1. LayoutInflater是做什么的
Instantiates a layout XML file into its corresponding {@link android.view.View}objects.
实例化一个布局XML文件转换为相应的{ @link android.view。视图对象};
在Android开发中LayoutInflater经常要用到，Fragment，adapter中的视图都是需要LayoutInflater将XML文件转换为View对象
2. 获取LayoutInflater对象
获取LayoutInflater对象有三种方法
（1）getLayoutInflater()
调用Activity的getLayoutInflater()
（2）LayoutInflater.from(context)
调用LayoutInflater的静态方法从context中获取LayoutInflater对象
（3）(LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)
调用系统服务获取LayoutInflater对象
3.将XML文件转换为View对象
最常用的做法
View view = layoutinflater.inflate(R.layout.XXX,null);
到此为止，上述的方法基本能实现布局从xml转换为想要的View对象
但是前几天在做RecyclerView的时候发现一个问题，如下
4.RecyclerView中item设置宽高无效
item_recyclerview.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="200dp"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:layout_marginTop="5dp"
android:layout_marginBottom="5dp"
android:padding="20dp"
android:background="#44ff0000">
<TextView
android:id="@+id/tv_item"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:textColor="#ffffff"
android:textSize="26sp"/>
</RelativeLayout>
recyclerview的adapter中布局转换
@Override
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType)
{
View view = lif.inflate(R.layout.item_recyclerview,null);
MyViewHolder myViewHolder = new MyViewHolder(view);
return myViewHolder;
}
简单的item布局，实现的效果如下
image.png
发现明明设置了宽度是match_parent，显示出来的效果确实有点像wrap_content
然后调整item的高度，发现也是不起作用的
在检查代码之后找不到出现问题的原因，百度了一波找到原因
※在布局转换的时候使用
View view = layoutinflater.inflate(R.layout.item_XXX,ViewGroup,false);
修改之后，效果如下
符合预期效果。
※问题：
（1）为什么使用三个参数的inflate方法就能实现效果？
（2）inflate三个参数和两个参数有什么区别？
5.LayoutInflater.inflate方法分析
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)
查看源码发现不管是三个参数还是两个参数的inflate方法
最终是实现了
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)
attachToRoot的值是根据 root != null来赋值
/**
* Inflate a new view hierarchy from the specified XML node. Throws
* {@link InflateException} if there is an error.
* <p>
* <em><strong>Important</strong></em> 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
* <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(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 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
final AttributeSet attrs = Xml.asAttributeSet(parser);
....
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);
}
}
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会失效
6.写后感
源码封装的太好了，代码跳来跳去，对于不经常看源码的我来说很容易看不下去。不过还是硬着头皮慢慢看下去，不要往深层去看，有些可以选择性忽略，只要看懂注释大概懂得，等知识累积的更多了相信再去重新解读源码会有新收获。
