首先我们看官方的说明:
The tag helps eliminate redundant view groups in your view hierarchy when including one layout within another. For example, if your main layout is a vertical LinearLayout in which two consecutive views can be re-used in multiple layouts, then the re-usable layout in which you place the two views requires its own root view. However, using another LinearLayout as the root for the re-usable layout would result in a vertical LinearLayout inside a vertical LinearLayout. The nested LinearLayout serves no real purpose other than to slow down your UI performance.
其实就是减少在include布局文件时的层级。<merge>标签是这几个标签中最让我费解的，大家可能想不到，标签竟然会是一个Activity，里面有一个LinearLayout对象。
使用merge来组织子元素可以减少布局的层级。例如我们在复用一个含有多个子控件的布局时，肯定需要一个ViewGroup来管理，例如这样 :
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<ImageView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:scaleType="center"
android:src="@drawable/golden_gate" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="20dip"
android:layout_gravity="center_horizontal|bottom"
android:padding="12dip"
android:background="#AA000000"
android:textColor="#ffffffff"
android:text="Golden Gate" />
</FrameLayout> 将该布局通过include引入时就会多引入了一个FrameLayout层级，此时结构如下 :
使用merge标签就会消除上图中蓝色的FrameLayout层级。示例如下 :
<merge xmlns:android="http://schemas.android.com/apk/res/android">
<ImageView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:scaleType="center"
android:src="@drawable/golden_gate" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="20dip"
android:layout_gravity="center_horizontal|bottom"
android:padding="12dip"
android:background="#AA000000"
android:textColor="#ffffffff"
android:text="Golden Gate" />
</merge> 那么它是如何实现的呢，我们还是看源码吧。相关的源码也是在LayoutInflater的inflate()函数中。
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context)mConstructorArgs[0];
mConstructorArgs[0] = mContext;
View result = root;
try {
// Look for the root node.
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription()
+ ": No start tag found!");
}
final String name = parser.getName();
// m如果是erge标签，那么调用rInflate进行解析
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");
}
// 解析merge标签
rInflate(parser, root, attrs, false);
} else {
// 代码省略
}
} catch (XmlPullParserException e) {
// 代码省略
}
return result;
}
}
void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
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)) {
parseRequestFocus(parser, parent);
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else if (TAG_1995.equals(name)) {
final View view = new BlinkLayout(mContext, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflate(parser, view, attrs, true);
viewGroup.addView(view, params);
} else { // 我们的例子会进入这里
final View view = createViewFromTag(parent, name, attrs);
// 获取merge标签的parent
final ViewGroup viewGroup = (ViewGroup) parent;
// 获取布局参数
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
// 递归解析每个子元素
rInflate(parser, view, attrs, true);
// 将子元素直接添加到merge标签的parent view中
viewGroup.addView(view, params);
}
}
if (finishInflate) parent.onFinishInflate();
} 其实就是如果是merge标签，那么直接将其中的子元素添加到merge标签parent中，这样就保证了不会引入额外的层级。
为什么跟布局如果是FrameLayout的可以用merge标签，就可以省略一个层级呢，其实是我们的Layout添加到系统的Window中的时候是添加到FrameLauyout布局中的，所以不需要再添加一层FrameLayout