序言：最近在做一个类似商城的app（电商类），然后我就用了阿里新开源的VLayout布局来做，简直不要太方便啊，有空的话我会写一篇关于VLayout的文章，它的底层也是RecyclerView，只不过对LayoutManager和Adapter进行了封装而已，但是我们用的时候也需要对它进行再一次封装，因为像addHeader和addFooter这些都是没有的，在这个过程中我偶然发现用LayoutInflater实例的header和footer宽高竟然没用，这让我很捉急（因为项目很赶），然后百度才发现LayoutInflater通过XML加载布局的时候会产生不同的现象。
1、使用方法
(1)、获取LayoutInflater实例有两种方法：
LayoutInflater layoutInflater = LayoutInflater.from(context);
LayoutInflater layoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
(2)、inflate方法：
layoutInflater.inflate(int layoutId, ViewGroup root);
/**
* layoutId 布局文件
* root 在该布局的最外层嵌套一层父布局，如果不需要就传null
* attachToRoot true表示将layout的view添加到root中
*/
layoutInflater.inflate(int layoutId, ViewGroup root, attachToRoot);
layoutInflater.inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)
注意：这里重点解释一下第三个参数attachToRoot，有以下几种情况：
(1)、如果root为null，无论attachToRoot为true或者false，效果都是一样的
(2)、如果root不为null，attachToRoot为true，表示将layout布局添加到root布局中
(3)、如果root不为null，attachToRoot为false，表示不将layout布局添加到root布局，若要添加则需要手动addView
(4)、如果root不为null，不设置attachToRoot(即调用两个参数的方法)，情况和(2)中一样
2、源码解析
前两种方法是最常用的加载布局的方法，最终都会调用最后一种方法来加载布局：
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
...
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 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);
}
}
...
// 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);
}
...
}
...
}
LayoutInflater是通过Pull解析（XmlPullParser解析器）方式来解析XML布局文件，解析出节点名之后，然后会调用rInflate方法，这个方法里面会遍历这个根布局下的子元素。然后会调用createViewFromTag方法，这个方法是干嘛的呢？顾名思义创建View的呗，创建View 之后会判断root是否为null，若不为null会为其生成一个params，然后设置给该View，如果root为null，则不会生成params，所以就没有宽高这些数据了。最后若root不为null会调用addView方法，将该生成的View添加到root布局中。
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("<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 (pendingRequestFocus) {
parent.restoreDefaultFocus();
}
if (finishInflate) {
parent.onFinishInflate();
}
}
细心的朋友可能会发现，上面两段代码都调用了同一个方法createViewFromTag，每次都会递归调用这个方法来创建这个View下的子元素并且添加到根布局中。那我们来看一下这个方法里面都干了些什么：
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs, boolean ignoreThemeAttr) {
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
// Apply a theme wrapper, if allowed and one is specified.
if (!ignoreThemeAttr) {
final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
final int themeResId = ta.getResourceId(0, 0);
if (themeResId != 0) {
context = new ContextThemeWrapper(context, themeResId);
}
ta.recycle();
}
if (name.equals(TAG_1995)) {
// Let's party like it's 1995!
return new BlinkLayout(context, attrs);
}
try {
View view;
if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) {
view = mFactory.onCreateView(name, context, attrs);
} else {
view = null;
}
if (view == null && mPrivateFactory != null) {
view = mPrivateFactory.onCreateView(parent, name, context, attrs);
}
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (InflateException e) {
throw e;
} catch (ClassNotFoundException e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
}
}
代码不是很多，这个方法把我们传进去的xml的节点名来生成一个View对象，如何生成的呢，调用了一个createView方法（onCreateView方法中也会调用createView方法）：
public final View createView(String name, String prefix, AttributeSet attrs) throws ClassNotFoundException, InflateException {
Constructor<? extends View> constructor = sConstructorMap.get(name);
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
...
final View view = constructor.newInstance(args);
...
}
看到这里大家都知道了吧，通过反射的方法来创建出最终的View。
好了，相信看到这里大家应该都能明白为什么有时候LayoutInflater出来的View宽高会失效了吧！
3、总结
LayoutInflater有两个加载布局的方法，分别是两个参数和三个参数的，这里有以下几种情况：
