * @param proposedContentOffset 停止滚动时的偏移量(正数)</span><br><span class="line"> * @param velocity 滚动速度</span><br><span class="line"> *</span><br><span class="line"> * @return 最终控件的偏移量</span><br><span class="line"> */</span><br><span class="line">- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity {</span><br><span class="line"> </span><br><span class="line"> // 滚动停止后控件在屏幕上的可视范围</span><br><span class="line"> CGRect viewRect ;</span><br><span class="line"> viewRect.origin = proposedContentOffset;</span><br><span class="line"> viewRect.size = self.collectionView.frame.size;</span><br><span class="line"> </span><br><span class="line"> // 在viewRect屏幕可见范围内的Cell Attributes数组</span><br><span class="line"> NSArray *viewAttributes = [self layoutAttributesForElementsInRect:viewRect];</span><br><span class="line"> </span><br><span class="line"> // 控件在屏幕中间位置的点（偏移量 + 控件宽度/2）</span><br><span class="line"> CGFloat center_X = proposedContentOffset.x + CGRectGetWidth(self.collectionView.bounds) / 2 ;</span><br><span class="line"> </span><br><span class="line"> // 通过center_X和可见范围cell的中点（上图红点）比对，判断那个cell离中间最近</span><br><span class="line"> CGFloat cell2CenterOffset = MAXFLOAT;</span><br><span class="line"> for (UICollectionViewLayoutAttributes *attribute in viewAttributes) {</span><br><span class="line"> if(ABS(attribute.center.x - center_X) < ABS(cell2CenterOffset)){</span><br><span class="line"> // 得到cell距中点的最小距离差</span><br><span class="line"> cell2CenterOffset = attribute.center.x - center_X;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> // 返回最终控件的偏移量</span><br><span class="line"> return CGPointMake(proposedContentOffset.x + cell2CenterOffset, proposedContentOffset.y);</span><br><span class="line"> </span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>例1：如上图cell_1距中点靠左，<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">CGRectGetWidth(self.collectionView.bounds) / 2 ＝ 187.5</span><br><span class="line">cell.center.x＝187.5，</span><br><span class="line">center_X＝14+187.5，</span><br><span class="line">cell2CenterOffset＝cell.center.x － center_X ＝ -14,</span><br></pre></td></tr></table></figure></p>
<p>说明偏移量proposedContentOffset.x需要减去14，即控件自动向右移动14的距离</p>
<p>例2:若cell_1距中点靠右，<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">CGRectGetWidth(self.collectionView.bounds) / 2 ＝ 187.5</span><br><span class="line">cell.center.x＝387.5，</span><br><span class="line">center.X＝172.5+187.5</span><br><span class="line">cell2CenterOffset＝cell.center.x － center_X ＝ 387.5 - 360 = 27.5</span><br></pre></td></tr></table></figure></p>
<p>说明偏移量proposedContentOffset.x需要增加27.5，即控件自动向左移动27.5的距离</p>
<h1 id="总结："><a href="#总结：" class="headerlink" title="总结："></a>总结：</h1><p>控件往左滚动，偏移量正数递增，通过得到屏幕中点相对于控件滚动的距离和每个cell在控件上的center.x比较，得到最小的距离，<br>cell在中点右边，cell.x－center_X > 0,则需要增加差值的偏移量，控件自动向左滑动差值距离<br>cell在中点的左边，cell.x - center_X < 0,则需要减少差值的偏移量，控件自动向右滑动差值距离</p>
<p><img src="https://raw.githubusercontent.com/KakaMic/BlogResource/master/collectionview2.jpeg" alt=""></p>
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