LayoutInflater源码分析

为什么要看这个类的源码呢？是因为在使用RecyclerView时，对Item进行getLayoutParams操作时，报了空指针异常。通过对源码的分析，才知道问题是出在了是否正确的使用了LayoutInflater。这是一个比较冷门的问题，也是一个比较容易被忽略的问题。但是我们每次将布局转化成View时，都会使用到这个类，所以想要好好分析一下，顺便记录下来，加深自己的理解和记忆，同时也希望可以帮助到其他的人。

首先我们来看一下出问题的代码：

看源码，首先从最常用的方法开始看，其实也可以说，就看常用的方法就够了。所以我们要从inflate()的源码开始看起。
// TODO 补充：源代码的位置
在LayoutInflater.java中，一共有4个重载的inflate(),我们一个一个慢慢的看。

// 第一个
/*** 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(@LayoutRes int resource, @Nullable ViewGroup root) {return inflate(resource, root, root != null);}// 第二个/*** 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, @Nullable ViewGroup root) {return inflate(parser, root, root != null);}

可以看到，第一个inflate(resource,root)是两个参数的inflate，它实际上是调用了三个参数的inflate(int resource, ViewGroup root, boolean attachToRoot)。第二个inflate(parser,root)的第一个参数是XmlPullParser，它调用的是三个参数的inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)。开始分析第三个inflate

//第三个/*** 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) + ")");}// 把resource转化为parser final XmlResourceParser parser = res.getLayout(resource);try {return inflate(parser, root, attachToRoot);} finally {parser.close();}}

查看源码发现，这个方法调用了三个参数的inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)。现在来重点分析这个方法：

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("<merge /> 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 xmlfinal 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 suppliedparams = 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;}}

在从上往下查看源码的过程中，我们看到了这样一句话：
params = root.generateLayoutParams(attrs);咦，我们的问题就是出在params上，params报空，因此我们往上看，看到了这一句： if (root != null) 原来问题出在了root上，查看我们出问题的代码，（写出出问题的代码）
再往下看：

 if (!attachToRoot) {// Set the layout params for temp if we are not// attaching. (If we are, we use addView, below)temp.setLayoutParams(params);}

也就是说，只有attachToRoot = false 时，才会将params设置到temp上去。

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