原理

通过layoutId,装换成xml对象，XmlResourceParser，将XmlResourceParser转成AttributeSet对象，根据XmlResourceParser的name得到view对象的类型，最后根据Constructor实例化view对象

几个概念

XmlResourceParser

• 将xml转换成java对象，就是XmlResourceParser
• Rescources class 中，根据resourceid，获得XmlResourceParser对象

@NonNullXmlResourceParser loadXmlResourceParser(@AnyRes int id, @NonNull String type)throws NotFoundException {final TypedValue value = obtainTempTypedValue();try {final ResourcesImpl impl = mResourcesImpl;impl.getValue(id, value, true);if (value.type == TypedValue.TYPE_STRING) {return impl.loadXmlResourceParser(value.string.toString(), id,value.assetCookie, type);}throw new NotFoundException("Resource ID #0x" + Integer.toHexString(id)+ " type #0x" + Integer.toHexString(value.type) + " is not valid");} finally {releaseTempTypedValue(value);}}

AttributeSet

• xml中标签对应的java对象，如layout_width等
• 利用Xml class中的方法，生成AttributeSet对象

public static AttributeSet asAttributeSet(XmlPullParser parser) {return (parser instanceof AttributeSet)? (AttributeSet) parser: new XmlPullAttributes(parser);}

Constructor

• 属于反射的一种方式，通过constructor可以获取某个具体的类。 提供有关类的单个构造函数的访问和访问的信息。
• 在xml解析成XmlPullParser时，将XmlPullParser解析成AttributeSet，获取AttributeSet的name，Constructor根据有包名的name，实例化一个view。

final View view = constructor.newInstance(args);

源码

layoutInflater

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);//画重点try {return inflate(parser, root, attachToRoot);} finally {parser.close();}}

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;//重点，通过设置Factory，可以达到将系统view替换成自定义view的目的if (mFactory2 != null) {view = mFactory2.onCreateView(parent, name, context, attrs);} else if (mFactory != null) {view = mFactory.onCreateView(name, context, attrs);} else {view = null;}if (view == null && mPrivateFactory != null) {view = mPrivateFactory.onCreateView(parent, name, context, attrs);}if (view == null) {final Object lastContext = mConstructorArgs[0];mConstructorArgs[0] = context;try {//重点，如果是系统控件，则会在name前面加上"android.view."if (-1 == name.indexOf('.')) {view = onCreateView(parent, name, attrs);} else {view = createView(name, null, attrs);}} finally {mConstructorArgs[0] = lastContext;}}return view;}...}

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 itclazz = 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);}}constructor = clazz.getConstructor(mConstructorSignature);constructor.setAccessible(true);sConstructorMap.put(name, constructor);} else {// If we have a filter, apply it to cached constructorif (mFilter != null) {// Have we seen this name before?Boolean allowedState = mFilterMap.get(name);if (allowedState == null) {// New class -- remember whether it is allowedclazz = mContext.getClassLoader().loadClass(prefix != null ? (prefix + name) : name).asSubclass(View.class);//重点，如果是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[] args = mConstructorArgs;args[1] = attrs;final View view = constructor.newInstance(args);//重点if (view instanceof ViewStub) {// Use the same context when inflating ViewStub later.final ViewStub viewStub = (ViewStub) view;viewStub.setLayoutInflater(cloneInContext((Context) args[0]));}return view;} ...}

Resources

public XmlResourceParser getLayout(@LayoutRes int id) throws NotFoundException {return loadXmlResourceParser(id, "layout");}

@NonNullXmlResourceParser loadXmlResourceParser(@AnyRes int id, @NonNull String type)throws NotFoundException {final TypedValue value = obtainTempTypedValue();try {final ResourcesImpl impl = mResourcesImpl;impl.getValue(id, value, true);if (value.type == TypedValue.TYPE_STRING) {return impl.loadXmlResourceParser(value.string.toString(), id,value.assetCookie, type);//重点}throw new NotFoundException("Resource ID #0x" + Integer.toHexString(id)+ " type #0x" + Integer.toHexString(value.type) + " is not valid");} finally {releaseTempTypedValue(value);}}