从RecyclerView的子View创建来聊聊View.inflate和LayoutInflat.from(context).inflate的区别
前言
聊聊RecyclerView子View的生成为什么需要使用LayoutInflat.from(context).inflate而不是View.inflate();
最近，用RecyclerView的时候，发现一个很奇怪的现象，就是使用View.inflate()生成的View有问题。但是换成LayoutInflate.from(context)却可以，详细的可以看下这篇博客
RecyclerView的item无法充满父布局的问题
这篇文章就来看看，两个到底有什么区别。
源码分析
View.inflate源码分析
/**
* Inflate a view from an XML resource. This convenience method wraps the {@link
* LayoutInflater} class, which provides a full range of options for view inflation.
*
* @param context The Context object for your activity or application.
* @param resource The resource ID to inflate
* @param root A view group that will be the parent. Used to properly inflate the
* layout_* parameters.
* @see LayoutInflater
*/
public static View inflate(Context context, @LayoutRes int resource, ViewGroup root) {
LayoutInflater factory = LayoutInflater.from(context);
return factory.inflate(resource, root);
}
从这里可以明显看到，走的其实也是LayoutInflater，接下来我们统一分析LayoutInflate
Layout.infalte().from()源码分析
LayoutInflater.from(context).inflate(R.layout.layout, parent, false)
#LayoutInflater.form()
/**
* 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;
}
可以很明显的看到，最后调用的其实都是context.getSystemService();
Context的getSystemService实现是虚方法，具体实现是在Activity里面
//LAYOUT_INFLATER_SERVICE值为layout_inflater
@Override
public Object getSystemService(@ServiceName @NonNull String name) {
if (getBaseContext() == null) {
throw new IllegalStateException(
"System services not available to Activities before onCreate()");
}
if (WINDOW_SERVICE.equals(name)) {
return mWindowManager;
} else if (SEARCH_SERVICE.equals(name)) {
ensureSearchManager();
return mSearchManager;
}
return super.getSystemService(name);
}
从这里看出来，其实这里走的是super.getSystemService()这里我们就暂时不往下跟了，先直接给结论，免得大家看的无聊，有时候，知道结果，反问原因，更能激起人们的兴趣。
这里走的是PhoneLayoutInflater。下面代码过长，直接略过即可，直接看下面的解析，会抓住本文的重点。
/**
* 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.,
* R.layout.main_page)
* @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);
}
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) + ")");
}
final 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 {
// 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("inflate里面涉及到的东西比较多，我们抓住重点，来说明我们今天的问题。
其实大家大概也能猜到，关键问题就在于LayoutParams，看看关键代码，这里一共涉及到两个核心点:
if (TAG_MERGE.equals(name)) {
}else{
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
//root就是传进来的parentView
if (root != null) {
// Create layout params that match root, if supplied
//这里涉及到一个知识点，就是如果传入的是Relativelayout,最后却添加到LinearLayout里面。这里可行吗？
//另外，这里的generateLayoutParams()其实几乎每一个ViewGroup都会重写自己的，并且生成自己的ViewGroup.LayoutParams。
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
//这里就是我们的核心代码1，如果有父View，但是暂时不添加，那么我们就设置params。
temp.setLayoutParams(params);
}
}
// Inflate all children under temp against its context.
//这里是去解析当前xml文件里面最外层View的子View，递归操作，很绕。
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.
//这里就是我们的核心代码2，当root!=null&&允许的时候才加入。
if (root != null && attachToRoot) {
root.addView(temp, params);
}
}
关键代码我已经在父View中给出了，那么，传入parentView但是未添加时，生成的LayoutParams什么时候消费呢？
关于generateLayoutParams()何时消费
这里生成的LayoutParams，毫无疑问，会在View添加到parentView的时候使用，那么，我们就来看下ViewGroup.addView():
public void addView(View child) {
addView(child, -1);
}
public void addView(View child, int index) {
if (child == null) {
throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
}
//看到了吗，没明显，这里就是拿到了之前使用generateLayoutParams()生成的LayoutParams。
LayoutParams params = child.getLayoutParams();
if (params == null) {
params = generateDefaultLayoutParams();
if (params == null) {
throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");
}
}
addView(child, index, params);
}
总结
由此可见，我们使用
LayoutInflater.from(context).inflate(R.layout.item, parent, false);
View.inflate(mContext,R.layout.item,null);
的区别就在于，传入ParentView会生成一个LayoutParams，但是不传入的话，并不会生成。
以后有时间我们再来聊聊，PhoneLayoutInflater这个类时怎么来的，这涉及到的就比较多，以后专门开一章。