LayoutInflater加载布局遇到的坑

问题：

Fragment，adapter中的视图都是需要LayoutInflater将XML文件转换为View对象，但是使用过程中发现加载后，RecyclerView中item设置宽高以及margin无效

案例：

layout_footer.xml

    

        ...

    

在Activity中加载footer

 View footerView = LayoutInflater.from(mContext).inflate(R.layout.layout_footer,null);

发现上面设置的android:layout_margin=“10”无效

解决：

在布局转换的时候使用

View view = layoutinflater.inflate(R.layout.XXX,ViewGroup,false);

分析：

为什么使用三个参数的inflate方法就能实现效果？

inflate有重载4个方法

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root)

public View inflate(XmlPullParser parser, @Nullable ViewGroup root)

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) 

最终都会走到

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)

 LayoutInflater类中的inflate源码

  /**
     * Inflate a new view hierarchy from the specified XML node. Throws
     * {@link InflateException} if there is an error.
     * 

     * Important   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
     *        attachToRoot is true), or else simply an object that
     *        provides a set of LayoutParams values for root of the returned
     *        hierarchy (if attachToRoot 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) {
        synchronized (mConstructorArgs) {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");

            final Context inflaterContext = mContext;
            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context) mConstructorArgs[0];
            mConstructorArgs[0] = inflaterContext;
            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(" can be used only with a valid "
                                + "ViewGroup root and attachToRoot=true");
                    }

                    rInflate(parser, root, inflaterContext, attrs, false);
                } else {
                    // Temp is the root view that was found in the xml
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);

                    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 against its context.
                    rInflateChildren(parser, temp, attrs, 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) {
                final InflateException ie = new InflateException(e.getMessage(), e);
                ie.setStackTrace(EMPTY_STACK_TRACE);
                throw ie;
            } catch (Exception e) {
                final InflateException ie = new InflateException(parser.getPositionDescription()
                        + ": " + e.getMessage(), e);
                ie.setStackTrace(EMPTY_STACK_TRACE);
                throw ie;
            } finally {
                // Don't retain static reference on context.
                mConstructorArgs[0] = lastContext;
                mConstructorArgs[1] = null;

                Trace.traceEnd(Trace.TRACE_TAG_VIEW);
            }

            return result;
        }
    }

1）第一个参数XmlPullParser parser

parser：xml文件
如果传入的是layout的id，会调用
final XmlResourceParser parser = res.getLayout(resource)；转换成xml文件

2）第二个参数root，第三个参数attachToRoot

root不为空
和第三个参数attachToRoot有关
① 如果attachToRoot为true
root成为inflate出来的view对象的父布局
② 如果attachToRoot为false
inflate出来的view的根布局提供LayoutParams参数的控件

当root不为空，attachToRoot为false时候。
获得xml文件中的属性
通过createViewFromTag生成View
再通过generateLayoutParams(attrs)生成layoutparams参数
设置到view中，使得view有layoutParams

 root为空（null）
不指定父布局，那么inflate出来的view对象的根布局的某些参数会失效，比如layout_width和layout_height，layout_margin会失效