在项目开发中，UI布局是我们都会遇到的问题，如果布局过于复杂，层级过深，不仅会影响阅读性，还会导致性能降低。Android官方给了几个优化的方法include、merge、ViewStub。这里我们我们简单的介绍下使用方法，注意事项，并从源码角度分析他们的好处，注意事项。
Include:
include是我们最常用的标签，它有点像C中的include头文件，我们把一套布局封装起来，等到使用的时候使用include标签引入即可。这样就提高了代码的复用性
不必每次都写一遍。先看下示例代码：
include文件：include_layout.xml
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent" android:layout_height="match_parent"
android:layout_marginTop="50dp"
android:id="@+id/my_layout_root_id">
<Button
android:id="@+id/back_btn"
android:layout_marginTop="50dp"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@mipmap/ic_launcher" />
<TextView
android:id="@+id/title_tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerVertical="true"
android:layout_marginLeft="20dp"
android:layout_toRightOf="@+id/back_btn"
android:gravity="center"
android:text="include"
android:textSize="18sp" />
</RelativeLayout>
在MainActivity的布局文件activity_main.xml中引用
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.zhangy.include_merge_viewstub.MainActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello World!" />
<include
android:id="@+id/my_layout"
layout="@layout/include_layout"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<include
android:id="@+id/my_merge_layout"
layout="@layout/merge_layout"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<ViewStub
android:id="@+id/view_stub"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:inflatedId="@+id/view_stub_layout"
android:layout="@layout/viewstub_layout" />
</LinearLayout>
MainActivity
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// View titleView = findViewById(R.id.my_layout_root_id) ;//这样会报错，因为我们重置了layout布局的id
View titleView = findViewById(R.id.my_layout) ;
ViewStub viewStub = (ViewStub)findViewById(R.id.view_stub) ;
TextView titleTextView = (TextView)titleView.findViewById(R.id.title_tv) ;
titleTextView.setText("yang");
viewStub.inflate();
viewStub.setOnInflateListener(new ViewStub.OnInflateListener() {
@Override
public void onInflate(ViewStub stub, View inflated) {
}
});
viewStub.setVisibility(View.VISIBLE);
}
}
include标签使用很简单但是需要注意以下两点：
- 这里我们设置了include标签的Id为include_layout,这个id会覆盖include文件：include_layout.xml中根标签的id：my_layout_root_id；所以当用findViewByid(R.id.my_layout_root_id)方法是找不到根View的，如果不加以注意会报空指针异常。
- 如果想再include标签中使用android：** 这些属性集，必须先layout_width、layout_height。否则这些属性不生效
接下来我们从源码角度分析这两个注意事项，Activity的setContentView方法最终会调到LayoutInflater的rInflate方法解析xml文件，我们看看rInflate方法
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
//获取xml深度
final int depth = parser.getDepth();
int type;
//迭代解析各个标签
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
if (type != XmlPullParser.START_TAG) {
continue;
}
//获取标签名
final String name = parser.getName();
if (TAG_REQUEST_FOCUS.equals(name)) {
parseRequestFocus(parser, parent);
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {//如果是include的标签
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else {
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflateChildren(parser, view, attrs, true);
viewGroup.addView(view, params);
}
}
if (finishInflate) {
parent.onFinishInflate();
}
}
这个方法其实就是遍历View树，并添加到根View中，当是include标签时调用parseInclude
private void parseInclude(XmlPullParser parser, Context context, View parent,
AttributeSet attrs) throws XmlPullParserException, IOException {
int type;
if (parent instanceof ViewGroup) {
...
//获取include中layout
int layout = attrs.getAttributeResourceValue(null, ATTR_LAYOUT, 0);
if (layout == 0) {
//include中没有设置layout，抛异常
final String value = attrs.getAttributeValue(null, ATTR_LAYOUT);
if (value == null || value.length() <= 0) {
throw new InflateException("You must specify a layout in the" + " include tag: <include layout=\"@layout/layoutID\" />");
}
...
} else {
//获取layout的xml解析器
final XmlResourceParser childParser = context.getResources().getLayout(layout);
try {
//获取layout的属性集
final AttributeSet childAttrs = Xml.asAttributeSet(childParser);
...
final String childName = childParser.getName();
if (TAG_MERGE.equals(childName)) {
...//merge标签
} else {
//得到include文件的根布局
final View view = createViewFromTag(parent, childName,
context, childAttrs, hasThemeOverride);
//得到include文件挂载的父容器
final ViewGroup group = (ViewGroup) parent;
//得到include标签的属性
final TypedArray a = context.obtainStyledAttributes(
attrs, R.styleable.Include);
//我们在使用include的时设置的Id
final int id = a.getResourceId(R.styleable.Include_id, View.NO_ID);
//我们在使用include的时设置的是否显示
final int visibility = a.getInt(R.styleable.Include_visibility, -1);
...
ViewGroup.LayoutParams params = null;
try {
//注释1.从我们设置的include标签中获取布局属性，必须先layout_width、layout_height 如果没设置，try catch异常，params为null
params = group.generateLayoutParams(attrs);
} catch (RuntimeException e) {
// Ignore, just fail over to child attrs.
}
if (params == null) {
//从include跟布局标签中获取布局属性
params = group.generateLayoutParams(childAttrs);
}
//设置布局参数。如果include标签中的params！=null则会替换layout根布局的布局参数，让其都失效
view.setLayoutParams(params);
//解析所有子控件
rInflateChildren(childParser, view, childAttrs, true);
//注释2.这里就将我们设置的include标签中的Id设置给layout根布局，改变了原有id
if (id != View.NO_ID) {
view.setId(id);
}
//设置VISIBLE属性
switch (visibility) {
case 0:
view.setVisibility(View.VISIBLE);
break;
case 1:
view.setVisibility(View.INVISIBLE);
break;
case 2:
view.setVisibility(View.GONE);
break;
}
//将根view添加到父控件中
group.addView(view);
}
} finally {
childParser.close();
}
}
} else {
throw new InflateException("<include /> can only be used inside of a ViewGroup");
}
LayoutInflater.consumeChildElements(parser);
}
该方法就是解析include标签，先解析include标签属性，再解析layout布局文件获得一View，如果include的params！=null就覆盖该View的原有的params，如果我们设置了include的id，则覆盖原有的id。然后再解析layout布局的子View。最终将这个view添加到父View parent上。注释1、2处分别说明我们使用时的注意事项原因。
merge:
merge标签可以减少层级布局，它是将merge标签下的子view直接添加到merge标签的parent中，这样就减少了不必要的层级。先看下示例代码
merge布局：
<merge xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<Button
android:id="@+id/back_btn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@mipmap/ic_launcher" />
<TextView
android:id="@+id/title_tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerVertical="true"
android:gravity="center"
android:text="我的title"
android:textSize="18sp" />
</merge>
merge标签使用见activity_main
merge标签的解析都会走到rInflate方法中
void rInflate(XmlPullParser parser, View parent, Context context,
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_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");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else {
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflateChildren(parser, view, attrs, true);
viewGroup.addView(view, params);//将merge标签下的子View直接添加到merge父容器中
}
}
if (finishInflate) {
parent.onFinishInflate();
}
}
以merge标签为跟标签都会调用viewGroup.addView(view, params)将其子View直接添加到merge父容器中，减少一层布局
需要注意的是，使用merge标签时LayoutInflate.inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)
root!=null attachToRoot == true;否则会抛InflateException异常
ViewStub ：
ViewStub标签最大的优点是当你需要时才会加载，使用他并不会影响UI初始化时的性能。各种不常用的布局想进度条、显示错误消息等可以使用ViewStub标签，以减少内存使用量，加快渲染速度。ViewStub是一个不可见的，大小为0的View，相当于一个“占位控件”。然后当ViewStub被设置为可见的时或调用了ViewStub.inflate()的时候，ViewStub所指向的布局就会被inflate实例化，且此布局文件直接将当前ViewStub替换掉，然后ViewStub的布局属性（layout_margin***、layout_width等）都会传给它所指向的布局。这样，就可以使用ViewStub在运行时动态显示布局，节约内存资源。先看示例代码：
viewstub_layout.xml:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<Button
android:layout_marginTop="50dp"
android:id="@+id/back_btn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@mipmap/ic_launcher" />
<TextView
android:id="@+id/title_tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerVertical="true"
android:gravity="center"
android:text="merge"
android:textSize="18sp" />
</LinearLayout>
使用方法见activity_main.xml：
显示加载的布局有两种方法调用inflate方法，或者设置VISIBLE即可 见MainActivity
ViewStub重新了setVisibility方法
public void setVisibility(int visibility) {
if (mInflatedViewRef != null) {//如果不是第一次，跟正常的View一样
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();//最后还是调用了inflate方法加载布局
}
}
}
我们来看看ViewStub的inflate方法
public View inflate() {
final ViewParent viewParent = getParent();
if (viewParent != null && viewParent instanceof ViewGroup) {
if (mLayoutResource != 0) {
final ViewGroup parent = (ViewGroup) viewParent;
final LayoutInflater factory;
if (mInflater != null) {
factory = mInflater;
} else {
factory = LayoutInflater.from(mContext);
}
//mLayoutResource就是我们在ViewStub标签中的layout布局
final View view = factory.inflate(mLayoutResource, parent, false);
//mInflatedId就是我们在ViewStub标签中的inflateId,如果我们设置了，则设置给view
if (mInflatedId != NO_ID) {
view.setId(mInflatedId);
}
//从父视图中查找ViewStub
final int index = parent.indexOfChild(this);
//注释1.把当前ViewStub对象从父视图中移除了
parent.removeViewInLayout(this);
final ViewGroup.LayoutParams layoutParams = getLayoutParams();
//注释2.得到ViewStub的LayoutParams布局参数对象，如果存在就把它赋给被inflate的布局对象,不存在就按脚标添加
if (layoutParams != null) {
parent.addView(view, index, layoutParams);
} else {
parent.addView(view, index);
}
mInflatedViewRef = new WeakReference<View>(view);
if (mInflateListener != null) {
mInflateListener.onInflate(this, view);//可以设置监听器在加载View前回调
}
return view;
} else {
throw new IllegalArgumentException("ViewStub must have a valid layoutResource");
}
} else {
throw new IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");
}
}
从注释1我们可以看出不能再次调用inflate方法，因为已经移除了ViewStub对象，得到的viewParent就为null，此时判断时候就会走else抛出一个IllegalStateException异常：ViewStub must have a non-null ViewGroup viewParent。
使用ViewStub要注意，ViewStub只是个“占位符”，达到延迟加载的效果，当它指向的layout被加载后，它就会被父容器移除，但是从注释2看到布局文件的layout params是以ViewStub为准，其他布局属性是以布局文件自身为准。
