经常用LayoutInflater加载布局，可是第三个参数true/false都木有搞懂，看看源码一探究竟。
一 首先，先要获取LayoutInflater的实例，这里有两种写法
LayoutInflater layoutInflater = LayoutInflater.from(context);
LayoutInflater layoutInflater = (LayoutInflater) context
.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
其实第一种就是第二种的简单写法，只是Android给我们做了一下封装而已。
LayoutInflater1.png
二 得到实例后，调用inflate（）方法来加载布局 方法的重载
inflate.png
不管调用那哪个方法，最后都会调用
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
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();
if (DEBUG) {
System.out.println("**************************");
System.out.println("Creating root view: "
+ name);
System.out.println("**************************");
}
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 {
// 采用pull解析方式来解析布局文件的， createViewFromTag方法内部调用createView()方法，然后使用反射的方式创建出View的实例并返回，这个只是创建一个根布局实例
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
if (DEBUG) {
System.out.println("-----> start inflating children");
}
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);
if (DEBUG) {
System.out.println("-----> done inflating children");
}
// We are supposed to attach all the views we found (int temp)
// to root. Do that now.
if (root != null && attachToRoot) {
root.addView(temp, params);
}
// Decide whether to return the root that was passed in or the
// top view found in xml.
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;
}
}
再查看rInflateChildren()中的rInflate()方法
创建View的实例 递归调用rInflateChildren() 将View添加到父布局中
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);
最后再来看看attachToRoot true or false？
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
如果root不为空，attachToRoot为false，将布局文件最外层的layout所有属性进行设置，当view被添加到父view中，layout会自动生效
if (root != null && attachToRoot) {
root.addView(temp, params);
}
如果root不为null，attachToRoot设为true，则会给加载的布局文件的指定一个父布局，即root。
// Decide whether to return the root that was passed in or the
// top view found in xml.
if (root == null || !attachToRoot) {
result = temp;
}
如果root为null，attachToRoot将失去作用，设置任何值都没有意义。
举个简单小例子，在布局中动态添加一个Button
mainLayout = (LinearLayout) findViewById(R.id.main_layout);
LayoutInflater layoutInflater = LayoutInflater.from(this);
View buttonLayout = layoutInflater.inflate(R.layout.button_layout, null);
mainLayout.addView(buttonLayout);
button_layout的布局
<Button xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button" >
</Button>
Button是wrap_content，但在实际项目里如果我们想改变Button的大小
android:layout_width="300dp"
android:layout_height="80dp"(然并卵)
原因就是 View必须存在于一个布局中
View buttonLayout = layoutInflater.inflate(R.layout.button_layout, null);第二个参数null,第三个参数就成了false
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
看上面的attachToRoot ，root不为空时，attachToRoot为false，将布局文件最外层的layout所有属性进行设置
让Button存在一个布局当中，按钮的终于可以变大了，这下总算是满足要求了吧。
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<Button
android:layout_width="300dp"
android:layout_height="80dp"
android:text="Button" >
</Button>
</RelativeLayout>
