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Nine-Patch-Affect-Padding-Principle.html
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<meta name="description" content="在上一篇文章记一次9.png的填坑之旅最后，我留了两个个问题：
为什么使用9.PNG设置背景之后会改变控件的padding属性值
在我所遇到的问题中9.PNG明明改变的是padding值为什么却产生了margin的效果
这篇文章就来解答这两个问题以及9.PNG使用过程中需要注意的地方。
各位同学如果没看过记一次9.png的填坑之旅这篇文章可以先去看看，因为我下面所讲的内容都是根据这篇文章">
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<meta property="og:description" content="在上一篇文章记一次9.png的填坑之旅最后，我留了两个个问题：
为什么使用9.PNG设置背景之后会改变控件的padding属性值
在我所遇到的问题中9.PNG明明改变的是padding值为什么却产生了margin的效果
这篇文章就来解答这两个问题以及9.PNG使用过程中需要注意的地方。
各位同学如果没看过记一次9.png的填坑之旅这篇文章可以先去看看，因为我下面所讲的内容都是根据这篇文章">
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<meta name="twitter:description" content="在上一篇文章记一次9.png的填坑之旅最后，我留了两个个问题：
为什么使用9.PNG设置背景之后会改变控件的padding属性值
在我所遇到的问题中9.PNG明明改变的是padding值为什么却产生了margin的效果
这篇文章就来解答这两个问题以及9.PNG使用过程中需要注意的地方。
各位同学如果没看过记一次9.png的填坑之旅这篇文章可以先去看看，因为我下面所讲的内容都是根据这篇文章">
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Android基础之9-Patch(9.PNG)与padding
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<p>在上一篇文章<a href="http://ittiger.cn/My-9.png-hold-the-fat-lady-tour.html">记一次9.png的填坑之旅</a>最后，我留了两个个问题：</p>
<ol>
<li>为什么使用<code>9.PNG</code>设置背景之后会改变控件的<code>padding</code>属性值</li>
<li>在我所遇到的问题中<code>9.PNG</code>明明改变的是<code>padding</code>值为什么却产生了<code>margin</code>的效果 </li>
</ol>
<p>这篇文章就来解答这两个问题以及<code>9.PNG</code>使用过程中需要注意的地方。</p>
<p>各位同学如果没看过<a href="http://ittiger.cn/My-9.png-hold-the-fat-lady-tour.html">记一次9.png的填坑之旅</a>这篇文章可以先去看看，因为我下面所讲的内容都是根据这篇文章中的问题来讲的。</p>
<a id="more"></a>
<h2 id="问题9-Patch图"><a href="#问题9-Patch图" class="headerlink" title="问题9-Patch图"></a>问题9-Patch图</h2><p>还是先来看下我所使用的<code>9.PNG</code>的图<br><img src="/img/View_9_PNG_3.png" alt=""><br>从图中可以看出中间那一片白色区域是内容区，而内容区到底部的黑边线之间还一段<font color="red"><strong>透明的区域</strong></font>，这段区域是不会显示内容的。</p>
<h2 id="结论猜想"><a href="#结论猜想" class="headerlink" title="结论猜想"></a>结论猜想</h2><p>看到这个透明区域联想到我上面遇到的问题</p>
<blockquote>
<p>明明改变的是<code>padding</code>却产生了<code>margin</code>的效果</p>
</blockquote>
<p>大家是不是会出现一个猜想，难道上篇文章中<code>padding</code>改变却出现<code>margin</code>效果是因为这个透明的区域导致的？</p>
<p>恭喜你，答对了，我前面遇到的问题中<code>padding</code>改变却产生<code>margin</code>效果就是因为这个透明区域导致的。</p>
<p>你可能又要接着问了，这是为什么呢？</p>
<p>不要走开，下面我就来说说其中的原因。</p>
<h2 id="一探setBackgroundResource"><a href="#一探setBackgroundResource" class="headerlink" title="一探setBackgroundResource"></a>一探setBackgroundResource</h2><p>我们来看看布局设置背景图片的方法<code>setBackgroundResource(int resid)</code>是如何实现的<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div></pre></td><td class="code"><pre><div class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setBackgroundResource</span><span class="params">(<span class="keyword">int</span> resid)</span> </span>{</div><div class="line"> <span class="keyword">if</span> (resid != <span class="number">0</span> && resid == mBackgroundResource) {</div><div class="line"> <span class="keyword">return</span>;</div><div class="line"> }</div><div class="line"> Drawable d = <span class="keyword">null</span>;</div><div class="line"> <span class="keyword">if</span> (resid != <span class="number">0</span>) {</div><div class="line"> d = mContext.getDrawable(resid);</div><div class="line"> }</div><div class="line"> setBackground(d);</div><div class="line"> mBackgroundResource = resid;</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>上面的代码可以看到调用了<code>setBackground(Drawable drawable)</code>这个方法，我们接着看<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setBackground</span><span class="params">(Drawable background)</span> </span>{</div><div class="line"> setBackgroundDrawable(background);</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>我们接着看<code>setBackgroundDrawable(Drawable drawable)</code>这个方法的实现<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setBackgroundDrawable</span><span class="params">(Drawable background)</span> </span>{</div><div class="line"> ...</div><div class="line"> <span class="keyword">if</span> (background != <span class="keyword">null</span>) {</div><div class="line"> Rect padding = sThreadLocal.get();</div><div class="line"> <span class="keyword">if</span> (padding == <span class="keyword">null</span>) {</div><div class="line"> padding = <span class="keyword">new</span> Rect();</div><div class="line"> sThreadLocal.set(padding);</div><div class="line"> }</div><div class="line"> resetResolvedDrawables();</div><div class="line"> background.setLayoutDirection(getLayoutDirection());</div><div class="line"> <span class="keyword">if</span> (background.getPadding(padding)) {</div><div class="line"> resetResolvedPadding();</div><div class="line"> <span class="keyword">switch</span> (background.getLayoutDirection()) {</div><div class="line"> <span class="keyword">case</span> LAYOUT_DIRECTION_RTL:</div><div class="line"> mUserPaddingLeftInitial = padding.right;</div><div class="line"> mUserPaddingRightInitial = padding.left;</div><div class="line"> internalSetPadding(padding.right, padding.top, padding.left, padding.bottom);</div><div class="line"> <span class="keyword">break</span>;</div><div class="line"> <span class="keyword">case</span> LAYOUT_DIRECTION_LTR:</div><div class="line"> <span class="keyword">default</span>:</div><div class="line"> mUserPaddingLeftInitial = padding.left;</div><div class="line"> mUserPaddingRightInitial = padding.right;</div><div class="line"> internalSetPadding(padding.left, padding.top, padding.right, padding.bottom);</div><div class="line"> }</div><div class="line"> mLeftPaddingDefined = <span class="keyword">false</span>;</div><div class="line"> mRightPaddingDefined = <span class="keyword">false</span>;</div><div class="line"> }</div><div class="line"> ...</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> ...</div><div class="line"> }</div><div class="line"> ...</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>上面的代码很明显可以看到当<code>background.getPadding(padding)</code>返回<code>true</code>的时候当前布局视图会通过<code>internalSetPadding(padding.left, padding.top, padding.right, padding.bottom)</code>给当前视图设置相应的<code>padding</code>值</p>
<h2 id="NinePatchDrawable"><a href="#NinePatchDrawable" class="headerlink" title="NinePatchDrawable"></a>NinePatchDrawable</h2><p>因为我们使用的是<code>9-Patch</code>，所以生成的<code>Drawable</code>对象是<code>NinePatchDrawable</code>实例，我们接着上面的源码流程继续查看<code>NinePatchDrawable.getPadding(Rect padding)</code>方法：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">getPadding</span><span class="params">(Rect padding)</span> </span>{</div><div class="line"> <span class="keyword">final</span> Rect scaledPadding = mPadding;</div><div class="line"> <span class="keyword">if</span> (scaledPadding != <span class="keyword">null</span>) {</div><div class="line"> <span class="keyword">if</span> (needsMirroring()) {</div><div class="line"> padding.set(scaledPadding.right, scaledPadding.top,</div><div class="line"> scaledPadding.left, scaledPadding.bottom);</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> padding.set(scaledPadding);</div><div class="line"> }</div><div class="line"> <span class="keyword">return</span> (padding.left | padding.top | padding.right | padding.bottom) != <span class="number">0</span>;</div><div class="line"> }</div><div class="line"> <span class="keyword">return</span> <span class="keyword">false</span>;</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>上面的方法很明显可以看出，如果当前<code>9-Patch</code>图存在<code>Padding</code>值的话，它会将该值设置到参数<code>Rect</code>中，而在前面<code>setBackgroundDrawable(Drawable drawable)</code>方法实现中方法<code>internalSetPadding(padding.left, padding.top, padding.right, padding.bottom)</code>里所传的参数<code>padding</code>就是这个地方的<code>Rect</code>参数实例。</p>
<h2 id="结论"><a href="#结论" class="headerlink" title="结论"></a>结论</h2><p>总结成一句话就是<font color="red">如果我们使用的<code>9-Patch</code>图生成的<code>NinePatchDrawable</code>对象存在<code>Padding</code>值，那么这些<code>Padding</code>值将会同时设置给使用该<code>9-Patch</code>作为背景图的<code>View</code>的<code>Padding</code>属性上</font></p>
<p>看了上面的结论，大家可能又会有一个疑问？？？</p>
<blockquote>
<p>我们使用的<code>9-Patch</code>图时，系统为我们生成<code>NinePatchDrawable</code>对象时产生的<code>Padding</code>是根据什么依据得来的呢？</p>
</blockquote>
<h2 id="9-Patch官方定义"><a href="#9-Patch官方定义" class="headerlink" title="9-Patch官方定义"></a>9-Patch官方定义</h2><p>OK，我们来看看<code>Google</code>官方对<code>9-Patch</code>图的定义和介绍<br><a href="https://developer.android.com/guide/topics/graphics/2d-graphics.html#nine-patch" target="_blank" rel="external">https://developer.android.com/guide/topics/graphics/2d-graphics.html#nine-patch</a></p>
<blockquote>
<p>The border is used to define the stretchable and static areas of the image. You indicate a stretchable section by drawing one (or more) 1-pixel-wide black line(s) in the left and top part of the border (the other border pixels should be fully transparent or white). You can have as many stretchable sections as you want: their relative size stays the same, so the largest sections always remain the largest.</p>
<p>You can also define an optional drawable section of the image (effectively, the padding lines) by drawing a line on the right and bottom lines. <font color="red">If a View object sets the NinePatch as its background and then specifies the View’s text, it will stretch itself so that all the text fits inside only the area designated by the right and bottom lines (if included). </font>If the padding lines are not included, Android uses the left and top lines to define this drawable area.</p>
<p>To clarify the difference between the different lines, the left and top lines define which pixels of the image are allowed to be replicated in order to stretch the image. The bottom and right lines define the relative area within the image that the contents of the View are allowed to lie within</p>
</blockquote>
<p><img src="/img/ninepatch_raw.png" alt=""></p>
<blockquote>
<p>This NinePatch defines one stretchable area with the left and top lines and the drawable area with the bottom and right lines. In the top image, the dotted grey lines identify the regions of the image that will be replicated in order to stretch the image. The pink rectangle in the bottom image identifies the region in which the contents of the View are allowed. If the contents don’t fit in this region, then the image will be stretched so that they do.</p>
</blockquote>
<p>上面这段几段文字是我引用的<code>Android</code>官方的介绍说明，里面介绍了<code>9-Patch</code>图的使用方式和要点。</p>
<h2 id="9-Patch图Padding的由来"><a href="#9-Patch图Padding的由来" class="headerlink" title="9-Patch图Padding的由来"></a>9-Patch图Padding的由来</h2><p>大家注意看段落中我标红的那一句话，这句话就是说：如果你使用<code>9-Patch</code>图给某个<code>View</code>设置<code>background</code>之后，这个<code>View</code>的所有内容仅仅适配显示在内容区域里，而内容区域范围是由<code>9-Patch</code>图<code>right and bottom lines</code>决定的。而<code>9-Patch</code>图内容区以外的部分是不可能无故消失的，所以就作为其<code>Padding</code>了，因此在我们使用<code>9-Patch</code>图作为背景图时，系统为我们生成的<code>NinePatchDrawable</code>对象的<code>Padding</code>值就是根据这得来的。</p>
<h2 id="回归问题"><a href="#回归问题" class="headerlink" title="回归问题"></a>回归问题</h2><p>知道上面的结论之后，我们再回过头来看我所遇到的问题，我们再看一遍我所使用的<code>9-Patch</code>图<br><img src="/img/View_9_PNG_3.png" alt=""><br>根据上面的结论可以发现这个图底部有一块透明的非内容区，因此我所使用的这个<code>9-Patch</code>图会产生一个<code>paddingBottom</code>值，而刚好这块区域又是透明，所以在使用的时候这块区域在视图上其实也是透明的，因此这块透明区域看到的效果其实是其父容器视图的内容，所以就造成了<code>margin</code>的效果。</p>
<p>讲到这里，我所遇到的那个问题就得到了完美的解答。</p>
<p>另外大家在使用<code>9-Patch</code>图的时候一定要注意其内容区域，如果<code>9-Patch</code>图制作不合适会产生一些莫名其妙的布局问题。</p>
<p>write by laohu<br>2016年11月18日</p>
<hr>
<p>原创文章，本文采用<a href="https://creativecommons.org/licenses/by/2.5/cn/" target="_blank" rel="external">知识共享署名 2.5(中国大陆许可协议)</a>进行许可，欢迎转载，但转载请注明来自<a href="ittiger.cn">ittiger.cn</a>，并保证转载后文章内容的完整性。本人(laohu)保留所有版权相关权利。</p>
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