Android LayoutInflater源码解析:你真的能正确使用吗?

版权声明:本文出自汪磊的博客,未经作者允许禁止转载。

好久没写博客了,最近忙着换工作,没时间写,工作刚定下来。稍后有时间会写一下换工作经历.接下来进入本篇主题,本来没想写LayoutInflater的,不过做项目的时候随手用了一下,运行发现了一些问题,稍微看了下源码,决定写篇博客当作记录一下吧。

一、LayoutInflater实例化方法(简单提一下)

activity中我们习惯直接调用getLayoutInflater()方法:

     @NonNull
public LayoutInflater getLayoutInflater() {
return getWindow().getLayoutInflater();
}

实际调用的是PhoneWindow里面的getLayoutInflater()方法:

      private LayoutInflater mLayoutInflater;

     @Override
public LayoutInflater getLayoutInflater() { return mLayoutInflater;
} public PhoneWindow(Context context) {
super(context);
mLayoutInflater = LayoutInflater.from(context);
}

看到了吧,实际调用的是LayoutInflater.from(context)方法进行的实例,LayoutInflater的from源码如下:

     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;
}

很简单,最本质就是调用context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)获取系统启动的时候就为我们创建好的服务。

至于实际中使用哪一种创建根据实际情况选择就好了,没什么多说的。

二、LayoutInflater的inflate方法源码解析(这才是重点)

inflate重载方法有四个,先看其中三个:

     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();
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();
}
}

最终调用的都是inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot),我们着重分析这个方法就可以了。

源码:

 /**
* 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 (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;
}
}

先不要着急上来就读源码逻辑,先看看人家写的注释,注释那么长不看浪费了,其实理解这个方法后两个参数root,attachToRoot这个方法也就理解的差不多了。接下来,是时候展示我英语6级的实力了:

 @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.)
翻译:root是一个可选的view,可以传入null,如果我们将attachToRoot设置为true,那么root即为生成视图的父类,如果attachToRoot
设置为false,root仅仅是一个对象,为返回的视图提供一系列LayoutParams参数。 @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.
翻译:大概意思就是attachToRoot可以决定生成的视图是否挂载到root这个参数上,如果设置为false,root这个参数仅仅用来为XML布局
生成正确的布局参数。 @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.
翻译:inflate方法返回值是生成视图的根布局,如果root参数被提供(传入的不是null)并且attachToRoot设置为true,则返回值就是
root参数,否则返回值就是XML布局的跟节点视图。 好了,以上就是大致的翻译,接下来分析实际源码;
28行,生成的attrs是XML布局根节点的布局参数集合。
31行,result默认就是我们传递进来的root参数,inflate方法最后返回的就是result。
64行,生成xml布局的根节点temp,如下布局:
 <?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:background="@android:color/holo_green_light"
android:layout_height="wrap_content" > <TextView
android:id="@+id/mytext"
android:layout_width="match_parent"
android:layout_height="60dp"
android:textColor="@android:color/holo_blue_dark"
android:textSize="30sp"
android:gravity="center"
android:text="ali" />
</FrameLayout>

我们调用inflate方法将这个布局转化为view的时候,那么temp即为FrameLayout,布局的根节点。

68-80行:如果我们传递进来的root不为null,则执行74行逻辑,根据attrs生成xml根布局节点的布局参数params,75行如果我们传递进来的

attachToRoot为false,表示xml布局生成的view不挂载到root上则执行78行逻辑,为temp即xml布局根节点设置布局参数params。

95-97行:如果我们传递的root不为null,并且参数attachToRoot为true,表示将xml生成的view挂载到root上则执行root.addView(temp, params)逻辑。

101-103行:如果我们传递进来的root为null,或者attachToRoot为false(表示我们自己没有想为xml布局设定父布局或者xml布局生成的view

不想挂载到root上)则result设置为temp,也就是xml布局的根节点。

123行:最后返回result。

三、个人总结

通过上面对参数的翻译以及源码的分析,相信你已经对inflate方法有一定了解,root就是我们想为xml布局生成的view指定一个父类,让其挂载上,root如果不为null,那么就会为xml布局的根节点生成布局参数。至于你到底想不想成为root的孩子,由attachToRoot决定,false表示不想,true表示想。如果你不想那么root还是会尽心为这辈子不能成为其孩子的view生成布局参数并且设置给view,如果想那么调用addView方法加载进来。

另外要说一点在任何我们不负责将View添加进ViewGroup的情况下都应该将attachToRoot设置为false,比如ListView,RecyclerView。

好了本片到此为止,希望读完本片你能有更深体会,而不是使用出现问题自己瞎搞,参数变来变去,蒙对了还挺高兴。

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