一、基础用法
inflate()
把传入的layout_res_id获得构建后的实例:
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val view = LayoutInflater.from(this).inflate(R.layout.layout, null)
先了解源码中几个频繁出现的TAG_*
字符串:
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TAG_MERGE = "merge";
TAG_INCLUDE = "include";
TAG_1995 = "blink";
TAG_REQUEST_FOCUS = "requestFocus";
TAG_TAG = "tag";
二、方法入口
inflate()
为指定的xml资源文件构建视图布局
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// resource: xml布局文件id
// root: 根布局
// attachToRoot: 是否加入到父布局
public View inflate(@LayoutRes int resource,
@Nullable ViewGroup root,
boolean attachToRoot) {
// 从Context获取Resources
final Resources res = getContext().getResources();
// 用Resources创建xml资源语义解析器,解析layout布局
final XmlResourceParser parser = res.getLayout(resource);
try {
// 返回创建的View
return inflate(parser, root, attachToRoot);
} finally {
// 关闭该语义解析器
parser.close();
}
}
下方方法的注释着重提到几点:
- 填充行为重度依赖编译过程预处理过的xml文件,以此提升查找性能;
- 运行期不可能用
XmlPullParser
调用inflate
方法处理普通xml; - 普通xml文件指开发可见的xml,此类文件没有经过预处理,所以运行期不能处理。
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public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
// mConstructorArgs作为同步对象,LayoutInflate实例同步执行
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
// mContext就是LayoutInflater(this)中的context.
final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
// mConstructorArgs[0]临时保存为lastContext
Context lastContext = (Context) mConstructorArgs[0];
// 用inflaterContext,即mContext赋值给mConstructorArgs[0]
mConstructorArgs[0] = inflaterContext;
View result = root;
try {
// 开始查找Root节点
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// 跳过非START_TAG或END_DOCUMENT标签
}
// 可能已找到START_TAG,也可能没有START_TAG而结束;
if (type != XmlPullParser.START_TAG) {
// 没有发现START_TAG意味xml内容非法,上层捕捉异常终止inflate
throw new InflateException(parser.getPositionDescription()
+ ": No start tag found!");
}
// 有START_TAG则解析根布局名
final String name = parser.getName();
if (TAG_MERGE.equals(name)) {
// merge只能在根布局非空且attachToRoot为true时使用
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
// 循环遍历子标签
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// 不是merge表明该标签是xml文件的根元素,命名为temp
// 例如name为"LinearLayout",则temp是LinearLayout实例
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
// 传进来的root不为空
if (root != null) {
// 根据root构建temp的LayoutParams
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// attachToRoot为false,从root获得LayoutParams并设到根元素temp
temp.setLayoutParams(params);
}
}
// 填充temp下所有子View.
rInflateChildren(parser, temp, attrs, true);
// xml构建的View添加到root中
if (root != null && attachToRoot) {
root.addView(temp, params);
}
// 如果temp不需要添加到root,则返回temp
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
final InflateException ie = new InflateException(e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(parser.getPositionDescription()
+ ": " + e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
// 在context中不保留静态引用
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
// root非空且attachToRoot为true时构建temp添加到root,返回root;
// 否则返回temp,temp可能设置了有关root的LayoutParams.
return result;
}
}
三、递归填充子视图
此方法用于递归填充非根的内部子视图。父context作为填充context,调用方法 rInflate
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final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}
四、填充子视图
r 原意为 recursive,深度递归xml布局初始化view,一并初始化此view的子view。
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void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
// 获取树最大深度
final int depth = parser.getDepth();
int type;
boolean pendingRequestFocus = false;
// 没遇到views本身的END_TAG或遍历子views时没遇到END_DOCUMENT就继续遍历
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();
// 此View设置了TAG_REQUEST_FOCUS
if (TAG_REQUEST_FOCUS.equals(name)) {
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) {
// 解析View的tag
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
// include仅能作为子元素
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs); // 解析include标签
} else if (TAG_MERGE.equals(name)) {
// 子视图不能是merge标签,merge只能用在根元素
throw new InflateException("<merge /> must be the root element");
} else {
// 通过createViewFromTag构建标签所指示的View
final View view = createViewFromTag(parent, name, context, attrs);
// 利用parent作为ViewGroup,构建出LayoutParams给子View使用
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
// rInflateChildren()会调用rInflate(),深度遍历初始化
rInflateChildren(parser, view, attrs, true);
// 构建成功的View添加到ViewGroup
viewGroup.addView(view, params);
}
}
if (pendingRequestFocus) {
parent.restoreDefaultFocus();
}
if (finishInflate) {
parent.onFinishInflate();
}
}
五、从tag构建视图
根据标签名创建view。AppCompatActivity 会给 LayoutInflater 注入 mFactory2 实例,然后构建工作会代理给 AppCompatDelegateImpl,实现填充 TextView 为 AppCompatTextView。
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View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
// ignoreThemeAttr为false则给context配置主题装饰器
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();
}
// TAG_1995返回BlinkLayout,即上世纪Disco舞厅灯光的闪烁感
if (name.equals(TAG_1995)) {
// Let's party like it's 1995!
return new BlinkLayout(context, attrs);
}
try {
View view;
if (mFactory2 != null) {
// mFactory2一般不为空,且转换TextView为AppCompatTextView
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) {
view = mFactory.onCreateView(name, context, attrs);
} else {
view = null;
}
// 如果上述工厂构建视图实例成功则完成构建
if (view == null && mPrivateFactory != null) {
view = mPrivateFactory.onCreateView(parent, name, context, attrs);
}
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
// 不包含符号'.'表示原生View,不是自定义如 <com.phatomvk.custom.view />
// onCreateView()调用createView(name, prefix = "android.view.", attrs)
// 示例:android.support.v7.widget.RecyclerView不会添加前缀
if (-1 == name.indexOf('.')) {
// 例:TextView全路径名为'android.view.TextView'
view = onCreateView(parent, name, attrs);
} else {
// 自定义View创建
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view; // 返回构建的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;
}
}
六、视图创建
全局静态HashMap缓存,缓存View的构造方法
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private static final HashMap<String, Constructor<? extends View>> sConstructorMap =
new HashMap<String, Constructor<? extends View>>();
View最终通过其全路径名在ClassLoader
中反射出对应的View
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public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
// 构建方法缓存HashMap
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) { // 该类没有缓存
// 从ClassLoader中反射类
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
// 未经许可的类不能实例化,并抛出异常
if (mFilter != null && clazz != null) {
// 指定类不允许被填充构建
boolean allowed = mFilter.onLoadClass(clazz);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
}
// 获取反射类的构造方法,并把构造方法添加到缓存
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
sConstructorMap.put(name, constructor);
} else {
// 已经缓存的构造方法还要经过mFilter的检查
if (mFilter != null) {
// 查看此name之前是否遇见过
Boolean allowedState = mFilterMap.get(name);
if (allowedState == null) {
// 获取类并记录此类是否允许使用
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) {
mConstructorArgs[0] = mContext;
}
Object[] args = mConstructorArgs;
args[1] = attrs;
// 创建类实例,args是自定义主题相关变量
final View view = constructor.newInstance(args);
// 类型是ViewStub
if (view instanceof ViewStub) {
// 使用同一个Context给ViewStub设置LayoutInflater
// 后面View填充是会使用此LayoutInflater
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
mConstructorArgs[0] = lastContext;
return view;
} catch (NoSuchMethodException e) {
// 无法找到构造方法
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + (prefix != null ? (prefix + name) : name), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (ClassCastException e) {
// 填充类不是View的子类
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Class is not a View " + (prefix != null ? (prefix + name) : name), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (ClassNotFoundException e) {
// 类加载失败,抛出异常
throw e;
} catch (Exception e) {
final InflateException ie = new InflateException(
attrs.getPositionDescription() + ": Error inflating class "
+ (clazz == null ? "<unknown>" : clazz.getName()), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
七、验证类加载器
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private final boolean verifyClassLoader(Constructor<? extends View> constructor) {
final ClassLoader constructorLoader = constructor.getDeclaringClass().getClassLoader();
if (constructorLoader == BOOT_CLASS_LOADER) {
// boot class loader是合法的类加载器
return true;
}
// in all normal cases (no dynamic code loading), we will exit the following loop on the
// first iteration (i.e. when the declaring classloader is the contexts class loader).
ClassLoader cl = mContext.getClassLoader();
do {
if (constructorLoader == cl) {
return true;
}
cl = cl.getParent();
} while (cl != null);
return false;
}
八、总结
整个过程最耗时间部分有两个:
- 读取并分析xml标签数据;
- 类反射获得全路径名的View实例;
从开始LayoutInflater.inflate
填充布局,调用rInflate
递归遍历子View,每个子View在createViewFromTag
通过全限定名调用createView
反射实例,层层处理结束后返回视图。
最后总结attachToRoot
:
root
为null,attachToRoot
设置值不起作用;root
不为null,attachToRoot
为true,给加载的布局文件指定父布局root;root
不为null,attachToRoot
为false,则设置布局文件最外层layout属性。当该view添加到父view时,这些layout属性起效;- 不设置
attachToRoot
参数且root不为null,attachToRoot
参数默认为true。
参考链接: