經常用LayoutInflater加載布局，可是第三個參數true/false都木有搞懂，看看源碼一探究竟。
一 首先，先要獲取LayoutInflater的實例，這里有兩種寫法
LayoutInflater layoutInflater = LayoutInflater.from(context);
LayoutInflater layoutInflater = (LayoutInflater) context
.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
其實第一種就是第二種的簡單寫法，只是Android給我們做了一下封裝而已。
LayoutInflater1.png
二 得到實例后，調用inflate（）方法來加載布局 方法的重載
inflate.png
不管調用那哪個方法，最后都會調用
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解析方式來解析布局文件的， createViewFromTag方法內部調用createView()方法，然后使用反射的方式創建出View的實例并返回，這個只是創建一個根布局實例
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
創建View的實例 遞歸調用rInflateChildren() 將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);
最后再來看看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不為空，attachToRoot為false，將布局文件最外層的layout所有屬性進行設置，當view被添加到父view中，layout會自動生效
if (root != null && attachToRoot) {
root.addView(temp, params);
}
如果root不為null，attachToRoot設為true，則會給加載的布局文件的指定一個父布局，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為null，attachToRoot將失去作用，設置任何值都沒有意義。
舉個簡單小例子，在布局中動態添加一個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，但在實際項目里如果我們想改變Button的大小
android:layout_width="300dp"
android:layout_height="80dp"(然并卵)
原因就是 View必須存在于一個布局中
View buttonLayout = layoutInflater.inflate(R.layout.button_layout, null);第二個參數null,第三個參數就成了false
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
看上面的attachToRoot ，root不為空時，attachToRoot為false，將布局文件最外層的layout所有屬性進行設置
讓Button存在一個布局當中，按鈕的終于可以變大了，這下總算是滿足要求了吧。
<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>
