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。
1、LayoutInflater 简介
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("<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
// 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("<include /> 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("<merge /> 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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