今天分析下经常使用加载布局的layoutInflater
我们在加载布局的时候都会主动或者被动的用到 LayoutInflater ，比如 Activity 的setContentView方法和Fragment的onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)回调等。LayoutInflater 的作用就是把布局文件xml实例化为相应的View组件。我们可以通过三种方法获取 LayoutInflater：
1.Activity.getLayoutInflater()；
2.LayoutInflater.from(context)；
3.context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)；
每个方法都和 Context 相关联，其中方法1和方法2最终都会通过方法3来实现。
获取到 LayoutInflater 后，通过调用inflate方法来实例化布局。而inflate方法由很多重载，我们常用的是inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)，所有 inflate 方法最终会调用到 inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)。下面就从这个方法入手，开始分析 LayoutInflater 的源码。
inflate方法
先看一下inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)的三个参数：
- XmlPullParser parser：很显然是一个 XML 解析器，这个解析器就是 LayoutInflater 所要加载的 XML 布局转化来的，通过 PULL 方式解析。
- ViewGroup root：装载要加载的 XML 布局的根容器，比如，在 Activity 的setContentView方法中就是 id 为android.R.id.content的 FrameLayout 根布局了。
- boolean attachToRoot：是否将所解析的布局添加到根容器中，同时也影响了所解析布局的宽高。
被广泛讨论的是root和attachToRoot的不同传参对被加载的布局文件的影响，下面看代码。
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final Context inflaterContext = mContext;
// 将parser转成AttributeSet接口，用来读取xml中设置的View属性
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root; // 此方法返回的View，默认是root
try {
// Look for the root node.
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
...
final String name = parser.getName(); // 获取当前的标签名
...
if (TAG_MERGE.equals(name)) { // 处理<merge>标签
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
// 递归处理
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// Temp is the root view that was found in the xml
// 创建View对象
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
...
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs); // 获取根View的宽高
if (!attachToRoot) { // 如果attachToRoot为false，则给根View设置宽高
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
...
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true); // 递归处理
...
// We are supposed to attach all the views we found (int temp)
// to root. Do that now.
if (root != null && attachToRoot) {
// 如果root不空，且attachToRoot为true，则将根View添加到容器中
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) {
// 如果root空或者attachToRoot为false，则将返回结果设置为根View
result = temp;
}
}
} catch (XmlPullParserException e) {
...
} catch (Exception e) {
...
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
// 要么是root，要么是创建的根View
return result;
}
}
从代码中可以看出root和attachToRoot不同传参的影响：
- 如果root不为null，attachToRoot设为true，则会将加载的布局添加到一个父布局中，即root，并且返回root；
- 如果root不为null，attachToRoot设为false，则会对布局文件最外层的所有layout属性进行设置，并且返回该布局的根View，当该view被添加到父view当中时，这些layout属性会自动生效；
- 如果root为null，attachToRoot将失去作用，设置任何值都没有意义，返回的也是要加载的布局的根View；
rInflate方法
从上面的方法中可以看到处理<merge>标签时会调用rInflate，处理子View时会调用rInflateChildren方法。其实rInflateChildren中调用的是rInflate，而rInflate也调用了rInflateChildren，从而形成了递归调用，也就是递归处理子View。
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)) {
// 处理<requestFocus>标签
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) {
// 处理<tag>标签
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
// 处理<include>标签
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) { // <merge>标签异常
throw new InflateException("<merge /> must be the root element");
} else { // 创建View对象
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); // 递归处理孩子节点
viewGroup.addView(view, params); // 将View添加到父布局中
}
}
if (pendingRequestFocus) { // 父布局处理焦点
parent.restoreDefaultFocus();
}
if (finishInflate) { // 结束加载
parent.onFinishInflate();
}
}
该方法中会处理<requestFocus>、<tag>、<include>、<merge>和普通View标签。其中：
- <requestFocus>是重新定位焦点的，调用的consumeChildElements方法其实没干什么事，只是简单的把该标签消费结束掉。
- <tag>标签一般很少用，它主要用来标记View，给View设置一个标签值，例如：
<TextView
android:id="@+id/tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello World!" >
<tag android:id="@+id/tag"
android:value="hello" />
</TextView>
findViewById(R.id.tv).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// 试一下tag标签
Toast.makeText(MainActivity.this, (String) v.getTag(R.id.tag), Toast.LENGTH_SHORT).show();
}
});
在ListView的自定义Adapter中，应该都有用到过View的setTag方法，即：使用ViewHolder来重复利用View。
parseViewTag方法：
private void parseViewTag(XmlPullParser parser, View view, AttributeSet attrs)
throws XmlPullParserException, IOException {
final Context context = view.getContext();
final TypedArray ta = context.obtainStyledAttributes(attrs, R.styleable.ViewTag);
// 读取tag的id
final int key = ta.getResourceId(R.styleable.ViewTag_id, 0);
// 读取tag的值
final CharSequence value = ta.getText(R.styleable.ViewTag_value);
// 给View设置该tag
view.setTag(key, value);
ta.recycle();
// 结束该标签（子View无效）
consumeChildElements(parser);
}
- <include>标签不能是根标签，parseInclude方法单独分析。
- <merge>标签只能是根标签，这里会抛异常。
parseInclude方法
private void parseInclude(XmlPullParser parser, Context context, View parent,
AttributeSet attrs) throws XmlPullParserException, IOException {
int type;
if (parent instanceof ViewGroup) { // 必须在ViewGroup里才有效
// 处理theme属性
...
// If the layout is pointing to a theme attribute, we have to
// massage the value to get a resource identifier out of it.
// 拿到layout指定的布局
int layout = attrs.getAttributeResourceValue(null, ATTR_LAYOUT, 0);
...
if (layout == 0) { // 必须是合法的id
final String value = attrs.getAttributeValue(null, ATTR_LAYOUT);
throw new InflateException("You must specify a valid layout "
+ "reference. The layout ID " + value + " is not valid.");
} else { // 类似于inflate的处理
// 拿到layout的解析器
final XmlResourceParser childParser = context.getResources().getLayout(layout);
try {
final AttributeSet childAttrs = Xml.asAttributeSet(childParser);
while ((type = childParser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty.
}
if (type != XmlPullParser.START_TAG) {
throw new InflateException(childParser.getPositionDescription() +
": No start tag found!");
}
// layout的根标签
final String childName = childParser.getName();
if (TAG_MERGE.equals(childName)) { // 处理<merge>
// The <merge> tag doesn't support android:theme, so
// nothing special to do here.
rInflate(childParser, parent, context, childAttrs, false);
} else { // 处理View
final View view = createViewFromTag(parent, childName,
context, childAttrs, hasThemeOverride);
final ViewGroup group = (ViewGroup) parent;
final TypedArray a = context.obtainStyledAttributes(
attrs, R.styleable.Include);
// 获取<include>里设置的id
final int id = a.getResourceId(R.styleable.Include_id, View.NO_ID);
// 获取<include>里设置的visibility
final int visibility = a.getInt(R.styleable.Include_visibility, -1);
a.recycle();
ViewGroup.LayoutParams params = null;
try { // 获取<include>里设置的宽高
params = group.generateLayoutParams(attrs);
} catch (RuntimeException e) {
// Ignore, just fail over to child attrs.
}
if (params == null) {
// 获取layout里设置的宽高
params = group.generateLayoutParams(childAttrs);
}
// <include>里设置的宽高优先于layout里设置的
view.setLayoutParams(params);
// Inflate all children.
rInflateChildren(childParser, view, childAttrs, true);
if (id != View.NO_ID) {
// include里设置的id优先级高
view.setId(id);
}
// include里设置的visibility优先级高
switch (visibility) {
case 0:
view.setVisibility(View.VISIBLE);
break;
case 1:
view.setVisibility(View.INVISIBLE);
break;
case 2:
view.setVisibility(View.GONE);
break;
}
group.addView(view);
}
} finally {
childParser.close();
}
}
} else {
throw new InflateException("<include /> can only be used inside of a ViewGroup");
}
LayoutInflater.consumeChildElements(parser);
}
- include里必须设置layout属性，且layout的id必须合法；
- include里设置的id优先级高于layout里设置的id，即：两者同时设置时，后者会失效；
- include里设置的width和height属性优先级高于layout里设置的宽高；
- include里设置的visibility属性优先级高于layout设置的visibility。
createViewFromTag方法
正常View标签都是通过createViewFromTag来创建对应的View对象的。
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
if (name.equals("view")) {
// 真正的View标签名存在class属性中
name = attrs.getAttributeValue(null, "class");
}
...
try {
View view;
if (mFactory2 != null) { // 先使用Factory2
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) { // 再使用Factory
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 {
// 通过标签名中是否包含'.'来区分是否为自定义View
if (-1 == name.indexOf('.')) {
// 处理系统View
view = onCreateView(parent, name, attrs);
} else { // 自定义View用的是全限定类名
// 处理自定义View
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (InflateException e) {
...
} catch (ClassNotFoundException e) {
...
} catch (Exception e) {
...
}
}
- 优先通过Factory2和Factory来创建View，这两个Factory等会再说；
- 通过标签名中是否包含'.'来区分待创建的View是自定义View还是系统View；
- 系统View会在onCreateView方法中添加android.view.前缀，然后交由createView处理。
createView方法
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
// 有缓存
Constructor<? extends View> constructor = sConstructorMap.get(name);
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
Class<? extends View> clazz = null;
try {
if (constructor == null) { // 第一次则通过反射创建constructor
// 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);
}
}
// 使用的是包含Context, AttributeSet这两个参数的构造函数
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
sConstructorMap.put(name, constructor); // 添加到缓存中
} else { // 命中缓存
// If we have a filter, apply it to cached constructor
if (mFilter != null) { // 先过滤
// 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;
// 反射创建View实例对象
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
// 如果是ViewStub则懒加载
// Use the same context when inflating ViewStub later.
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
mConstructorArgs[0] = lastContext;
return view;
}
...
}
通过反射待创建View的构造函数（两个参数：Context和AttributeSet的构造函数）来实例化View对象，如果是ViewStub对象还会进行懒加载。
LayoutInflater.Factory/Factory2
通过以上流程，使用LayoutInflater的infalte方法加载布局文件的整体流程就分析完了。但出现了Factory2和Factory类，它们会优先创建View，我们来看看着两个类到底是什么！
它们都是LayoutInflater的内部类——两个接口：
public interface Factory {
public View onCreateView(String name, Context context, AttributeSet attrs);
}
public interface Factory2 extends Factory {
public View onCreateView(View parent, String name, Context context, AttributeSet attrs);
}
Factory2继承了Factory，增加了一个带View parent参数的onCreateView重载方法。它们是在createViewFromTag中被调用的，默认为null，说明开发人员可以自定义这两个Factory，则通过它们可以改造待加载XML布局中的View标签，来使用自定义规则创建View。
来看一下它们的设置方法：
public void setFactory(Factory factory) {
if (mFactorySet) {
throw new IllegalStateException("A factory has already been set on this LayoutInflater");
}
// 和setFactory2类似
...
}
}
public void setFactory2(Factory2 factory) {
if (mFactorySet) { // 只能设置一次
throw new IllegalStateException("A factory has already been set on this LayoutInflater");
}
if (factory == null) {
throw new NullPointerException("Given factory can not be null");
}
mFactorySet = true;
if (mFactory == null) {
mFactory = mFactory2 = factory;
} else { // 合并原有的Factory
mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);
}
}
可以看到Factory和Factory2只能设置一次，否则会抛异常。
这两个Factory的区别是什么？
Factory2 是API 11 被加进来的；
Factory2 继承自 Factory，也就说现在直接使用Factory2即可；
Factory2 可以对创建 View 的 Parent 进行操作；
总结
LayoutInflater的相关分析就这么多，文章有点长，慢慢看吧！
- LayoutInflater的inflate的过程的核心方法是：createViewFromTag 和 createView 方法；
- LayoutInflater通过PULL解析器来解析XML布局文件，通过反射来创建View对象；
- LayoutInflater.Factory只能设置一次，可以用来替换View；
