安卓源码分析——LayoutInflater(上)
安卓源码分析——LayoutInflater(下)
1 前言
在安卓源码分析——LayoutInflater(上)中，我们通过对LayoutInflater的源码分析得出最终获取到的LayoutInfalter对象为PhoneLayoutInflater。那么接下来我们将继续剖析LayoutInflater是如何加载布局的。
2 PhoneLayoutInflater
public class PhoneLayoutInflater extends LayoutInflater {
//安卓内置View类型的包名的前缀
//TextViwe的全路径名称为android.widget.TextView
//WebView的全路径名为android.webkit.WebView
//android.app.为新增的前缀名
private static final String[] sClassPrefixList = {
"android.widget.",
"android.webkit.",
"android.app."
};
public PhoneLayoutInflater(Context context) {
super(context);
}
protected PhoneLayoutInflater(LayoutInflater original, Context newContext) {
super(original, newContext);
}
@Override protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {
for (String prefix : sClassPrefixList) {
try {
View view = createView(name, prefix, attrs);
if (view != null) {
return view;
}
} catch (ClassNotFoundException e) {
}
}
return super.onCreateView(name, attrs);
}
public LayoutInflater cloneInContext(Context newContext) {
return new PhoneLayoutInflater(this, newContext);
}
}
PhoneLayoutInflater主要是重写了LayoutInflater的onCreateView()方法。并调用了createView()方法。
3 布局加载inflate
LayoutInflater加载布局使用inflate()方法。
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
// 获取资源文件
final Resources res = getContext().getResources();
if (DEBUG) {
Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
+ Integer.toHexString(resource) + ")");
}
// 获取Xml文件的解析器, Resources类中调用ResourcesImpl的loadXmlResourceParser()方法
//ResourcesImpl的loadXmlResourceParser()方法调用的XmlBlock的newParser()方法。
//XmlBlock中newParser()方法调用nativeCreateParseState()本地方法最终创建Xml解析器
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
public View inflate(XmlPullParser parser, @Nullable ViewGroup root) {
return inflate(parser, root, root != null);
}
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
//此处为同步代码
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 {
// 寻找Xml文件的根节点
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
}
//若没有根节点则抛出异常
if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription()
+ ": No start tag found!");
}
//获取Xml解析器名称
final String name = parser.getName();
// 解析merge标签
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");
}
//如果是merge标签调用rInflate()方法解析
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// 如果不是merge标签，直接解析布局中的视图
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
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
//如果attachToRoot为false则为temp设置布局参数
temp.setLayoutParams(params);
}
}
// 加载标签内部子布局
rInflateChildren(parser, temp, attrs, true);
// 如果root不为空并且attachToRoot为true则将temp添加至root视图
if (root != null && attachToRoot) {
root.addView(temp, params);
}
// 如果root为空并且attachToRoot为false则直接返回temp视图
if (root == null || !attachToRoot) {
result = temp;
}
}
}
//省略部分代码
return result;
}
}
4 rInflate 方法
rInflate()方法用于加载merge标签的布局。
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
//获取树的深度，采用的是树的深度优先遍历方法
final int depth = parser.getDepth();
int type;
boolean pendingRequestFocus = false;
//按照顺序解析布局元素
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)) { // 解析include标签
if (parser.getDepth() == 0) {
//如果树深度为0，且根为include 则抛出此异常
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
//如果merge标签不为根，则抛出此异常
throw new InflateException("<merge /> must be the root element");
} else {
//根据元素标签名称进行解析
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
//解析内部子标签
rInflateChildren(parser, view, attrs, true);
将解析结果View添加至其父布局中
viewGroup.addView(view, params);
}
}
//省略部分代码
}
rInflate()通过深度优先搜索方式构造View树，每解析到一个View就会递归加载下去，直至该路径下的View都加载完毕，然后将此路径下的View添加至父布局中。rInflate()加载元素调用了createViewFromTag()方法。
5 createViewFromTag方法
createViewFromTag()用于解析具体某个View元素。
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;
//用户可以通过设置LayoutInflater的Factory来自行解析View，但是大部分开发人员没有这么做
//mFactory2、mFactory、mPrivateFactory默认为空值，因此可以暂时忽略此处代码。
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('.')) {
//解析内置View，因为内置的View中不包含.字符
view = onCreateView(parent, name, attrs);
} else {
// 解析自定义View
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
}
//省略部分代码
}
通过分析发现onCreateView()最终仍是调用createView()实现View的解析的。因此我们只需查看createView()方法。
6 createView
createView()实现View的最终创建过程。
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
//从缓存中获取构造方法
Constructor<? extends View> constructor = sConstructorMap.get(name);
//缓存中构造方法不为空，但是判断类加载是不是由LayoutInflater委派的，则从缓存中删除此构造方法
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);
}
//从 Class对象中获取构造方法
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
//将构造方法加入缓存
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) {
// 新的类型，则加载该类，并记录其加载状态
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;
//通过反射的方式实现View对象的创建
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;
}
//省略catch 和 finally代码块
}
createView()根据类名通过反射的方式实现View对象的创建。并且在创建同时将View类的构造函数添加至缓存，可以提高View的加载效率。
7 小结
布局的加载首先判断根布局的标签，根据不同根布局分开解析。不带有子布局的直接解析，带有子布局的递归解析。根据Xml解析出的标签名称得出类名，最终对于View对象的创建根据类名通过反射的方式实现，最终实现布局的完全加载。
