LayoutInflater源码分析

一、基本用法

LayoutInflater inflater;a）inflater = LayoutInflater.from(context);  b）inflater =  (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE); inflater.inflate(......);

查看LayoutInflater的构造器源码，我们会发现a中方法调用了b方法，也就是最终都是通过b方法得到LayoutInflater的实例的：

    /**     * Obtains the LayoutInflater from the given context.     */    public static LayoutInflater from(Context context) {        LayoutInflater LayoutInflater =                (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);        if (LayoutInflater == null) {            throw new AssertionError("LayoutInflater not found.");        }        return LayoutInflater;    }

二、inflate(...)加载布局流程

关于inflate（）有多个重载的方法，我们只看其中的两个就可以了，因为最终都是通过第二个方法来完成View的Inflate的。

  public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {        return inflate(resource, root, root != null);//印证了下面的结论4    }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 {                // 查找根节点                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("**************************");                }//merge标签，如果root是null，则抛出异常；否则，解析merge标签下的子标签                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");                    }//inflate root下的所有子View                    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) {//印证了下面的的结论3                            // 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) {//印证了下面的结论2                        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 (Exception 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;        }    }

第一个inflate方法，我们主要看第三个参数，我们可以发现，如果root不为null，则attachToRoot默认是为TRUE的。

关于root和attachToRoot两个参数之间的关系，借用郭神总结的：

1. 如果root为null，attachToRoot将失去作用，设置任何值都没有意义。

2. 如果root不为null，attachToRoot设为true，则会给加载的布局文件的指定一个父布局，即root。

3. 如果root不为null，attachToRoot设为false，则会将布局文件最外层的所有layout属性进行设置，当该view被添加到父view当中时，这些layout属性会自动生效。

4. 在不设置attachToRoot参数的情况下，如果root不为null，attachToRoot参数默认为true。

第二个方法整体流程：

一句话，inflate根标签，紧接着通过递归的方式，inflate所有的子标签，并将生成的View其添加到parent ViewGroup中，当遍历完整个View 树的时候，整个的inflate流程就结束了 。我们重点看rInflate(...)方法：

   void rInflate(XmlPullParser parser, View parent, Context context,            AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {        final int depth = parser.getDepth();        int type;        while (((type = parser.next()) != XmlPullParser.END_TAG ||                parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {            if (type != XmlPullParser.START_TAG) {                continue;            }            final String name = parser.getName();                        if (TAG_REQUEST_FOCUS.equals(name)) {                parseRequestFocus(parser, parent);            } else if (TAG_TAG.equals(name)) {                parseViewTag(parser, parent, attrs);            } else if (TAG_INCLUDE.equals(name)) {                if (parser.getDepth() == 0) {                    throw new InflateException("<include /> cannot be the root element");                }                parseInclude(parser, context, parent, attrs);            } else if (TAG_MERGE.equals(name)) {                throw new InflateException("<merge /> must be the root element");            } else {                final View view = createViewFromTag(parent, name, context, attrs);                final ViewGroup viewGroup = (ViewGroup) parent;                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);                rInflateChildren(parser, view, attrs, true);                viewGroup.addView(view, params);            }        }        if (finishInflate) {            parent.onFinishInflate();        }    }

在这个方法中，重点看最后几行代码：

final View view = createViewFromTag(parent, name, context, attrs);                final ViewGroup viewGroup = (ViewGroup) parent;                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);                rInflateChildren(parser, view, attrs, true);                viewGroup.addView(view, params);

这几句代码的含义就是首先根据标签的名字创建View（createViewFromTag），然后将其添加到parent中（viewGroup.addView(view, params);），通过递归遍历子标签（rInflateChildren），并重复这一过程。注意rInflateChildren内部也是调用的rInflate方法。

最终，当所有的布局标签inflate完毕，可以看到会回调onFinishInflate（）方法。