Android之 inflate() 方法总结

文章目录

一、引言
二、源码分析

2.1 源码跟踪
2.2 结论

一、引言

inflater.inflate(R.layout.layout_inflate_test,null);inflater.inflate(R.layout.layout_inflate_test, root,false);inflater.inflate(R.layout.layout_inflate_test, root,true);

做Android这么久，经常会看到上面三个方法，只知道这是通过布局资源id解析xml文件并返回View用的，但具体什么时候该用哪种参数的方法，还是懵懵懂懂。

以下从源码角度分析一下inflate()方法。

二、源码分析

2.1 源码跟踪

先看一下inflate()涉及的相关源码（以Android API 29为例）：

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {  return inflate(resource, root, root != null);}public View inflate(XmlPullParser parser, @Nullable ViewGroup root) {  return inflate(parser, root, root != null);}public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {  final Resources res = getContext().getResources();//...  View view = tryInflatePrecompiled(resource, res, root, attachToRoot);  if (view != null) {    return view;  }  XmlResourceParser parser = res.getLayout(resource);  try {    return inflate(parser, root, attachToRoot);  } finally {    parser.close();  }}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 {      advanceToRootNode(parser);      final String name = parser.getName();//...      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) {//...          // 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);          }        }//...                // Inflate all children under temp against its context.        rInflateChildren(parser, temp, attrs, true);//...        // 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(        getParserStateDescription(inflaterContext, attrs)        + ": " + 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;  }}

以上虽然有四个重载方法，但仔细观察，发现万剑归宗，最后的调用都指向一个方法：inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot),我们只需要跟进这个方法便可。
Android之 inflate() 方法总结_第1张图片

从上图可以看出以上四个方法总体上可以分为两类：第一类的第一个参数为布局资源id；第二类第一个参数为xml文件解析器XmlPullParser。
第一类调用第二类的关键代码如下：
XmlResourceParser parser = res.getLayout(resource);try {    return inflate(parser, root, attachToRoot);} finally {    parser.close();}

跟进inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)方法：

final Context inflaterContext = mContext;final AttributeSet attrs = Xml.asAttributeSet(parser);Context lastContext = (Context) mConstructorArgs[0];mConstructorArgs[0] = inflaterContext;View result = root;

这段代码主要通过XmlPuulPaser获取xml的属性集，以及初始化一些数据，把root参数（可能为null）赋值给返回结果。继续分析：

advanceToRootNode(parser); //       ↓（实现）private void advanceToRootNode(XmlPullParser parser)  throws InflateException, IOException, XmlPullParserException {  // 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!");  }}

这里的代码含义是遍历寻找根节点，继续分析：

final String name = parser.getName();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);}

当根节点的标签是merge时，如果root为null或者attachToRoot为false会直接抛异常，也就是当根标签为merge的时候必须使用inflater.inflate(R.layout.layout_inflate_test, root,true);这种形式，不然会报错，你可以自己试验一下。实验结果：

然后看rInflate(parser, root, inflaterContext, attrs, false);方法的实现：

void rInflate(XmlPullParser parser, View parent, Context context,              AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {  final int depth = parser.getDepth();  int type;  boolean pendingRequestFocus = false;  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)) {      pendingRequestFocus = true;      consumeChildElements(parser);    } else if (TAG_TAG.equals(name)) {      parseViewTag(parser, parent, attrs);    } else if (TAG_INCLUDE.equals(name)) {      if (parser.getDepth() == 0) {        throw new InflateException(" cannot be the root element");      }      parseInclude(parser, context, parent, attrs);    } else if (TAG_MERGE.equals(name)) {      throw new InflateException(" 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 (pendingRequestFocus) {    parent.restoreDefaultFocus();  }  if (finishInflate) {    //个人理解：这里的应该是控制递归调用结束的一个非常关键的因素（即非`ViewGroup`类型的view不会再被递归）    parent.onFinishInflate();  }}

该方法递归遍历xml文件，根据标签Tag的名称通过createViewFromTag(parent, name, context, attrs);方法并利用反射创建子View，并添加到ViewGroup parent上。然后继续分析：

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) {//...    // 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);    }  }  //...  // Inflate all children under temp against its context.  rInflateChildren(parser, temp, attrs, true);  //...  // 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;  }}

之前的merge是特殊情况，常见的是else中的情况，首先会通过createViewFromTag(root, name, inflaterContext, attrs);方法生成一个叫temp的root view，下面的代码是关键：

if (root != null) {  //...  // 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);  }}

如果root != null的话，根据root创建相应的LayoutParams params；如果attachToRoot 为 false的话，就把这个布局参数设置给temp，再继续分析：

// Inflate all children under temp against its context.rInflateChildren(parser, temp, attrs, true);//...// 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;}

首先是递归解析子布局并生成相应的View，如果root != null && attachToRoot的话，就把temp以root的布局参数params添加到root布局中；如果root == null 或者 attachToRoot为false，则忽略root的布局参数信息，直接将temp赋值给result并最终返回该结果。

2.2 结论

引用郭神的结论：

resourceId	Root	attachToRoot	效果
存在	null	设置无意义	`attachToRoot`失去意义
存在	!=null	true	会给加载的布局文件指定一个父布局`root`
存在	!=null	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
存在	!=null	未设置(true)	会给加载的布局文件指定一个父布局`root`

补充说明：

如果root不为null，布局文件最外层的layout关于LayoutParams设置的属性和其他属性都会被保留下来，attachToRoot设为true，则会给加载的布局文件的指定一个父布局，我们不需要自己再addView，否则会报错;

attachToRoot设为false，需要我们自己addView，root为null时，被加载的布局LayoutParams的属性会被改变，但是其它属性例如背景颜色什么的会被保留。

本文参考：Android inflate方法总结

文章目录

一、引言

二、源码分析

2.1 源码跟踪

2.2 结论

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