Android LayoutInflate深度解析给你带来全新的认识

1、题外话

相信大家对LayoutInflate都不陌生，特别在ListView的Adapter的getView方法中基本都会出现，使用inflate方法去加载一个布局，用于ListView的每个Item的布局。Inflate有三个参数，我在初学Android的时候这么理解的：

对于Inflate的三个参数(int resource, ViewGroup root, boolean attachToRoot)

如果inflate(layoutId, null )则layoutId的最外层的控件的宽高是没有效果的

如果inflate(layoutId, root, false ) 则认为和上面效果是一样的

如果inflate(layoutId, root, true ) 则认为这样的话layoutId的最外层控件的宽高才能正常显示

如果你也这么认为，那么你有就必要好好阅读这篇文章，因为这篇文章首先会验证上面的理解是错误的，然后从源码角度去解释，最后会从ViewGroup与View的角度去解释。

2、实践是验证真理的唯一标准

下面我写一个特别常见的例子来验证上面的理解是错误的，一个特别简单的ListView，每个Item中放一个按钮：

Activity的布局文件：

<span class="tag">&lt;<span class="title">ListView</span> <span class="attribute">xmlns:android</span>=<span class="value">"http://schemas.android.com/apk/res/android"</span>
<span class="attribute">xmlns:tools</span>=<span class="value">"http://schemas.android.com/tools"</span>
<span class="attribute">android:id</span>=<span class="value">"@+id/id_listview"</span>
<span class="attribute">android:layout_width</span>=<span class="value">"fill_parent"</span>
<span class="attribute">android:layout_height</span>=<span class="value">"wrap_content"</span> &gt;</span>
<span class="tag">&lt;/<span class="title">ListView</span>&gt;</span>

ListView的Item的布局文件：

<span class="tag">&lt;<span class="title">Button</span> <span class="attribute">xmlns:android</span>=<span class="value">"http://schemas.android.com/apk/res/android"</span>
  <span class="attribute">xmlns:tools</span>=<span class="value">"http://schemas.android.com/tools"</span>
  <span class="attribute">android:id</span>=<span class="value">"@+id/id_btn"</span>
  <span class="attribute">android:layout_width</span>=<span class="value">"120dp"</span>
  <span class="attribute">android:layout_height</span>=<span class="value">"120dp"</span> &gt;</span>

<span class="tag">&lt;/<span class="title">Button</span>&gt;</span>

ListView的适配器：

<span class="keyword">package</span> com.example.zhy_layoutinflater;

<span class="keyword">import</span> java.util.List;

<span class="keyword">import</span> android.content.Context;
<span class="keyword">import</span> android.view.LayoutInflater;
<span class="keyword">import</span> android.view.View;
<span class="keyword">import</span> android.view.ViewGroup;
<span class="keyword">import</span> android.widget.BaseAdapter;
<span class="keyword">import</span> android.widget.Button;

<span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">MyAdapter</span> <span class="keyword">extends</span> <span class="title">BaseAdapter</span>
{</span>

  <span class="keyword">private</span> LayoutInflater mInflater;
  <span class="keyword">private</span> List&lt;String&gt; mDatas;

  <span class="keyword">public</span> MyAdapter(Context context, List&lt;String&gt; datas)
  {
    mInflater = LayoutInflater.from(context);
    mDatas = datas;
  }

  <span class="annotation">@Override</span>
  <span class="keyword">public</span> <span class="keyword">int</span> getCount()
  {
    <span class="keyword">return</span> mDatas.size();
  }

  <span class="annotation">@Override</span>
  <span class="keyword">public</span> Object getItem(<span class="keyword">int</span> position)
  {
    <span class="keyword">return</span> mDatas.get(position);
  }

  <span class="annotation">@Override</span>
  <span class="keyword">public</span> <span class="keyword">long</span> getItemId(<span class="keyword">int</span> position)
  {
    <span class="keyword">return</span> position;
  }

  <span class="annotation">@Override</span>
  <span class="keyword">public</span> View getView(<span class="keyword">int</span> position, View convertView, ViewGroup parent)
  {

    ViewHolder holder = <span class="keyword">null</span>;
    <span class="keyword">if</span> (convertView == <span class="keyword">null</span>)
    {
      holder = <span class="keyword">new</span> ViewHolder();
      convertView = mInflater.inflate(R.layout.item, <span class="keyword">null</span>);
<span class="comment">//			convertView = mInflater.inflate(R.layout.item, parent ,false);</span>
<span class="comment">//			convertView = mInflater.inflate(R.layout.item, parent ,true);</span>
      holder.mBtn = (Button) convertView.findViewById(R.id.id_btn);
      convertView.setTag(holder);
    } <span class="keyword">else</span>
    {
      holder = (ViewHolder) convertView.getTag();
    }

    holder.mBtn.setText(mDatas.get(position));

    <span class="keyword">return</span> convertView;
  }

  <span class="keyword">private</span> <span class="keyword">final</span> <span class="class"><span class="keyword">class</span> <span class="title">ViewHolder</span>
  {</span>
    Button mBtn;
  }
}

主Activity:

<span class="keyword">package</span> com.example.zhy_layoutinflater;

<span class="keyword">import</span> java.util.Arrays;
<span class="keyword">import</span> java.util.List;

<span class="keyword">import</span> android.app.Activity;
<span class="keyword">import</span> android.os.Bundle;
<span class="keyword">import</span> android.widget.ListView;

<span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">MainActivity</span> <span class="keyword">extends</span> <span class="title">Activity</span>
{</span>

  <span class="keyword">private</span> ListView mListView;
  <span class="keyword">private</span> MyAdapter mAdapter;
  <span class="keyword">private</span> List&lt;String&gt; mDatas = Arrays.asList(<span class="string">"Hello"</span>, <span class="string">"Java"</span>, <span class="string">"Android"</span>);

  <span class="annotation">@Override</span>
  <span class="keyword">protected</span> <span class="keyword">void</span> onCreate(Bundle savedInstanceState)
  {
    <span class="keyword">super</span>.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mListView = (ListView) findViewById(R.id.id_listview);
    mAdapter = <span class="keyword">new</span> MyAdapter(<span class="keyword">this</span>, mDatas);
    mListView.setAdapter(mAdapter);

  }

}

好了，相信大家对这个例子都再熟悉不过了，没啥好说的，我们主要关注getView里面的inflate那行代码：下面我依次把getView里的写成：

1、convertView = mInflater.inflate(R.layout.item, null);

2、convertView = mInflater.inflate(R.layout.item, parent ,false);

3、convertView = mInflater.inflate(R.layout.item, parent ,true);

分别看效果图：

图1：

图2：

图3：

FATAL EXCEPTION: main
java.lang.UnsupportedOperationException: 
addView(View, LayoutParams) <span class="keyword">is</span> <span class="keyword">not</span> supported <span class="keyword">in</span> AdapterView

嗯，没错没有图3，第三种写法会报错。

由上面三行代码的变化，产生3个不同的结果，可以看到

inflater(resId, null )的确不能正确处理宽高的值，但是inflater(resId,parent,false)并非和inflater(resId, null )效果一致，它可以看出完美的显示了宽和高。

而inflater(resId,parent,true)报错了（错误的原因在解析源码的时候说）。

由此可见：文章开始提出的理解是绝对错误的。

3、源码解析

下面我通过源码来解释，这三种写法真正的差异

这三个方法，最终都会执行下面的代码：

<span class="keyword">public</span> View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
    synchronized (mConstructorArgs) {
      final AttributeSet attrs = Xml.asAttributeSet(parser);
      Context lastContext = (Context)mConstructorArgs[<span class="number"></span>];
      mConstructorArgs[<span class="number"></span>] = mContext;
      View result = root;

      <span class="keyword">try</span> {
        <span class="comment">// Look for the root node.</span>
        <span class="keyword">int</span> type;
        <span class="keyword">while</span> ((type = parser.next()) != XmlPullParser.START_TAG &&
            type != XmlPullParser.END_DOCUMENT) {
          <span class="comment">// Empty</span>
        }

        <span class="keyword">if</span> (type != XmlPullParser.START_TAG) {
          <span class="keyword">throw</span> <span class="keyword">new</span> InflateException(parser.getPositionDescription()
              + <span class="string">": No start tag found!"</span>);
        }

        final String name = parser.getName();
        
        <span class="keyword">if</span> (DEBUG) {
          System.<span class="keyword">out</span>.println(<span class="string">"**************************"</span>);
          System.<span class="keyword">out</span>.println(<span class="string">"Creating root view: "</span>
              + name);
          System.<span class="keyword">out</span>.println(<span class="string">"**************************"</span>);
        }

        <span class="keyword">if</span> (TAG_MERGE.equals(name)) {
          <span class="keyword">if</span> (root == <span class="keyword">null</span> || !attachToRoot) {
            <span class="keyword">throw</span> <span class="keyword">new</span> InflateException(<span class="string">"&lt;merge /&gt; can be used only with a valid "</span>
                + <span class="string">"ViewGroup root and attachToRoot=true"</span>);
          }

          rInflate(parser, root, attrs, <span class="keyword">false</span>);
        } <span class="keyword">else</span> {
          <span class="comment">// Temp is the root view that was found in the xml</span>
          View temp;
          <span class="keyword">if</span> (TAG_1995.equals(name)) {
            temp = <span class="keyword">new</span> BlinkLayout(mContext, attrs);
          } <span class="keyword">else</span> {
            temp = createViewFromTag(root, name, attrs);
          }

          ViewGroup.LayoutParams <span class="keyword">params</span> = <span class="keyword">null</span>;

          <span class="keyword">if</span> (root != <span class="keyword">null</span>) {
            <span class="keyword">if</span> (DEBUG) {
              System.<span class="keyword">out</span>.println(<span class="string">"Creating params from root: "</span> +
                  root);
            }
            <span class="comment">// Create layout params that match root, if supplied</span>
            <span class="keyword">params</span> = root.generateLayoutParams(attrs);
            <span class="keyword">if</span> (!attachToRoot) {
              <span class="comment">// Set the layout params for temp if we are not</span>
              <span class="comment">// attaching. (If we are, we use addView, below)</span>
              temp.setLayoutParams(<span class="keyword">params</span>);
            }
          }

          <span class="keyword">if</span> (DEBUG) {
            System.<span class="keyword">out</span>.println(<span class="string">"-----&gt; start inflating children"</span>);
          }
          <span class="comment">// Inflate all children under temp</span>
          rInflate(parser, temp, attrs, <span class="keyword">true</span>);
          <span class="keyword">if</span> (DEBUG) {
            System.<span class="keyword">out</span>.println(<span class="string">"-----&gt; done inflating children"</span>);
          }

          <span class="comment">// We are supposed to attach all the views we found (int temp)</span>
          <span class="comment">// to root. Do that now.</span>
          <span class="keyword">if</span> (root != <span class="keyword">null</span> && attachToRoot) {
            root.addView(temp, <span class="keyword">params</span>);
          }

          <span class="comment">// Decide whether to return the root that was passed in or the</span>
          <span class="comment">// top view found in xml.</span>
          <span class="keyword">if</span> (root == <span class="keyword">null</span> || !attachToRoot) {
            result = temp;
          }
        }

      } <span class="keyword">catch</span> (XmlPullParserException e) {
        InflateException ex = <span class="keyword">new</span> InflateException(e.getMessage());
        ex.initCause(e);
        <span class="keyword">throw</span> ex;
      } <span class="keyword">catch</span> (IOException e) {
        InflateException ex = <span class="keyword">new</span> InflateException(
            parser.getPositionDescription()
            + <span class="string">": "</span> + e.getMessage());
        ex.initCause(e);
        <span class="keyword">throw</span> ex;
      } <span class="keyword">finally</span> {
        <span class="comment">// Don't retain static reference on context.</span>
        mConstructorArgs[<span class="number"></span>] = lastContext;
        mConstructorArgs[<span class="number">1</span>] = <span class="keyword">null</span>;
      }

      <span class="keyword">return</span> result;
    }
  }

第6行：首先声明了View result = root ；//最终返回值为result

第43行执行了：temp = createViewFromTag(root, name, attrs);创建了View

然后直接看48-59：

<span class="keyword">if</span>(root==<span class="keyword">null</span>)
{
 <span class="keyword">params</span> = root.generateLayoutParams(attrs);
        <span class="keyword">if</span> (!attachToRoot)
 {
   temp.setLayoutParams(<span class="keyword">params</span>);
 }
}

可以看到，当root为null,attachToRoot为false时，为temp设置了LayoutParams.

继续往下，看73-75行：

<span class="keyword">if</span> (root != <span class="keyword">null</span> && attachToRoot)
{
root.addView(temp, <span class="keyword">params</span>);
}

当root不为null，attachToRoot为true时，将tmp按照params添加到root中。

然后78-81行：

<span class="keyword">if</span> (root == <span class="keyword">null</span> || !attachToRoot) { 
result = temp; 
}

如果root为null，或者attachToRoot为false则，将temp赋值给result。

最后返回result。





从上面的分析已经可以看出：

Inflate(resId , null ) 只创建temp ,返回temp

Inflate(resId , parent, false )创建temp，然后执行temp.setLayoutParams(params);返回temp

Inflate(resId , parent, true ) 创建temp，然后执行root.addView(temp, params);最后返回root

由上面已经能够解释：

Inflate(resId , null )不能正确处理宽和高是因为：layout_width,layout_height是相对了父级设置的，必须与父级的LayoutParams一致。而此temp的getLayoutParams为null

Inflate(resId , parent,false ) 可以正确处理，因为temp.setLayoutParams(params);这个params正是root.generateLayoutParams(attrs);得到的。

Inflate(resId , parent,true )不仅能够正确的处理，而且已经把resId这个view加入到了parent，并且返回的是parent，和以上两者返回值有绝对的区别，还记得文章前面的例子上，MyAdapter里面的getView报的错误:

java.lang.UnsupportedOperationException: 
addView(View, LayoutParams) <span class="keyword">is</span> <span class="keyword">not</span> supported <span class="keyword">in</span> AdapterView

这是因为源码中调用了root.addView(temp, params);而此时的root是我们的ListView，ListView为AdapterView的子类:

直接看AdapterView的源码：

<span class="annotation">@Override</span>
  <span class="keyword">public</span> <span class="keyword">void</span> addView(View child) {
        <span class="keyword">throw</span> <span class="keyword">new</span> UnsupportedOperationException(<span class="string">"addView(View) is not supported in AdapterView"</span>);
  }

可以看到这个错误为啥产生了。

4、进一步的解析

上面我根据源码得出的结论可能大家还是有一丝的迷惑，我再写个例子论证我们上面得出的结论：

主布局文件：

<span class="tag">&lt;<span class="title">Button</span> <span class="attribute">xmlns:android</span>=<span class="value">"http://schemas.android.com/apk/res/android"</span>
  <span class="attribute">xmlns:tools</span>=<span class="value">"http://schemas.android.com/tools"</span>
  <span class="attribute">android:id</span>=<span class="value">"@+id/id_btn"</span>
  <span class="attribute">android:layout_width</span>=<span class="value">"120dp"</span>
  <span class="attribute">android:layout_height</span>=<span class="value">"120dp"</span>
  <span class="attribute">android:text</span>=<span class="value">"Button"</span> &gt;</span>
<span class="tag">&lt;/<span class="title">Button</span>&gt;</span>

主Activity:

<span class="keyword">package</span> com.example.zhy_layoutinflater;

<span class="keyword">import</span> android.app.ListActivity;
<span class="keyword">import</span> android.os.Bundle;
<span class="keyword">import</span> android.util.Log;
<span class="keyword">import</span> android.view.LayoutInflater;
<span class="keyword">import</span> android.view.View;
<span class="keyword">import</span> android.view.ViewGroup;

<span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">MainActivity</span> <span class="keyword">extends</span> <span class="title">ListActivity</span>
{</span>

  <span class="keyword">private</span> LayoutInflater mInflater;

  <span class="annotation">@Override</span>
  <span class="keyword">protected</span> <span class="keyword">void</span> onCreate(Bundle savedInstanceState)
  {
    <span class="keyword">super</span>.onCreate(savedInstanceState);

    mInflater = LayoutInflater.from(<span class="keyword">this</span>);

    View view1 = mInflater.inflate(R.layout.activity_main, <span class="keyword">null</span>);
    View view2 = mInflater.inflate(R.layout.activity_main,
        (ViewGroup)findViewById(android.R.id.content), <span class="keyword">false</span>);
    View view3 = mInflater.inflate(R.layout.activity_main,
        (ViewGroup)findViewById(android.R.id.content), <span class="keyword">true</span>);

    Log.e(<span class="string">"TAG"</span>, <span class="string">"view1 = "</span> + view1  +<span class="string">" , view1.layoutParams = "</span> + view1.getLayoutParams());
    Log.e(<span class="string">"TAG"</span>, <span class="string">"view2 = "</span> + view2  +<span class="string">" , view2.layoutParams = "</span> + view2.getLayoutParams());
    Log.e(<span class="string">"TAG"</span>, <span class="string">"view3 = "</span> + view3  );

  }

}

可以看到我们的主Activity并没有执行setContentView，仅仅执行了LayoutInflater的3个方法。

注：parent我们用的是Activity的内容区域：即android.R.id.content，是一个FrameLayout，我们在setContentView(resId)时，其实系统会自动为了包上一层FrameLayout（id=content）。

按照我们上面的说法：

view1的layoutParams 应该为null





view2的layoutParams 应该不为null，且为FrameLayout.LayoutParams





view3为FrameLayout，且将这个button添加到Activity的内容区域了（因为R.id.content代表Actvity内容区域）

下面看一下输出结果，和Activity的展示：

<span class="number">07</span>-<span class="number">27</span> <span class="number">14</span><span class="symbol">:</span><span class="number">17</span><span class="symbol">:</span><span class="number">36.703</span><span class="symbol">:</span> <span class="constant">E</span>/<span class="constant">TAG</span>(<span class="number">2911</span>)<span class="symbol">:</span> view1 = android.widget.<span class="constant">Button</span><span class="variable">@429d1660</span> , view1.layoutParams = null
<span class="number">07</span>-<span class="number">27</span> <span class="number">14</span><span class="symbol">:</span><span class="number">17</span><span class="symbol">:</span><span class="number">36.703</span><span class="symbol">:</span> <span class="constant">E</span>/<span class="constant">TAG</span>(<span class="number">2911</span>)<span class="symbol">:</span> view2 = android.widget.<span class="constant">Button</span><span class="variable">@42a0e120</span> , view2.layoutParams = android.widget.<span class="constant">FrameLayout</span><span class="variable">$LayoutParams</span><span class="variable">@42a0e9a0</span>
<span class="number">07</span>-<span class="number">27</span> <span class="number">14</span><span class="symbol">:</span><span class="number">17</span><span class="symbol">:</span><span class="number">36.703</span><span class="symbol">:</span> <span class="constant">E</span>/<span class="constant">TAG</span>(<span class="number">2911</span>)<span class="symbol">:</span> view3 = android.widget.<span class="constant">FrameLayout</span><span class="variable">@42a0a240</span>

效果图：

可见，虽然我们没有执行setContentView，但是依然可以看到绘制的控件，是因为

View view3 = mInflater.inflate(R.layout.activity_main,(ViewGroup)findViewById(android.R.id.content), true);

这个方法内部已经执行了root.addView(temp , params); 上面已经解析过了。





也可以看出：和我们的推测完全一致，到此已经完全说明了inflate3个重载的方法的区别。相信大家以后在使用时也能选择出最好的方式。不过下面准备从ViewGroup和View的角度来说一下，为啥layoutParams为null，就不能这确的处理。

5、从ViewGroup和View的角度来解析

如果大家对自定义ViewGroup和自定义View有一定的掌握，肯定不会对onMeasure方法陌生： ViewGroup的onMeasure方法所做的是：

为childView设置测量模式和测量出来的值。

如何设置呢？就是根据LayoutParams。





如果childView的宽为：LayoutParams. MATCH_PARENT，则设置模式为MeasureSpec.EXACTLY，且为childView计算宽度。





如果childView的宽为：固定值（即大于0），则设置模式为MeasureSpec.EXACTLY，且将lp.width直接作为childView的宽度。





如果childView的宽为：LayoutParams. WRAP_CONTENT，则设置模式为：MeasureSpec.AT_MOST





高度与宽度类似。

View的onMeasure方法：





主要做的就是根据ViewGroup传入的测量模式和测量值，计算自己应该的宽和高：





一般是这样的流程：





如果宽的模式是AT_MOST：则自己计算宽的值。





如果宽的模式是EXACTLY：则直接使用MeasureSpec.getSize(widthMeasureSpec);

对于最后一块，如果不清楚，不要紧，以后我会在自定义ViewGroup和自定义View时详细讲解的。

大概就是这样的流程，真正的绘制过程肯定比这个要复杂，就是为了说明如果View的宽和高如果设置为准确值，则一定依赖于LayoutParams，所以我们的inflate(resId,null)才没能正确处理宽和高。

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