在上篇文章中我们学习了setContentView的源码，还记得其中的LayoutInflater吗？本篇文章就来学习下LayoutInflater。

@Override

public void setContentView(int resId) {

ensureSubDecor();

ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);

contentParent.removeAllViews();

LayoutInflater.from(mContext).inflate(resId, contentParent);

mOriginalWindowCallback.onContentChanged();

}

备注：本文基于 Android 8.1.0。

Instantiates a layout XML file into its corresponding View objects. It is never used directly. Instead, use Activity.getLayoutInflater() or Context.getSystemService(Class) to retrieve a standard LayoutInflater instance that is already hooked up to the current context and correctly configured for the device you are running on.

翻译过来就是：LayoutInflater 的作用就是将XML布局文件实例化为相应的 View 对象，需要通过Activity.getLayoutInflater() 或 Context.getSystemService(Class) 来获取与当前Context已经关联且正确配置的标准LayoutInflater。

总共有三种方法来获取 LayoutInflater：

Activity.getLayoutInflater()；

Context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) ;

LayoutInflater.from(context);

事实上，这三种方法之间是有关联的：

Activity.getLayoutInflater() 最终会调用到 PhoneWindow 的构造方法，实际上最终调用的就是方法三；

而方法三最终会调用到方法二 Context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) ；

2、inflate 方法解析

image

LayoutInflater 的 inflate 方法总共有四个，属于重载的关系，最终都会调用到 inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) 方法。

备注：以下源码中有七条备注。

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;

// ① 寻找布局的根节点，判断布局的合理性

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 (TAG_MERGE.equals(name)) {

// ② 如果是Merge标签，则必须依附于一个RootView，否则抛出异常

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

throw new InflateException(" 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

// ③ 根据节点名来创建View对象

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

// ④ 如果设置的Root不为null，则根据当前标签的参数生成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)

// ⑤ 如果不是attachToRoot ，则对这个Tag和创建出来的View设置LayoutParams；注意：此处的params只有当被添加到一个Viewz中的时候才会生效；

temp.setLayoutParams(params);

}

}

if (DEBUG) {

System.out.println("-----> start inflating children");

}

// Inflate all children under temp against its context.

// ⑥ inflate children tag

rInflateChildren(parser, temp, attrs, true);

// We are supposed to attach all the views we found (int temp)

// to root. Do that now.

if (root != null && attachToRoot) {

// ⑦ 如果Root不为null且是attachToRoot，则添加创建出来的View到Root 中

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) {

......

}

return result;

}

}

备注：根据以上源码，我们也可以分析出来 inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) 不同参数值带来的影响：

如果root为null，attachToRoot将失去作用，设置任何值都没有意义；

如果root不为null，attachToRoot设为true，则会给加载的布局文件的指定一个父布局，即root；

如果root不为null，attachToRoot设为false，则会将布局文件最外层的所有layout属性进行设置，当该view被添加到父view当中时，这些layout属性会自动生效；

在不设置attachToRoot参数的情况下，如果root不为null，attachToRoot参数默认为true；

3、rInflate 方法解析

以上代码中我们还有两个方法没有分析：rInflate 和 rInflateChildren ；而 rInflateChildren 实际上是调用了rInflate；

备注：以下源码中有六条备注。

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

AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {

final int depth = parser.getDepth();

int type;

boolean pendingRequestFocus = false;

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)) {

pendingRequestFocus = true;

consumeChildElements(parser);

} else if (TAG_TAG.equals(name)) {

parseViewTag(parser, parent, attrs);

} else if (TAG_INCLUDE.equals(name)) {

if (parser.getDepth() == 0) {

// ① 如果这里出现了include标签，就会抛出异常

throw new InflateException(" cannot be the root element");

}

parseInclude(parser, context, parent, attrs);

} else if (TAG_MERGE.equals(name)) {

// ② 同理如果这里出现了merge标签，也会抛出异常

throw new InflateException(" must be the root element");

} else {

// ③ 最重要的方法在这里，createViewFromTag

final View view = createViewFromTag(parent, name, context, attrs);

final ViewGroup viewGroup = (ViewGroup) parent;

final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);

// ④如果当前View是ViewGroup(包裹了别的View)则在此处inflate其所有的子View

rInflateChildren(parser, view, attrs, true);

// ⑤添加inflate出来的view到parent中

viewGroup.addView(view, params);

}

}

if (pendingRequestFocus) {

parent.restoreDefaultFocus();

}

if (finishInflate) {

// ⑥如果inflate结束，则回调parent的onFinishInflate方法

parent.onFinishInflate();

}

}

总结：

首先进行View的合理性校验，include、merge等标签；

通过 createViewFromTag 创建出 View 对象；

如果是 ViewGroup，则重复以上步骤；

add View 到相应的 parent 中；

4、createViewFromTag 方法解析

备注：以下源码中有六条备注。

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

boolean ignoreThemeAttr) {

if (name.equals("view")) {

name = attrs.getAttributeValue(null, "class");

}

// Apply a theme wrapper, if allowed and one is specified.

if (!ignoreThemeAttr) {

final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);

final int themeResId = ta.getResourceId(0, 0);

if (themeResId != 0) {

context = new ContextThemeWrapper(context, themeResId);

}

ta.recycle();

}

if (name.equals(TAG_1995)) {

// Let's party like it's 1995!

return new BlinkLayout(context, attrs);

}

try {

View view;

if (mFactory2 != null) {

// ① 有mFactory2，则调用mFactory2的onCreateView方法

view = mFactory2.onCreateView(parent, name, context, attrs);

} else if (mFactory != null) {

// ② 有mFactory，则调用mFactory的onCreateView方法

view = mFactory.onCreateView(name, context, attrs);

} else {

view = null;

}

if (view == null && mPrivateFactory != null) {

// ③ 有mPrivateFactory，则调用mPrivateFactory的onCreateView方法

view = mPrivateFactory.onCreateView(parent, name, context, attrs);

}

if (view == null) {

// ④ 走到这步说明三个Factory都没有，则开始自己创建View

final Object lastContext = mConstructorArgs[0];

mConstructorArgs[0] = context;

try {

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

// ⑤ 如果View的name中不包含 '.' 则说明是系统控件，会在接下来的调用链在name前面加上 'android.view.'

view = onCreateView(parent, name, attrs);

} else {

// ⑥ 如果name中包含 '.' 则直接调用createView方法，onCreateView 后续也是调用了createView

view = createView(name, null, attrs);

}

} finally {

mConstructorArgs[0] = lastContext;

}

}

return view;

} catch (InflateException e) {

throw e;

}

}

总结：

createViewFromTag 方法比较简单，首先尝试通过 Factory 来创建View；

如果没有 Factory 的话则通过 createView 来创建View；

5、createView 方法解析

备注：以下源码中有三条备注。

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

throws ClassNotFoundException, InflateException {

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

if (constructor != null && !verifyClassLoader(constructor)) {

constructor = null;

sConstructorMap.remove(name);

}

Class extends View> clazz = null;

try {

Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);

if (constructor == null) {

// Class not found in the cache, see if it's real, and try to add it

clazz = mContext.getClassLoader().loadClass(

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

if (mFilter != null && clazz != null) {

boolean allowed = mFilter.onLoadClass(clazz);

if (!allowed) {

failNotAllowed(name, prefix, attrs);

}

}

// ① 反射获取这个View的构造器

constructor = clazz.getConstructor(mConstructorSignature);

constructor.setAccessible(true);

// ② 缓存构造器

sConstructorMap.put(name, constructor);

} else {

// If we have a filter, apply it to cached constructor

if (mFilter != null) {

// Have we seen this name before?

Boolean allowedState = mFilterMap.get(name);

if (allowedState == null) {

// New class -- remember whether it is allowed

clazz = mContext.getClassLoader().loadClass(

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

boolean allowed = clazz != null && mFilter.onLoadClass(clazz);

mFilterMap.put(name, allowed);

if (!allowed) {

failNotAllowed(name, prefix, attrs);

}

} else if (allowedState.equals(Boolean.FALSE)) {

failNotAllowed(name, prefix, attrs);

}

}

}

Object lastContext = mConstructorArgs[0];

if (mConstructorArgs[0] == null) {

// Fill in the context if not already within inflation.

mConstructorArgs[0] = mContext;

}

Object[] args = mConstructorArgs;

args[1] = attrs;

// ③ 使用反射创建 View 对象，这样一个 View 就被创建出来了

final View view = constructor.newInstance(args);

if (view instanceof ViewStub) {

// Use the same context when inflating ViewStub later.

final ViewStub viewStub = (ViewStub) view;

viewStub.setLayoutInflater(cloneInContext((Context) args[0]));

}

mConstructorArgs[0] = lastContext;

return view;

} catch (ClassCastException e) {

}

}

总结：

createView 方法也比较简单，通过反射来创建的 View 对象；

6、总结

通过本文我们学习到 LayoutInflater 创建 View的过程，也知道了 inflate 方法不同参数的意义，以及开发中遇到的一些异常在源码中的根源。可以看到从布局中 inflate 一个个具体的 View 的过程其实也很简单：

通过 XML 的 Pull 解析方式获取 View 的标签；

通过标签以反射的方式来创建 View 对象；

如果是 ViewGroup 的话则会对子 View 遍历并重复以上步骤，然后 add 到父 View 中；

与之相关的几个方法：inflate ——》 rInflate ——》 createViewFromTag ——》 createView ；

参考

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