LayoutInflater想必对于Android开发者来说都不陌生，它主要用于加载布局，在Fragment的onCreateView方法、ListView Adapter的getView方法等许多地方都可以见到它的身影，它也就是将xml文件转换成一个View供我们使用。那么它是怎么将xml文件转成View的尼？带着这个疑问往下看。
LayoutInflater的使用
LayoutInflater是一个抽象类，所以我们无法直接通过new来创建LayoutInflater对象，但系统给我们提供了如下几种方式来获取LayoutInflater对象。
LayoutInflater.from(context)- 在Activity内部调用getLayoutInflater()方法
LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
前面两种方式最终都是调用的第三种方式context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);，它返回的是一个继承自LayoutInflater的类——PhoneLayoutInflater，这个类很简单，主要是重写了onCreateView与cloneInContext这两个方法。代码如下：
public class PhoneLayoutInflater extends LayoutInflater {
private static final String[] sClassPrefixList = {
"android.widget.",
"android.webkit.",
"android.app."
};
/**
* Instead of instantiating directly, you should retrieve an instance
* through {@link Context#getSystemService}
*
* @param context The Context in which in which to find resources and other
* application-specific things.
*
* @see Context#getSystemService
*/
public PhoneLayoutInflater(Context context) {
super(context);
}
protected PhoneLayoutInflater(LayoutInflater original, Context newContext) {
super(original, newContext);
}
/** Override onCreateView to instantiate names that correspond to the
widgets known to the Widget factory. If we don't find a match,
call through to our super class.
*/
@Override protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {
for (String prefix : sClassPrefixList) {
try {
//如果是android.widget、android.webkit、android.app这三个包下的控件，则直接调用createView
View view = createView(name, prefix, attrs);
if (view != null) {
return view;
}
} catch (ClassNotFoundException e) {
// In this case we want to let the base class take a crack
// at it.
}
}
return super.onCreateView(name, attrs);
}
public LayoutInflater cloneInContext(Context newContext) {
return new PhoneLayoutInflater(this, newContext);
}
}
拿到PhoneLayoutInflater对象后就能够调用inflate来加载布局，由于PhoneLayoutInflater并没有重写inflate这个方法，所以还是调用的LayoutInflater里面的inflate方法，inflate有多个不同的重载方法。代码如下：
//方法一
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root)
//方法二
public View inflate(XmlPullParser parser, @Nullable ViewGroup root)
//方法三
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)
//方法四
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)
方法一跟方法三想必大家都很熟悉，但它们的最终都是调用了都是方法四。
LayoutInflater源码分析
前面说了加载布局的最终实现是方法四，但方法四具体是怎么来实现的尼？先来看一下代码。
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;
//找到根节点
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
//没有拿到根节点
if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription()
+ ": No start tag found!");
}
//拿到根节点的名称，如果是LinearLayout则name为LinearLayout
final String name = parser.getName();
//如果根节点是merge
if (TAG_MERGE.equals(name)) {
//如果根节点是merge且不附加到任何View上则会抛异常
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
//开始解析布局并创建View对象
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// Temp is the root view that was found in the xml
//创建View对象，通过反射创建的
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);
}
}
// Inflate all children under temp against its context.
//解析并创建所有的子View
rInflateChildren(parser, temp, attrs, true);
// We are supposed to attach all the views we found (int temp)
// to root. Do that now.
//直接添加到父View中，这时候外面就不能在调用root.addView了，否则抛异常
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.
//直接返回生成的View对象
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
...
}
return result;
}
}
代码也比较简单，首先就是解析xml文件。常用的xml解析方式有DOM，SAX和PULL三种方式。DOM不适合xml文档较大，内存较小的场景，所以不适用于手机这样内存有限的移动设备上。SAX和PULL类似，都具有解析速度快，占用内存少的优点，而相对之下，PULL的操作方式更为简单易用，所以，Android系统内部在解析各种xml时都用的是PULL解析器。然后拿到根节点，如果根节点为merge且满足root == null || !attachToRoot这个条件就会抛异常，否则就去遍历所有子节点并创建View对象。
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();
//获取焦点
if (TAG_REQUEST_FOCUS.equals(name)) {
pendingRequestFocus = true;
consumeChildElements(parser);
//如果是Tag标签
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
//如果是include标签
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
//如果是merge标签
} else if (TAG_MERGE.equals(name)) {
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);
//添加到父View
viewGroup.addView(view, params);
}
}
if (pendingRequestFocus) {
parent.restoreDefaultFocus();
}
if (finishInflate) {
parent.onFinishInflate();
}
}
如果根节点不是merge，则创建根对象，然后遍历所有子节点并创建对象。遍历子节点代码如下：
final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}
最后根据root与attachToRoot来判断是否添加到传递过来的父View，到此，基本上LayoutInflater加载布局的流程就出来，主要有以下几步：
- 解析xml文件
- 创建根节点对象（如果是merge的话则直接遍历所有子节点）
- 遍历所有子节点并创建子View
LayoutInflater加载xml文件的流程到此就梳理完了，但是还有一个很重要问题，就是View是如何创建的尼？上面只是说了View是通过createViewFromTag这个方法创建的，那这个方法的具体实现是什么尼？createViewFromTag最终调用的是如下方法
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) {
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 (InflateException e) {
throw e;
} catch (ClassNotFoundException e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
}
}
通常情况下，自定义工厂mFactory2、mFactory和私有工厂mPrivateFactory是空的，当Activity继承自AppCompatActivity时，才会存在自定义Factory。所以就在onCreateView与createView中创建View，当是系统控件时，会调用onCreateView来补全控件名称，然后最终调用creatView方法。
//补全控件名称
protected View onCreateView(String name, AttributeSet attrs)
throws ClassNotFoundException {
return createView(name, "android.view.", attrs);
}
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);
}
}
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 = 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;
//通过反射来创建View对象
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 (NoSuchMethodException e) {
...
} catch (ClassCastException e) {
...
} catch (ClassNotFoundException e) {
...
} catch (Exception e) {
...
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
到这里是不是就恍然大悟了啊，原来LayoutInflater里创建View跟Activity一样也是通过反射创建的。当activity继承自AppCompatActivity时，则有自定义的mFactory实现。其实现在AppCompatDelegateImplV9这个类中，具体实现如下：
@Override
public View createView(View parent, final String name, @NonNull Context context,
@NonNull AttributeSet attrs) {
if (mAppCompatViewInflater == null) {
mAppCompatViewInflater = new AppCompatViewInflater();
}
boolean inheritContext = false;
if (IS_PRE_LOLLIPOP) {
inheritContext = (attrs instanceof XmlPullParser)
// If we have a XmlPullParser, we can detect where we are in the layout
? ((XmlPullParser) attrs).getDepth() > 1
// Otherwise we have to use the old heuristic
: shouldInheritContext((ViewParent) parent);
}
return mAppCompatViewInflater.createView(parent, name, context, attrs, inheritContext,
IS_PRE_LOLLIPOP, /* Only read android:theme pre-L (L+ handles this anyway) */
true, /* Read read app:theme as a fallback at all times for legacy reasons */
VectorEnabledTintResources.shouldBeUsed() /* Only tint wrap the context if enabled */
);
}
最终是调用了mAppCompatViewInflater的createView方法，再来看createView方法，
public final View createView(View parent, final String name, @NonNull Context context,
@NonNull AttributeSet attrs, boolean inheritContext,
boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {
final Context originalContext = context;
// We can emulate Lollipop's android:theme attribute propagating down the view hierarchy
// by using the parent's context
if (inheritContext && parent != null) {
context = parent.getContext();
}
if (readAndroidTheme || readAppTheme) {
// We then apply the theme on the context, if specified
context = themifyContext(context, attrs, readAndroidTheme, readAppTheme);
}
if (wrapContext) {
context = TintContextWrapper.wrap(context);
}
View view = null;
// We need to 'inject' our tint aware Views in place of the standard framework versions
//这里是来替换系统控件的实现
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;
case "CheckBox":
view = new AppCompatCheckBox(context, attrs);
break;
case "RadioButton":
view = new AppCompatRadioButton(context, attrs);
break;
case "CheckedTextView":
view = new AppCompatCheckedTextView(context, attrs);
break;
case "AutoCompleteTextView":
view = new AppCompatAutoCompleteTextView(context, attrs);
break;
case "MultiAutoCompleteTextView":
view = new AppCompatMultiAutoCompleteTextView(context, attrs);
break;
case "RatingBar":
view = new AppCompatRatingBar(context, attrs);
break;
case "SeekBar":
view = new AppCompatSeekBar(context, attrs);
break;
}
if (view == null && originalContext != context) {
// If the original context does not equal our themed context, then we need to manually
// inflate it using the name so that android:theme takes effect.
//这里是不是很熟悉啊，也是通过反射来创建的
view = createViewFromTag(context, name, attrs);
}
if (view != null) {
// If we have created a view, check its android:onClick
checkOnClickListener(view, attrs);
}
return view;
}
到此，LayoutInflater中，系统控件如何创建的也就梳理完毕了。其实自定义Factory一个十分有用的使用场景就是实现应用换肤，有兴趣的读者可以参考Android-Skin-Loader中的具体细节。
参考
Android LayoutInflater源码解析
Android LayoutInflater源码解析:你真的能正确使用吗？
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