LayoutInflater.inflate()之attachToRoot

将XML布局文件转化为View或者Widget时，经常使用LayoutInfater.infate()&&View.inflate()
View.inflate()源代码如下：

 /**     * Inflate a view from an XML resource.  This convenience method wraps the {@link     * LayoutInflater} class, which provides a full range of options for view inflation.     *     * @param context The Context object for your activity or application.     * @param resource The resource ID to inflate     * @param root A view group that will be the parent.  Used to properly inflate the     * layout_* parameters.     * @see LayoutInflater     */    public static View inflate(Context context, @LayoutRes int resource, ViewGroup root) {        LayoutInflater factory = LayoutInflater.from(context);        return factory.inflate(resource, root);    }

可见View.inflate()是通过LayoutInfater.inflate()实现了xml文件到View或者Widget的转化的。下面讲述一下LayoutInfater.inflate()中关于attachToRoot参数的注意事项。

    /**     * Inflate a new view hierarchy from the specified xml resource. Throws     * {@link InflateException} if there is an error.     *      * @param resource ID for an XML layout resource to load (e.g.,     *        <code>R.layout.main_page</code>)     * @param root Optional view to be the parent of the generated hierarchy (if     *        <em>attachToRoot</em> is true), or else simply an object that     *        provides a set of LayoutParams values for root of the returned     *        hierarchy (if <em>attachToRoot</em> is false.)     * @param attachToRoot Whether the inflated hierarchy should be attached to     *        the root parameter? If false, root is only used to create the     *        correct subclass of LayoutParams for the root view in the XML.     * @return The root View of the inflated hierarchy. If root was supplied and     *         attachToRoot is true, this is root; otherwise it is the root of     *         the inflated XML file.     */    public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {        final Resources res = getContext().getResources();        if (DEBUG) {            Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("                    + Integer.toHexString(resource) + ")");        }        final XmlResourceParser parser = res.getLayout(resource);        try {            return inflate(parser, root, attachToRoot);        } finally {            parser.close();        }    }

这一段注释有点长，再摘抄一下，大家请看下面的内容：

 * @param attachToRoot Whether the inflated hierarchy should be attached to     *        the root parameter? If false, root is only used to create the     *        correct subclass of LayoutParams for the root view in the XML.

如果attachToRoot=true,则布局文件将转化为View并绑定到root，然后返回root作为根节点的整个View;如果attachToRoot=false,则布局文件转化为View但不绑定到root，返回以布局文件根节点为根节点的View。在这两种情况下，root的LayoutParams***都*会影响布局文件的显示样式。下面分情况对attachToRoot的参数的设置进行讲解。

attachToRoot=true

• 将一个xml布局文件放入组件

item_linearout.xml

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:id="@+id/ll"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:background="@android:color/white"    android:orientation="vertical" ></LinearLayout>

非常简单的布局，注意它的背景颜色是白色！
item_btn.xml

<Button xmlns:android="http://schemas.android.com/apk/res/android"    android:id="@+id/btn"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:background="#ff0000" ></Button>

注意他的背景是红色
MainActivity主要代码：

@Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.item_linearout);        final LinearLayout outLayout = (LinearLayout) findViewById(R.id.ll);        LayoutInflater inflater = LayoutInflater.from(this);        inflater.inflate(R.layout.item_btn, outLayout, true);    }

运行效果

ok，将他放入了linearlaout中。

• 自定义组件
  view_with_merge_tag.xml文件

<?xml version="1.0" encoding="utf-8"?><merge xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent"     >    <TextView        android:layout_width="match_parent"        android:layout_height="match_parent"        android:background="#0f0"        android:text="Hello World!" >    </TextView></merge>

为了减少布局参差的嵌套，这是用了merge.注意其背景会绿色。
CustomLinear.java

package com.example.androidtest;import android.content.Context;import android.util.AttributeSet;import android.view.LayoutInflater;import android.widget.LinearLayout;public class CustomLinear extends LinearLayout {    public CustomLinear(Context context, AttributeSet attrs) {        super(context, attrs);        init();    }    public CustomLinear(Context context) {        super(context);        init();    }    private void init() {        LayoutInflater inflater = LayoutInflater.from(getContext());        inflater.inflate(R.layout.view_with_merge_tag, this);    }}

MainActivity.java主要代码

    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(new CustomLinear(this));    }

运行效果：

ok！

attachToRoot =false

attacheToRoot就是暂时不将布局文件放入root中。现在修改第一个例子，将attachToRoot设置为false,实现同样的效果。
MainActivity的主要代码：

@Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.item_linearout);        final LinearLayout outLayout = (LinearLayout) findViewById(R.id.ll);        LayoutInflater inflater = LayoutInflater.from(this);        Button button = (Button) inflater.inflate(R.layout.item_btn, outLayout,                false);        outLayout.addView(button);    }

这里将attachToRoot设置为false,没有将button立即加入linearlayout中，稍后的addView，将button添加到了linearlayout中。
布局文件转换为View,不都是为了放在组件，显示在界面上吗？为何还要多此一举，分两步来做这样的事情呢？Android中有些组件有其自身的”add View”机制，如：Fragment.onCreateView

package com.example.androidtest;import android.app.Fragment;import android.os.Bundle;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;public class CustomFragment extends Fragment {    @Override    public View onCreateView(LayoutInflater inflater, ViewGroup container,            Bundle savedInstanceState) {        View view = inflater.inflate(R.layout.item_btn, container, false);        return view;    }}

在Activty中，

@Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.item_linearout);        FragmentManager fragmentManager = getFragmentManager();        FragmentTransaction fragmentTransaction = fragmentManager                .beginTransaction();        fragmentTransaction.replace(R.id.ll, new CustomFragment());        fragmentTransaction.commit();}

“add View”的逻辑由FragmentTransaction 进行处理了，如果我们这里将attachToRoot设置为true，就会有IllegalStateException.

所以组件自己有“add View”的机制，我们就不要再画蛇添足了。
可能会有疑问既然没有将创建的View 添加到root中，为什么还要添加root参数呢？直接使用

        View view = inflater.inflate(R.layout.item_btn,null);

不是更简单吗？如果这样写，lint会给出警告
Avoid passing null as the view root (needed to resolve layout parameters on the inflated layout’s root element)。
xml布局文件在解析成View的时候，需要root的布局信息（View的布局参数要受到root的限制）。如果不填写root,使用xml布局文件生成View的时候就会使用默认的LayoutParams.而这个不一定是符合要求的，View可能比设定的要小。

不设置root

有些时候无法明确的知道View的root，eg.当你实例化一个AlterDialog.

@Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.item_linearout);        LayoutInflater inflater = LayoutInflater.from(this);        AlertDialog.Builder dialogBuilder = new AlertDialog.Builder(this);        View customView = inflater.inflate(R.layout.item_btn, null);        dialogBuilder.setView(customView);        dialogBuilder.show();        }

在这种情况下，传递null 就可以了。通用的规则是，如果可以知道root，一定要传递这个参数。

结论

• 如果知道root，一定要传，尽量避免传null
• 当不需要将布局文件生成的View添加到组件中时（组件有其自身的添加逻辑），attachToRoot设置成false.
• 当View已经添加到一个root中时，attachToRoot设置成false.
• 自定义组件应该将attachToRoot设置成true.

翻译地址

https://www.bignerdranch.com/blog/understanding-androids-layoutinflater-inflate/