LayoutInflater之inflate

inflate方法参数

总结

方法

• View inflate(@LayoutRes int resource, @Nullable ViewGroup root)
• View inflate(XmlPullParser parser, @Nullable ViewGroup root)
• View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)
• View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {

参数

• int resource：需要inflate的布局文件资源id
• ViewGroup root: 这个inflate出来的View的父容器
• boolean attachToRoot：是否将inflate出来的View添加到指定的root父容器中
• XmlPullParser parser：XML解析器

解释

• root = null

  也就是未指定inflate View的父容器，那么肯定无法将View绑定到指定父容器中，从而inflate View也就丢失了其LayoutParams属性，，返回的View也就是最终inflate出来的Veiw

• root != null, attachToRoot = true

• root != null, attachToRoot = false

  inflate View 会有LayoutParams属性，即在resources布局文件中设置的width与height属性会被设置进来，

  true 与 false 的区别：

  • true 执行 root.addView(View,params)，false不执行
  • true 返回的 View 的是 root，false 返回的是 inflate 出来的 View

• 一般项目中需求很少有不需要LayoutParams的场景，故父容器 root 需要指定的，而且在AS lint提示中也能体现出来的，如图：

inflate_whenRootIsNull_lint.png

• 使用RecyclerView 添加header、footer、emptyView时，
  这样inflate：view = inflater.inflate(layoutId, viewGroup, false);
  inflater.inflate(layoutId, null)会丢失LayoutParams导致渲染出来后布局错乱
  如果没有现成的viewgroup可以new一个

源码解析

 /**
     * 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, @Nullable ViewGroup root, boolean attachToRoot) {
    ...
    
    //初始化返回的View为指定的父容器root
    View result = root;
    ...
    final View temp = createViewFromTag(root, name, inflaterContext, attrs);

    ViewGroup.LayoutParams params = null;

    //从此处的判断可以知道只有root！=null的情况下，才会执行解析inflate View的LayoutParams属性
    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);
        }
    }
    
    // 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;
    }

}