相信大家已经对这三位大哥不陌生了。
- include：一般xml代码View的复用，减少同样布局的cv操作
- merge：提高xml的View组件复用，且减少不必要层级
- ViewStub：某些View不是必须加载的，通过ViewStub实现延时加载
通过LayoutInflater分析include和merge
总所周知xml布局代码，最终会调用LayoutInflater.inflate方法。先去看下inflate()具体流程。所以include和merge直接通过LayoutInflater去解析更加清晰明了
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
//无关代码将会省略，展示代码并非LayoutInflater.inflate方法所有代码
synchronized (mConstructorArgs) {
try {
if (TAG_MERGE.equals(name)) {
//inflate Merge标签下的所有View
rInflate(parser, root, inflaterContext, attrs, false);
} else {
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
//解析xml中的View，最终回落rInflate
rInflateChildren(parser, temp, attrs, true);
if (root != null && attachToRoot) {
root.addView(temp, params);
}
if (root == null || !attachToRoot) {
result = temp;
}
}
}//忽略相关catch代码
return result;
}
}
可以看到inflate代码逻辑比较简单，如果是merge标签则通过rInflate方法解析merge所有view。否则通过rInflateChildren方法去解析，而rInflateChildren最终调用rInflate方法。
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();
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)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
//解析include标签-----
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else {
//merge标签最终回落到这里---------
final View view = createViewFromTag(parent, name, context, attrs);
//merge标签最终布局为其调用的父布局
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
//继续inflate其children view
rInflateChildren(parser, view, attrs, true);
// 添加至父布局
viewGroup.addView(view, params);
}
}
if (pendingRequestFocus) {
parent.restoreDefaultFocus();
}
if (finishInflate) {
parent.onFinishInflate();
}
}
从上面可以看出merge最终会将merge中所有view添加至父布局中。
而include标签最终起调parseInclude方法。
private void parseInclude(XmlPullParser parser, Context context, View parent,
AttributeSet attrs) throws XmlPullParserException, IOException {
int type;
if (parent instanceof ViewGroup) {//include 最外层元素必须要viewgroup类型
int layout = attrs.getAttributeResourceValue(null, ATTR_LAYOUT, 0);
if (layout == 0) {
layout = context.getResources().getIdentifier(
value.substring(1), "attr", context.getPackageName());
}
if (layout == 0) {
throw new InflateException("You must specify a valid layout "
+ "reference. The layout ID " + value + " is not valid.");
} else {
final XmlResourceParser childParser = context.getResources().getLayout(layout);
try {
final AttributeSet childAttrs = Xml.asAttributeSet(childParser);
if (TAG_MERGE.equals(childName)) {
// The <merge> tag doesn't support android:theme, so
// nothing special to do here.
rInflate(childParser, parent, context, childAttrs, false);
} else {
final View view = createViewFromTag(parent, childName,
context, childAttrs, hasThemeOverride);
final ViewGroup group = (ViewGroup) parent;
final TypedArray a = context.obtainStyledAttributes(
attrs, R.styleable.Include);
final int id = a.getResourceId(R.styleable.Include_id, View.NO_ID);
final int visibility = a.getInt(R.styleable.Include_visibility, -1);
a.recycle();
//优先使用父布局中的layout信息，没有的话使用include中的信息
ViewGroup.LayoutParams params = null;
try {
params = group.generateLayoutParams(attrs);
} catch (RuntimeException e) {
}
if (params == null) {
params = group.generateLayoutParams(childAttrs);
}
view.setLayoutParams(params);
// Inflate all children.
rInflateChildren(childParser, view, childAttrs, true);
//若父布局中有id则覆盖include内的id
if (id != View.NO_ID) {
view.setId(id);
}
group.addView(view);
}
} finally {
childParser.close();
}
}
} else {
throw new InflateException("<include /> can only be used inside of a ViewGroup");
}
LayoutInflater.consumeChildElements(parser);
}
这里可以看出include布局中的宽高信息及id默认使用父布局定义的。所以include中定义的id及宽高信息会被调用的父布局定义给覆盖。且父布局中也只能定义对应的LayoutParams信息。
ViewStub
public final class ViewStub extends View
可以看出ViewStub本质上还是一个view。只是在构建方法中默认将自己设为GON及忽略了绘制过程，并且在measure时默认将自己设为0像素大小
public void setVisibility(int visibility) {
if (mInflatedViewRef != null) {
View view = mInflatedViewRef.get();
if (view != null) {
view.setVisibility(visibility);
} else {
throw new IllegalStateException("setVisibility called on un-referenced view");
}
} else {
super.setVisibility(visibility);
if (visibility == VISIBLE || visibility == INVISIBLE) {
inflate();
}
}
}
public View inflate() {
final ViewParent viewParent = getParent();
if (viewParent != null && viewParent instanceof ViewGroup) {
if (mLayoutResource != 0) {
final ViewGroup parent = (ViewGroup) viewParent;
//inflate 指定的layout
final View view = inflateViewNoAdd(parent);
//移除自己并将inflate的View添加进parent中
replaceSelfWithView(view, parent);
mInflatedViewRef = new WeakReference<>(view);
if (mInflateListener != null) {
mInflateListener.onInflate(this, view);
}
return view;
} else {
throw new IllegalArgumentException("ViewStub must have a valid layoutResource");
}
} else {
throw new IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");
}
}
可以看出通过setVisibility且visibility == VISIBLE || visibility == INVISIBLE时也是会起调inflate()方法进行懒加载。
inflate过后ViewStub的parent将为空，所以可以通过ViewStub.getParent()是否等于空去判断ViewStub是否已经inflate，或者通过setOnInflateListener去监听。
