我们知道，在Activity#setContentView()中会调用PhoneWindow#setContentView()。而在PhoneWindow#setContentView()中有这么一句mLayoutInflater.inflate(layoutResID, mContentParent)。这行代码的作用是将我们的activity_main.xml填充到mContentParent中去。详见：setContentView源码解析。在写adapter的时候，也经常写mInflater.inflate(layoutResID, null)。那么，这行代码怎么就将xml文件转换成了View或者ViewGroup了呢？

获取LayoutInflater对象无非以下两种方式：

LayoutInflater.from(Context context);

LayoutInflater LayoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

其实这俩是同一种方式，首先看下LayoutInflater#from()

源码位置：frameworks/base/core/java/android/view/LayoutInflater.java

LayoutInflater#from()

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;

}

第一种获取LayoutInflater对象的方式，不过就是对第二种方式的一个简单封装。实际上还是一回事。Context的实现类是ContextImpl，跟进。

源码位置：frameworks/base/core/java/android/app/ContextImpl.java

ContextImpl#getSystemService()

@Override

public Object getSystemService(String name) {

return SystemServiceRegistry.getSystemService(this, name);

}

跟进。

源码位置：frameworks/base/core/java/android/app/SystemServiceRegistry.java

SystemServiceRegistry#getSystemService()

public static Object getSystemService(ContextImpl ctx, String name) {

ServiceFetcher> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);

return fetcher != null ? fetcher.getService(ctx) : null;

}

直接从全局变量SYSTEM_SERVICE_FETCHERS中依据名字就get到了fetcher，之后依据fetcher直接get到了LayoutInflater对象。大写的懵B~原来啊，在SystemServiceRegistry中有个静态代码块，先看下这部分。

static {

...

registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,

new CachedServiceFetcher() {

@Override

public LayoutInflater createService(ContextImpl ctx) {

return new PhoneLayoutInflater(ctx.getOuterContext());

}});

...

}

private static void registerService(String serviceName, Class serviceClass,

ServiceFetcher serviceFetcher) {

SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);

SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);

}

static abstract class CachedServiceFetcher implements ServiceFetcher {

private final int mCacheIndex;

public CachedServiceFetcher() {

mCacheIndex = sServiceCacheSize++;

}

@Override

@SuppressWarnings("unchecked")

public final T getService(ContextImpl ctx) {

final Object[] cache = ctx.mServiceCache;

synchronized (cache) {

// Fetch or create the service.

Object service = cache[mCacheIndex];

if (service == null) {

service = createService(ctx);

cache[mCacheIndex] = service;

}

return (T)service;

}

}

public abstract T createService(ContextImpl ctx);

}

这里连续贴了两个方法和一个抽象内部类CachedServiceFetcher。由于在抽象方法CachedServiceFetcher#createService()的具体实现中返回的是PhoneLayoutInflater，所以后文中使用的一直是PhoneLayoutInflater的对象。获取LayoutInflater对象(其实是其子类PhoneLayoutInflater对象)之后，调用LayoutInflater#inflate()。跟进。

源码位置：frameworks/base/core/java/android/view/LayoutInflater.java

LayoutInflater#inflate()

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {

return inflate(resource, root, root != null);

}

这里以setContentView中的mLayoutInflater.inflate(layoutResID, mContentParent)为例，顺带也会讲解adapter中mInflater.inflate(layoutResID,null)这种情况。也就是root参数为null和不为null两种情况。root==null，则第三个参数为false.root!=null，则第三个参数为true。跟进。

LayoutInflater#inflate()

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {

final Resources res = getContext().getResources();

final XmlResourceParser parser = res.getLayout(resource);

try {

return inflate(parser, root, attachToRoot);

} finally {

parser.close();

}

}

跟进。

LayoutInflater#inflate()

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {

synchronized (mConstructorArgs) {

...

View result = root;

try {

...

// 获取根节点的字符串，例如LinearLayout

final String name = parser.getName();

// 根节点merge开头

if (TAG_MERGE.equals(name)) {

...

} else {

// 创建根视图View

final View temp = createViewFromTag(root, name, inflaterContext, attrs);

ViewGroup.LayoutParams params = null;

if (root != null) {

// 获取LayoutParams

params = root.generateLayoutParams(attrs);

if (!attachToRoot) {

// 应用LayoutParams到根节点View

temp.setLayoutParams(params);

}

}

// 遍历解析子View，并添加到根节点temp中

rInflateChildren(parser, temp, attrs, true);

// root不为空，直接将根节点View添加到root中

if (root != null && attachToRoot) {

root.addView(temp, params);

}

// root等于null,直接返回根节点temp

if (root == null || !attachToRoot) {

result = temp;

}

}

}catch (Exception e) {

...

}

return result;

}

}

上面每一步都有注释，下面重点看下生成根节点View的createViewFromTag()和遍历生成子View的rInflateChildren()方法。

LayoutInflater#createViewFromTag()

private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {

return createViewFromTag(parent, name, context, attrs, false);

}

View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,

boolean ignoreThemeAttr) {

...

if (-1 == name.indexOf('.')) {

view = onCreateView(parent, name, attrs);

} else {

view = createView(name, null, attrs);

}

...

return view;

}

跟进。

LayoutInflater#createView()

public final View createView(String name, String prefix, AttributeSet attrs)

throws ClassNotFoundException, InflateException {

Constructor extends View> constructor = sConstructorMap.get(name);

Class extends View> clazz = null;

try {

if (constructor == null) {

clazz = mContext.getClassLoader().loadClass(

prefix != null ? (prefix + name) : name).asSubclass(View.class);

...

constructor = clazz.getConstructor(mConstructorSignature);

constructor.setAccessible(true);

sConstructorMap.put(name, constructor);

} else {

...

}

final View view = constructor.newInstance(args);

return view;

} catch (Exception e) {

...

}

}

sConstructorMap是个HashMap>对象。首先依据根节点的名字，例如LinearLayout去查找缓存的构造器，如果是第一次执行，肯定返回null。如果返回为null，则通过反射出构造方法，并强制设置可访问，之后存进sConstructorMap中。如果缓存中有构造器，那么直接取出。最后调用newInstance反射出根节点View实例。得到根节点View实例之后，接着设置属性，最后调用rInflateChildren()遍历创建子View。跟进。

LayoutInflater#rInflateChildren()

final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,

boolean finishInflate) throws XmlPullParserException, IOException {

rInflate(parser, parent, parent.getContext(), attrs, finishInflate);

}

parent参数是根节点View。这里只是简单转发给rInflate()方法处理。跟进。

LayoutInflater#rInflateChildren()

void rInflate(XmlPullParser parser, View parent, Context context,

AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {

final int depth = parser.getDepth();

int type;

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)) {

...

}

...

} 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 (finishInflate) {

parent.onFinishInflate();

}

}

遍历体现在While循环上，name为子节点View的名称，例如：TextView，RelativeLayout等。几个以tag、include等开头的子节点走最上面几个if的逻辑，我们的重点在于寻常View走的else逻辑。可以看到：首先，和创建根节点View调用同一个方法createViewFromTag()创建子View，紧接着设置子View的参数，然后调用递归调用rInflateChildren()方法再去测量子节点的所有View，最后才将子节点添加到父布局，这个父布局可能是根节点，也可能是某个子节点。遍历结束之后，所有子View也添加到布局当中并设置好相应的布局参数。

至此，LayoutInflater.from().inflate()源码解析结束~

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