LayoutInflater
开头先附一段LayoutInflater类的注释简介
/**
* Instantiates a layout XML file into its corresponding {@link android.view.View}
* objects. It is never used directly. Instead, use
* {@link android.app.Activity#getLayoutInflater()} or
* {@link Context#getSystemService} 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.
*
* To create a new LayoutInflater with an additional {@link Factory} for your
* own views, you can use {@link #cloneInContext} to clone an existing
* ViewFactory, and then call {@link #setFactory} on it to include your
* Factory.
*
* For performance reasons, view inflation relies heavily on pre-processing of
* XML files that is done at build time. Therefore, it is not currently possible
* to use LayoutInflater with an XmlPullParser over a plain XML file at runtime;
* it only works with an XmlPullParser returned from a compiled resource
* (R.<em>something</em> file.)
*/
这是LayoutInflater开头的一段介绍，我们能看到几个重要的信息：
- LayoutInfalter的作用是把XML转化成对应的View对象，需要用
Activity#getLayoutInflater()或者getSystemService获取，会绑定当前的Context - 如果需要使用自定义的
Factory查看替换加载信息，需要用cloneInContext去克隆一个ViewFactory，然后调用setFactory设置自定义的Factory - 性能原因，inflate会在build阶段进行，无法用来加载一个外部文本XML文件，只能加载R.xxx.xxx这种处理好的资源文件。
//LayoutInflater.java
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
//root是否为null来决定attachToRoot是否为true。
return inflate(resource, root, root != null);
}
public View inflate(XmlPullParser parser, @Nullable ViewGroup root) {
return inflate(parser, root, root != null);
}
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();
}
}
//三个inflate方法最终都会调用到下面这个三个参数的inflate方法。
/**
* parser XML节点包含了View的层级描述
* root 需要attached到的根目录，如果attachToRoot为true则root必须不为null。
* attachToRoot 加载的层级是否需要attach到rootView，
* return attachToRoot为true，就返回root，反之false就返回加载的XML文件的根节点View。
*/
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
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;
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)) {
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
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);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
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) {
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;
}
}
inflate方法使用XmlPullParser解析XML文件，并根据得到的标签名执行不同的逻辑：
- 首先如果是
merge标签，会走rInflate方法，方法前面带r的说明是recurse递归方法 - 如果不是
merge标签，执行createViewFromTag，根据传入的name和attrs获取到name对应的rootView并且添加到root里面。
针对merge标签，如果是merge标签必须有root并且必须attachToRoot==true，否则直接抛异常，所以我们得知merge必须作为root标签使用，并且不能用在子标签中①，rInflate方法中也会针对merge标签进行检查，保证merge标签不会出现在子标签中，后面会有介绍。
检查通过则调用rInflate(parser, root, inflaterContext, attrs, false)方法，递归遍历root的层级，解析加载childrenView挂载到parentView下面，rinflate详细解析可以看rinflate。
如果不是merge标签则调用createViewFromTag(root, name, inflaterContext, attrs)，这个方法的作用是加载名字为name的view，根据name反射方式创建对应的View，根据传入的attrs构造Params设置给View，返回创建好的View。
当然这只是创建了一个View，需要再调用rInflateChildren(parser, temp, attrs, true)，这个方法也是一个递归方法，它的作用是根据传入的parser包含的层级，加载此层级的子View并挂载到temp下面。
createViewFromTag
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.
// 如果传入的attr中包含theme属性，则使用此attr中的theme。
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) {
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 {
if (-1 == name.indexOf('.')) {
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (Exception e) {
...
}
}
先看当前标签的attr属性里面是否设置了theme，如果设置了就用当前标签的theme属性，绑定到context上面。 这里很有意思的是特殊判断了一个TAG_1995，也就是blink，一个将包裹的内容每隔500ms显示隐藏的一个标签，怎么看都像个彩蛋~
然后调用mFactory2的onCreateView,如果没有设置mFactory2就尝试mFactory，否则调用mPrivateFactory,mFactory2和mFactory后面再说，这里先往后走。
如果还是没有加载到view，先判断name，看名字里是不是有.，如果没有就表明是Android原生的View，最终都会调用到createView方法，onCreateView最终会调用到createView(name, "android.view.", attrs);，会在View名字天面添加"android.view."前缀。
下面是默认的createView的实现：
@Nullable
public final View createView(@NonNull Context viewContext, @NonNull String name,
@Nullable String prefix, @Nullable AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Objects.requireNonNull(viewContext);
Objects.requireNonNull(name);
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) {
// Class not found in the cache, see if it's real, and try to add it
clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
mContext.getClassLoader()).asSubclass(View.class);
if (mFilter != null && clazz != null) {
boolean allowed = mFilter.onLoadClass(clazz);
if (!allowed) {
failNotAllowed(name, prefix, viewContext, attrs);
}
}
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
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 = Class.forName(prefix != null ? (prefix + name) : name, false,
mContext.getClassLoader()).asSubclass(View.class);
boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
mFilterMap.put(name, allowed);
if (!allowed) {
failNotAllowed(name, prefix, viewContext, attrs);
}
} else if (allowedState.equals(Boolean.FALSE)) {
failNotAllowed(name, prefix, viewContext, attrs);
}
}
}
Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = viewContext;
Object[] args = mConstructorArgs;
args[1] = attrs;
try {
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]));
}
return view;
} finally {
mConstructorArgs[0] = lastContext;
}
} catch (Exception e) {
...
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
这个方法可以看到View是怎么创建出来的，用类的全限定名拿到class信息，有一个sConstructorMap缓存类的constructor，如果能拿到有效的构造器就不再重复创建来提升效率，如果没有缓存的构造器，就反射得到构造器并添加到sConstructorMap中以便后面使用。这里有个mFilter来提供自定义选项，用户可以自定义哪些类不允许构造。
拿到构造器之后，实际上newInstance是调用了两View个参数的构造方法。第一个参数是Context，第二个参数是attrs，这样我们就得到了需要加载的View。
这里可以结合LayoutInflater.Factory2一起来看，Activity实际上是实现了LayoutInflater.Factory2接口的：
//Activity.java
public View onCreateView(@NonNull String name, @NonNull Context context,
@NonNull AttributeSet attrs) {
return null;
}
所以我们可以直接在Activity里面重写onCreateView方法，这样就可以根据View的名字来实现我们的一些操作，比如换肤的操作，比如定义一个名字来表示某种自定义View。可以看这样一个用法：
<PlaceHolder
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="center"
android:text="include LinearLayout"
android:textColor="#fff"
android:textSize="15sp" />
然后我们在重写的onCreateView里面判断name：
@Override
public View onCreateView(String name, Context context, AttributeSet attrs) {
if ("PlaceHolder".equals(name)) {
return new TextView(this, attrs);
}
return super.onCreateView(name, context, attrs);
}
这样其实就不拘泥于名字可以自己创建对应的View，这样其实可以用在多个module依赖的时候，如果在moduleA中得不到moduleB的某个自定义View，可以使用一个这样的方式来在moudleA中暂时的用来做一个占位标记，在moduleB中做一个判断。
同样的，通过上面的代码我们知道LayoutInflater是通过反射拿到构造方法来创建View的，那众所周知反射是有性能损耗的，那么我们可以在onCreateView方法中判断名字直接new出来，当然也可以跟AppcompatActivity里面做的一样，做一些兼容的操作来替换成不同版本的View:
public final View createView(View parent, final String name, @NonNull Context context,
View view = null;
switch (name) {
case "TextView":
view = new AppCompatTextView(context, attrs);
break;
case "ImageView":
view = new AppCompatImageView(context, attrs);
break;
case "Button":
view = new AppCompatButton(context, attrs);
break;
case "EditText":
view = new AppCompatEditText(context, attrs);
break;
case "Spinner":
view = new AppCompatSpinner(context, attrs);
break;
case "ImageButton":
view = new AppCompatImageButton(context, attrs);
break;
...
}
...
return view;
}
还没有展开说rinflate，篇幅限制，放到另外一篇文章中去分析，rinflate源码分析。
流程图如下：
总结
- LayoutInfalter的作用是把XML转化成对应的View对象，需要用
Activity#getLayoutInflater()或者getSystemService获取 - 加载时先判断是否是merge标签，merge标签走递归方法rinflate，否则走createViewFromTag
- createViewFromTag作用是根据xml标签的名字去加载对应的View，使用的是反射的方法
- LayoutInflater.Factory2是设计出来灵活构造View的接口，可以用来实现换肤或者替换View的功能，同时也是AppcompatActivity用来做兼容和版本替换的接口