听FackBook工程师讲Custom ViewGroups

原文链接 : [Custom ViewGroups][1]
原文作者 : [Sriram Ramani][2]

Android提供了几个ViewGroups如LinearLayout, RelativeLayout, FrameLayout来固定child Views的位置。在这些普通的ViewGroups中有多种使用选择。例如：LinearLayout几乎支持HTML Flexbox的所有特性(除了包装)。在view之间你可以选择是否显示分割线(dividers),并且基于最大的child测量所有的children。RelativeLayout是一种限制性的解决方案。这些layouts都已经足够好了，但是当你的UI非常复杂的时候它们还能很好的解决么？

ViewGroup with a ProfilePhoto, Title, Subtitle and Menu button.

上面的这种布局在Facebook app中是非常常见的。有头像、其它的view垂直摆在它的右侧、还有一个可选操作的view在最右边。这个布局可以通过使用LinearLayout嵌套或者一个RelativeLayout这样的普通ViewGroup实现。我们看一下当分别使用这两种布局的情况下在measure时会发生什么。

使用LinearLayout完成布局的示例

``` <LinearLayout android:layout_width="match_parent" android:layout_height="wrap_content">

<span class="pl-k">&lt;</span><span class="pl-smi">ProfilePhoto</span>
    android<span class="pl-k">:</span>layout_width<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>40dp<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_height<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>40dp<span class="pl-pds">"</span></span><span class="pl-k">/</span><span class="pl-k">&gt;</span>

<span class="pl-k">&lt;</span><span class="pl-smi">LinearLayout</span>
    android<span class="pl-k">:</span>layout_width<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>0dp<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_height<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>wrap_content<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_weight<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>1<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>orientation<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>vertical<span class="pl-pds">"</span></span><span class="pl-k">&gt;</span>

    <span class="pl-k">&lt;</span><span class="pl-smi">Title</span>
        android<span class="pl-k">:</span>layout_width<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>match_parent<span class="pl-pds">"</span></span>
        android<span class="pl-k">:</span>layout_height<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>wrap_content<span class="pl-pds">"</span></span><span class="pl-k">/</span><span class="pl-k">&gt;</span>

    <span class="pl-k">&lt;</span><span class="pl-smi">Subtitle</span>
        android<span class="pl-k">:</span>layout_width<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>match_parent<span class="pl-pds">"</span></span>
        android<span class="pl-k">:</span>layout_height<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>wrap_content<span class="pl-pds">"</span></span><span class="pl-k">/</span><span class="pl-k">&gt;</span>

<span class="pl-k">&lt;</span><span class="pl-k">/</span><span class="pl-smi">LinearLayout</span><span class="pl-k">&gt;</span>

<span class="pl-k">&lt;</span><span class="pl-smi">Menu</span>
    android<span class="pl-k">:</span>layout_width<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>20dp<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_height<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>20dp<span class="pl-pds">"</span></span><span class="pl-k">/</span><span class="pl-k">&gt;</span>

</LinearLayout>

</div>

在Nexus 5设备上measure发生时的情况如下

    > LinearLayout [horizontal]       [w: 1080  exactly,       h: 1557  exactly    ]
        > ProfilePhoto                [w: 120   exactly,       h: 120   exactly    ]
        > LinearLayout [vertical]     [w: 0     unspecified,   h: 0     unspecified]
            > Title                   [w: 0     unspecified,   h: 0     unspecified]
            > Subtitle                [w: 0     unspecified,   h: 0     unspecified]
            > Title                   [w: 222   exactly,       h: 57    exactly    ]
            > Subtitle                [w: 222   exactly,       h: 57    exactly    ]
        > Menu                        [w: 60    exactly,       h: 60    exactly    ]
        > LinearLayout [vertical]     [w: 900   exactly,       h: 1557  at_most    ]
            > Title                   [w: 900   exactly,       h: 1557  at_most    ]
            > Subtitle                [w: 900   exactly,       h: 1500  at_most    ]       
    

ProfilePhoto和Menu只被测量了一次，因为它们有明确的宽高值。垂直的LinearLayout被测量了两次。第一次的时候，父LinearLayout要求以UNSPECIFIED spec的方式来测量。导致了垂直的LinearLayout也以这种方式测量它的子view.此时它在它们返回值的基础上以EXACTLY spec的方式测量它的子view，但是它还没有结束。一旦在测量ProfilePhoto和Menu之后，父布局知道可用于垂直的LinearLayout的尺寸大小。以AT_MOST height对Title 和 Subtitle测量之后导致了第二次传值。显然，每一个TextView (Title and Subtitle)被测量3次。第二次传值创建或者废弃Layouts，这些操作是昂贵的。如果想ViewGroup发挥更好的性能，首要的工作就是免去对TextViews的测量传值工作。

使用RelativeLayout效果会不会好一些?

<div class="highlight highlight-java">

<span class="pl-k">&lt;</span><span class="pl-smi">ProfilePhoto</span>
    android<span class="pl-k">:</span>layout_width<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>40dp<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_height<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>40dp<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_alignParentTop<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>true<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_alignParentLeft<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>true<span class="pl-pds">"</span></span><span class="pl-k">/</span><span class="pl-k">&gt;</span>

<span class="pl-k">&lt;</span><span class="pl-smi">Menu</span>
    android<span class="pl-k">:</span>layout_width<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>20dp<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_height<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>20dp<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_alignParentTop<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>true<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_alignParentRight<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>true<span class="pl-pds">"</span></span><span class="pl-k">/</span><span class="pl-k">&gt;</span>

<span class="pl-k">&lt;</span><span class="pl-smi">Title</span>
    android<span class="pl-k">:</span>layout_width<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>wrap_content<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_height<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>wrap_content<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_toRightOf<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>@id/profile_photo<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_toLeftOf<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>@id/menu<span class="pl-pds">"</span></span><span class="pl-k">/</span><span class="pl-k">&gt;</span>

<span class="pl-k">&lt;</span><span class="pl-smi">Subtitle</span>
    android<span class="pl-k">:</span>layout_width<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>wrap_content<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_height<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>wrap_content<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_below<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>@id/title<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_toRightOf<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>@id/profile_photo<span class="pl-pds">"</span></span>
    android<span class="pl-k">:</span>layout_toLeftOf<span class="pl-k">=</span><span class="pl-s"><span class="pl-pds">"</span>@id/menu<span class="pl-pds">"</span></span><span class="pl-k">/</span><span class="pl-k">&gt;</span>

</RelativeLayout>

</div>

测量情况如下：

    > RelativeLayout                  [w: 1080  exactly,   h: 1557  exactly]
        > Menu                        [w: 60    exactly,   h: 1557  at_most]
        > ProfilePhoto                [w: 120   exactly,   h: 1557  at_most]
        > Title                       [w: 900   exactly,   h: 1557  at_most]
        > Subtitle                    [w: 900   exactly,   h: 1557  at_most]
        > Title                       [w: 900   exactly,   h: 1557  at_most]
        > Subtitle                    [w: 900   exactly,   h: 1500  at_most]
        > Menu                        [w: 60    exactly,   h: 60    exactly]
        > ProfilePhoto                [w: 120   exactly,   h: 120   exactly]
    

正如先前提到的，RelativeLayout是通过solving constraints(分解约束。译者认为就是一层一层的测量)进行测量，上面的布局中ProfilePhoto和Menu没有依赖其它的参照物(siblings)，因此它们先被测量(with an AT_MOST height).这时Title(2个约束)和Subtitle(3个约束)才会被测量。此时所有view明确了自己想要的尺寸大小。RelativeLayout使用这些信息第二次传值给Title, Subtitle, Menu和ProfilePhoto。再重复一遍，每个view被测量了两次，因此这种方案稍佳。如果你和上面的LinearLayout例子相比较一下，最后用于测量所有leaf Views所使用的MeasureSpec是相同的-因此最后的输出结果是一样的。

怎么样才能免去对子view的测量传值呢？自定义一个ViewGroup是不是会有帮助？让我们分析一下这个布局。Title 和 Subtitle 总是在ProfilePhoto的左侧在Menu按钮的右侧。如果我们手工解决这个问题，需要计算出ProfilePhoto和Menu按钮的尺寸，并且使用剩下的尺寸再来计算Title 和 Subtitle。这时对每个view只进行一次测量传值。我们叫这种布局为ProfilePhotoLayout。

<div class="highlight highlight-java">

public class ProfilePhotoLayout extends ViewGroup {

<span class="pl-k">private</span> <span class="pl-smi">ProfilePhoto</span> mProfilePhoto;
<span class="pl-k">private</span> <span class="pl-smi">Menu</span> mMenu;
<span class="pl-k">private</span> <span class="pl-smi">Title</span> mTitle;
<span class="pl-k">private</span> <span class="pl-smi">Subtitle</span> mSubtitle;

<span class="pl-k">@Override</span>
<span class="pl-k">protected</span> <span class="pl-k">void</span> <span class="pl-en">onMeasure</span>(<span class="pl-k">int</span> <span class="pl-v">widthMeasureSpec</span>, <span class="pl-k">int</span> <span class="pl-v">heightMeasureSpec</span>) {
    <span class="pl-c">// 1. Setup initial constraints.</span>
    <span class="pl-k">int</span> widthConstraints <span class="pl-k">=</span> getPaddingLeft() <span class="pl-k">+</span> getPaddingRight();
    <span class="pl-k">int</span> heightContraints <span class="pl-k">=</span> getPaddingTop() <span class="pl-k">+</span> getPaddingBottom();
    <span class="pl-k">int</span> width <span class="pl-k">=</span> <span class="pl-c1"></span>;
    <span class="pl-k">int</span> height <span class="pl-k">=</span> <span class="pl-c1"></span>;

    <span class="pl-c">// 2. Measure the ProfilePhoto</span>
    measureChildWithMargins(
        mProfilePhoto,
        widthMeasureSpec,
        widthConstraints,
        heightMeasureSpec,
        heightConstraints);

    <span class="pl-c">// 3. Update the contraints.</span>
    widthConstraints <span class="pl-k">+=</span> mProfilePhoto<span class="pl-k">.</span>getMeasuredWidth();
    width <span class="pl-k">+=</span> mProfilePhoto<span class="pl-k">.</span>getMeasuredWidth();
    height <span class="pl-k">=</span> <span class="pl-smi">Math</span><span class="pl-k">.</span>max(mProfilePhoto<span class="pl-k">.</span>getMeasuredHeight(), height);

    <span class="pl-c">// 4. Measure the Menu.</span>
    measureChildWithMargins(
        mMenu,
        widthMeasureSpec,
        widthConstraints,
        heightMeasureSpec,
        heightConstraints);

    <span class="pl-c">// 5. Update the constraints.</span>
    widthConstraints <span class="pl-k">+=</span> mMenu<span class="pl-k">.</span>getMeasuredWidth();
    width <span class="pl-k">+=</span> mMenu<span class="pl-k">.</span>getMeasuredWidth();
    height <span class="pl-k">=</span> <span class="pl-smi">Math</span><span class="pl-k">.</span>max(mMenu<span class="pl-k">.</span>getMeasuredHeight(), height);

    <span class="pl-c">// 6. Prepare the vertical MeasureSpec.</span>
    <span class="pl-k">int</span> verticalWidthMeasureSpec <span class="pl-k">=</span> <span class="pl-smi">MeasureSpec</span><span class="pl-k">.</span>makeMeasureSpec(
        <span class="pl-smi">MeasureSpec</span><span class="pl-k">.</span>getSize(widthMeasureSpec) <span class="pl-k">-</span> widthConstraints,
        <span class="pl-smi">MeasureSpec</span><span class="pl-k">.</span>getMode(widthMeasureSpec));

    <span class="pl-k">int</span> verticalHeightMeasureSpec <span class="pl-k">=</span> <span class="pl-smi">MeasureSpec</span><span class="pl-k">.</span>makeMeasureSpec(
        <span class="pl-smi">MeasureSpec</span><span class="pl-k">.</span>getSize(heightMeasureSpec) <span class="pl-k">-</span> heightConstraints,
        <span class="pl-smi">MeasureSpec</span><span class="pl-k">.</span>getMode(heightMeasureSpec));

    <span class="pl-c">// 7. Measure the Title.</span>
    measureChildWithMargins(
        mTitle,
        verticalWidthMeasureSpec,
        <span class="pl-c1"></span>,
        verticalHeightMeasureSpec,
        <span class="pl-c1"></span>);

    <span class="pl-c">// 8. Measure the Subtitle.</span>
    measureChildWithMargins(
        mSubtitle,
        verticalWidthMeasureSpec,
        <span class="pl-c1"></span>,
        verticalHeightMeasureSpec,
        mTitle<span class="pl-k">.</span>getMeasuredHeight());

    <span class="pl-c">// 9. Update the sizes.</span>
    width <span class="pl-k">+=</span> <span class="pl-smi">Math</span><span class="pl-k">.</span>max(mTitle<span class="pl-k">.</span>getMeasuredWidth(), mSubtitle<span class="pl-k">.</span>getMeasuredWidth());
    height <span class="pl-k">=</span> <span class="pl-smi">Math</span><span class="pl-k">.</span>max(mTitle<span class="pl-k">.</span>getMeasuredHeight() <span class="pl-k">+</span> mSubtitle<span class="pl-k">.</span>getMeasuredHeight(), height);

    <span class="pl-c">// 10. Set the dimension for this ViewGroup.</span>
    setMeasuredDimension(
        resolveSize(width, widthMeasureSpec),
        resolveSize(height, heightMeasureSpec));
}

<span class="pl-k">@Override</span>
<span class="pl-k">protected</span> <span class="pl-k">void</span> <span class="pl-en">measureChildWithMargins</span>(
    <span class="pl-smi">View</span> <span class="pl-v">child</span>,
    <span class="pl-k">int</span> <span class="pl-v">parentWidthMeasureSpec</span>,
    <span class="pl-k">int</span> <span class="pl-v">widthUsed</span>,
    <span class="pl-k">int</span> <span class="pl-v">parentHeightMeasureSpec</span>,
    <span class="pl-k">int</span> <span class="pl-v">heightUsed</span>) {
    <span class="pl-smi">MarginLayoutParams</span> lp <span class="pl-k">=</span> (<span class="pl-smi">MarginLayoutParams</span>) child<span class="pl-k">.</span>getLayoutParams();

    <span class="pl-k">int</span> childWidthMeasureSpec <span class="pl-k">=</span> getChildMeasureSpec(
        parentWidthMeasureSpec,
        widthUsed <span class="pl-k">+</span> lp<span class="pl-k">.</span>leftMargin <span class="pl-k">+</span> lp<span class="pl-k">.</span>rightMargin,
        lp<span class="pl-k">.</span>width);

    <span class="pl-k">int</span> childHeightMeasureSpec <span class="pl-k">=</span> getChildMeasureSpec(
        parentHeightMeasureSpec,
        heightUsed <span class="pl-k">+</span> lp<span class="pl-k">.</span>topMargin <span class="pl-k">+</span> lp<span class="pl-k">.</span>bottomMargin,
        lp<span class="pl-k">.</span>height);

    child<span class="pl-k">.</span>measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

}

</div>

我们来分析一下代码。我们从已知的约束条件开始 — 所有边的内边距，另外还需要考虑的约束是使用固定值的控件的高和宽。Android提供了一个帮助方法-measureChildWithMargins()用于测量ViewGroup内的子view.然而它总是添加padding作为约束条件的一部分。因此我们复写这个方法自己来管理这些约束条件。从测量ProfilePhoto开始，测量完成后更新一下constraints。对menu按钮的测量亦是如此。 现在还剩下Title和Subtitle的宽度没有测量。Android还提供了另外一个帮助方法-makeMeasureSpec()，用于构造MeasureSpec,传入相应的size和mode返回一个MeasureSpec。接下来我们传入Title 和 Subtitle可用的width 和 height及相应的MeasureSpecs来测量Title 和 Subtitle。最后更新一下ViewGroup的尺寸。在这一步可以明确每个view都只被测量一次。

    > ProfilePhotoLayout              [w: 1080  exactly,   h: 1557  exactly]
        > ProfilePhoto                [w: 120   exactly,   h: 120   exactly]
        > Menu                        [w: 60    exactly,   h: 60    exactly]
        > Title                       [w: 900   exactly,   h: 1557  at_most]
        > Subtitle                    [w: 900   exactly,   h: 1500  at_most]
    

性能上是不是提升了？Facebook app中你看见的大多数布局都使用了这种布局，并且经证明确实提高了性能。我把没有提到的onLayout()方法留给读者作为练习。

你喜欢解决这种Android UI 工程问题么? Facebook在这方面缺少专业的人才。

 [1]: https://sriramramani.wordpress.com/2015/05/06/custom-viewgroups/
 [2]: https://sriramramani.wordpress.com/

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