LayoutInflater.inflate()浅析

AndroidView 2016-04

几个问题

• inflate()方法中第三个Boolean型参数实现什么功能
• 请举例常见的几种情况
• 什么情况下应该设置为true，什么情况下应该设置为false

源码

/**
 * 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.
 * @return The root View of the inflated hierarchy. If root was supplied,
 *         this is the root View; otherwise it is the root of the inflated
 *         XML file.
 */
public View inflate(int resource, ViewGroup root) {
    return inflate(resource, root, root != null);
}
/**
 * Inflate a new view hierarchy from the specified xml node. Throws
 * {@link InflateException} if there is an error. *
 * <p>
 * <em><strong>Important</strong></em>&nbsp;&nbsp;&nbsp;For performance
 * reasons, view inflation relies heavily on pre-processing of XML files
 * that is done at build time. Therefore, it is not currently possible to
 * use LayoutInflater with an XmlPullParser over a plain XML file at runtime.
 * 
 * @param parser XML dom node containing the description of the view
 *        hierarchy.
 * @param root Optional view to be the parent of the generated hierarchy.
 * @return The root View of the inflated hierarchy. If root was supplied,
 *         this is the root View; otherwise it is the root of the inflated
 *         XML file.
 */
public View inflate(XmlPullParser parser, ViewGroup root) {
    return inflate(parser, root, root != null);
}
/**
 * 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(int resource, 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();
    }
}
/**
 * Inflate a new view hierarchy from the specified XML node. Throws
 * {@link InflateException} if there is an error.
 * <p>
 * <em><strong>Important</strong></em>&nbsp;&nbsp;&nbsp;For performance
 * reasons, view inflation relies heavily on pre-processing of XML files
 * that is done at build time. Therefore, it is not currently possible to
 * use LayoutInflater with an XmlPullParser over a plain XML file at runtime.
 * 
 * @param parser XML dom node containing the description of the view
 *        hierarchy.
 * @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(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
    synchronized (mConstructorArgs) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
        final AttributeSet attrs = Xml.asAttributeSet(parser);
        Context lastContext = (Context)mConstructorArgs[0];
        mConstructorArgs[0] = mContext;
        View result = root;
        try {
            // Look for the root node.
            int type;
            while ((type = parser.next()) != XmlPullParser.START_TAG &&
                    type != XmlPullParser.END_DOCUMENT) {
                // Empty
            }
            if (type != XmlPullParser.START_TAG) {
                throw new InflateException(parser.getPositionDescription()
                        + ": No start tag found!");
            }
            final String name = parser.getName();
            if (DEBUG) {
                System.out.println("**************************");
                System.out.println("Creating root view: "
                        + name);
                System.out.println("**************************");
            }
            if (TAG_MERGE.equals(name)) {
                if (root == null || !attachToRoot) {
                    throw new InflateException("<merge /> can be used only with a valid "
                            + "ViewGroup root and attachToRoot=true");
                }
                rInflate(parser, root, attrs, false, false);
            } else {
                // Temp is the root view that was found in the xml
                final View temp = createViewFromTag(root, name, attrs, false);
                ViewGroup.LayoutParams params = null;
                if (root != null) {
                    if (DEBUG) {
                        System.out.println("Creating params from root: " +
                                root);
                    }
                    // Create layout params that match root, if supplied
                    params = root.generateLayoutParams(attrs);
                    if (!attachToRoot) {
                        // Set the layout params for temp if we are not
                        // attaching. (If we are, we use addView, below)
                        temp.setLayoutParams(params);
                    }
                }
                if (DEBUG) {
                    System.out.println("-----> start inflating children");
                }
                // Inflate all children under temp
                rInflate(parser, temp, attrs, true, true);
                if (DEBUG) {
                    System.out.println("-----> done inflating children");
                }
                // We are supposed to attach all the views we found (int temp)
                // to root. Do that now.
                if (root != null && attachToRoot) {
                    root.addView(temp, params);
                }
                // Decide whether to return the root that was passed in or the
                // top view found in xml.
                if (root == null || !attachToRoot) {
                    result = temp;
                }
            }
        } catch (XmlPullParserException e) {
            InflateException ex = new InflateException(e.getMessage());
            ex.initCause(e);
            throw ex;
        } catch (IOException e) {
            InflateException ex = new InflateException(
                    parser.getPositionDescription()
                    + ": " + e.getMessage());
            ex.initCause(e);
            throw ex;
        } finally {
            // Don't retain static reference on context.
            mConstructorArgs[0] = lastContext;
            mConstructorArgs[1] = null;
        }
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        return result;
    }
}

分析问题

回顾一下上面刚开始提及的几个问题。

• inflate()方法中第三个Boolean型参数实现什么功能
• 请举例常见的几种情况
• 什么情况下应该设置为true，什么情况下应该设置为false

• 第一个问题：上面代码中L159 - L161很清楚的解释了Boolean型参数的具体功能

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

所以小结下来就是这样：

Button button = (Button) inflater.inflate(R.layout.custom_button, mLinearLayout, false);
mLinearLayout.addView(button);

===========================等价于==========================

Button button = (Button) inflater.inflate(R.layout.custom_button, mLinearLayout, true);

• 第二个问题 开发中常见的例子
  先看一下在Fragment中最常见的

@Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        rootView = inflater.inflate(R.layout.webview_style_inner, container, false);
        rootWebView = (FanliWebviewContainer) rootView.findViewById(R.id.webviewContainer);
        return rootView;
    }

上面的第三行很多人也会这么写

rootView = inflater.inflate(R.layout.webview_style_inner, null);

问题：这两种写法，你更倾向于哪种，有什么区别？为什么大部分情况下这两种写法都能正常工作，而参数取为true则直接抛IllegalStateException异常。

如果ViewGroup传值为null，最起码的区别就是下面的警告

那么出现这种警告以后，我们有必要甩它吗？如果不care会怎样？

你的App不会挂掉，但是可能会出现异常。当你的子View没有正确的LayoutParams时，它会自己通过generateDefaultLayoutParams)计算。
你可能并不想要这些默认的LayoutParams。你在XML指定的LayoutParams会被忽略。我们可能已经指定了子View要填充父元素的宽度，但父View又wrap_content导致最终的View小很多。

所以很多人随手将ViewGroup故意置为null，为将来的bug埋下隐患，所以这里的原则就是有ViewGroup的地方还是老老实实的传下去。

问题：上面留下来的，为什么第三个参数传值为true会抛IllegalStateException异常？

原因是Fragment中将View添加到Activity，是由FragmentManager来负责添加，删除，替换等，所以这些都是由FragmentManager来管理的，不需要我们手动添加View到ViewGroup，如果手动去添加，势必会导致重复添加子View，所以会抛出IllegalStateException异常就好解释了。

那么还有其他情况和此种情况类似的吗? 这里仅仅举个栗子，在BaseAdapter中getView()方法里面

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    ViewHolder holder;
    if (convertView == null) {
        holder = new ViewHolder();
        convertView = mInflater.inflate(R.layout.list_fanli_record_item, parent， false);
        holder.creatTimeView = (TangFontTextView) convertView.findViewById(R.id.create_time);
        convertView.setTag(holder);
    } else {
        holder = (ViewHolder) convertView.getTag();
    }
    return convertView;
}

上面是ListView的数据适配器Adapter中适配View并重用View的典型代码，其中L7就符合上面Fragment的情况。ListView负责决定什么时候展示它的子View，这个不由我们决定。所以小结一下结论就是：在任何我们不负责将View添加进ViewGroup的情况下都应该将attachToRoot设置为false。

总结

总结一下结论如下：

• 如果有ViewGroup作为根元素，那么请将它传递下去。
• 避免将null作为根ViewGroup传入。
• 当我们不负责将layout文件的View添加进ViewGroup时，请设置attachToRoot参数为false。
• 不要在View已经被添加进ViewGroup时传入true，否则会抛IllegalStateException异常。
• 自定义View时请将attachToRoot设置为true，以免丢失父容器信息，默认不传改属性值，如果父容器不为空则该值为true。