Android(安卓)LayoutInflater的获取及inflate()参数和返回值

Android LayoutInflater是我们在Android App开发中特别是自定义view时候经常常用的一个类。也许你开发中没有特别注意到它，但其实我们都直接或间接的使用它了。这里主要说明Android LayoutInflater的获取方式及其inflate()参数和返回值的关系。

LayoutInflater获取

LayoutInflater layoutInflater = (LayoutInflater) getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
LayoutInflater layoutInflater = LayoutInflater.from(Context context)
LayoutInflater layoutInflater = activity.getLayoutInflater();
后面2种方式最后都还是调用到getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE)。
而LayoutInflater 是一个抽象类，真正实现的是policy底下的PhoneLayoutInflater。
PhoneLayoutInflater是在SystemServiceRegistry.java注册到SystemService中的：

registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,                new CachedServiceFetcher() {            @Override            public LayoutInflater createService(ContextImpl ctx) {                return new PhoneLayoutInflater(ctx.getOuterContext());            }});

LayoutInflater.inflate()方法

View inflate(@LayoutRes int resource, @Nullable ViewGroup root)
View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)
View inflate(XmlPullParser parser, @Nullable ViewGroup root)
View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)
其中，关于XmlPullParser 在API文档中有这样一段话：

Important 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.

大意是：基于性能考虑，Android的Layout文件在编译时候做了预处理；因此，在运行时给LayoutInflater传递一个未预处理的xml的XmlPullParser是行不通地。

LayoutInflater.inflate()的返回值

 * @param root Optional view to be the parent of the generated hierarchy (if *        attachToRoot is true), or else simply an object that *        provides a set of LayoutParams values for root of the returned *        hierarchy (if attachToRoot 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.

说白了，inflate()方法的返回值取决于参数root和attachToRoot：

if(root!=null && attachToRoo) return root;
else return root of the inflated XML

LayoutInflater获取

LayoutInflater.inflate()方法

LayoutInflater.inflate()的返回值

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