LayoutInflater.inflate源码详解
LayoutInflater的inflate方法相信大家都不陌生,在Fragment的onCreateView中或者在BaseAdapter的getView方法中我们都会经常用这个方法来实例化出我们需要的View.
假设我们有一个需要实例化的布局文件menu_item.xml:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
<TextView
android:id="@+id/id_menu_title_tv"
android:layout_width="match_parent"
android:layout_height="300dp"
android:gravity="center_vertical"
android:textColor="@android:color/black"
android:textSize="16sp"
android:text="@string/menu_item"/>
</LinearLayout>我们想在BaseAdapter的getView()方法中对其进行实例化,其实例化的方法有三种,分别是:
- 2个参数的方法:
convertView = mInflater.inflate(R.layout.menu_item, null);- 3个参数的方法(attachToRoot=false):
convertView = mInflater.inflate(R.layout.menu_item, parent, false);- 3个参数的方法(attachToRoot=true):
convertView = mInflater.inflate(R.layout.menu_item, parent, true);究竟我们应该用哪个方法进行实例化View,这3个方法又有什么区别呢？如果有同学对三个方法的区别还不是特别清楚,那么就和我一起从源码的角度来分析一下这个问题吧.
源码
inflate
我们先来看一下两个参数的inflate方法,源码如下:
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}从代码我们看出,其实两个参数的inflate方法根据父布局parent是否为null作为第三个参数来调用三个参数的inflate方法,三个参数的inflate方法源码如下:
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
// 获取当前应用的资源集合
final Resources res = getContext().getResources();
// 获取指定资源的xml解析器
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
// 返回View之前关闭parser资源
parser.close();
}
}这里需要解释一下,我们传入的资源布局id是无法直接实例化的,需要借助XmlResourceParser.
而XmlResourceParser是借助Android的pull解析方法是解析布局文件的.继续跟踪inflate方法源码:
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
// 获取上下文对象,即LayoutInflater.from传入的Context.
final Context inflaterContext = mContext;
// 根据parser构建XmlPullAttributes.
final AttributeSet attrs = Xml.asAttributeSet(parser);
// 保存之前的Context对象.
Context lastContext = (Context) mConstructorArgs[0];
// 赋值为传入的Context对象.
mConstructorArgs[0] = inflaterContext;
// 注意,默认返回的是父布局root.
View result = root;
try {
// 查找xml的开始标签.
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
// 如果没有找到有效的开始标签,则抛出InflateException异常.
if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription()
+ ": No start tag found!");
}
// 获取控件名称.
final String name = parser.getName();
// 特殊处理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");
}
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// 实例化我们传入的资源布局的view
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
// 如果传入的parent不为空.
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// 创建父类型的LayoutParams参数.
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// 如果实例化的View不需要添加到父布局上,则直接将根据父布局生成的params参数设置
// 给它即可.
temp.setLayoutParams(params);
}
}
// 递归的创建当前布局的所有控件
rInflateChildren(parser, temp, attrs, true);
// 如果传入的父布局不为null,且attachToRoot为true,则将实例化的View加入到父布局root中
if (root != null && attachToRoot) {
root.addView(temp, params);
}
// 如果父布局为null或者attachToRoot为false,则将返回值设置成我们实例化的View
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
InflateException ex = new InflateException(e.getMessage());
ex.initCause(e);
throw ex;
} catch (Exception e) {
InflateException ex = new InflateException(
parser.getPositionDescription()
+ ": " + e.getMessage());
ex.initCause(e);
throw ex;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
return result;
}
}上述代码中的关键部分我已经加入了中文注释.从上述代码中我们还可以发现,我们传入的布局文件是通过createViewFromTag来实例化每一个子节点的.
createViewFromTag
函数源码如下:
/**
* 方便调用5个参数的方法,ignoreThemeAttr的值为false.
*/
private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
return createViewFromTag(parent, name, context, attrs, false);
}
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();
}
// 特殊处理“1995”这个标签(ps: 平时我们写xml布局文件时基本没有使用过).
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);
ie.initCause(e);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name);
ie.initCause(e);
throw ie;
}
}在createViewFromTag方法中,最终是通过createView方法利用反射来实例化view控件的.
createView
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
// 以View的name为key, 查询构造函数的缓存map中是否已经存在该View的构造函数.
Constructor<? extends View> constructor = sConstructorMap.get(name);
Class<? extends View> clazz = null;
try {
// 构造函数在缓存中未命中
if (constructor == null) {
// 通过类名去加载控件的字节码
clazz = mContext.getClassLoader().loadClass(prefix != null ? (prefix + name) : name).asSubClass(View.class);
// 如果有自定义的过滤器并且加载到字节码,则通过过滤器判断是否允许加载该View
if (mFilter != null && clazz != null) {
boolean allowed = mFilter.onLoadClass(clazz);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
}
// 得到构造函数
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
// 缓存构造函数
sConstructorMap.put(name, constructor);
} else {
if (mFilter != null) {
// 过滤的map是否包含了此类名
Boolean allowedState = mFilterMap.get(name);
if (allowedState == null) {
// 重新加载类的字节码
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);
}
}
}
// 实例化类的参数数组(mConstructorArgs[0]为Context, [1]为View的属性)
Object[] args = mConstructorArgs;
args[1] = attrs;
// 通过构造函数实例化View
final View view = constructor.newInstance(args);
if (View instanceof ViewStub) {
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context)args[0]))
}
return view;
} catch (NoSunchMethodException e) {
// ......
} catch (ClassNotFoundException e) {
// ......
} catch (Exception e) {
// ......
} finally {
// ......
}
}总结
通过学习了inflate函数源码,我们再回过头去看BaseAdapter的那三种方法,我们可以得出的结论是:
- 第一种方法使用不够规范, 且会导致实例化View的LayoutParams属性失效.(ps: 即layout_width和layout_height等参数失效, 因为源码中这种情况的LayoutParams为null).
- 第二种是最正确,也是最标准的写法.
- 第三种由于attachToRoot为true,所以返回的View其实是父布局ListView,这显然不是我们想要实例化的View.因此,第三种写法是错误的.
