入口就从这个setContentView方法进入了
public class DemoActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
super.setContentView(R.layout.view_demo);
}
}
Activity中的setContentView方法
public void setContentView(int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
getWindow()获取的是 com.android.internal.policy.impl.PhoneWindow
这个对象是PhoneWindow中的方法。
@Override
public void setContentView(int layoutResID) {
if (mContentParent == null) {
installDecor();
} else {
mContentParent.removeAllViews();
}
mLayoutInflater.inflate(layoutResID, mContentParent);
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
installDecor初始化DecorView，DecorView是什么呢？
布局文件如下
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/view_demo_root"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
</LinearLayout>
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
super.setContentView(R.layout.view_demo);
View root_view = findViewById(R.id.view_demo_root);
while (root_view != null) {
Log.d("ViewDemo", root_view.getClass().toString());
root_view = (View) root_view.getParent();
}
}
Log打印：
class android.widget.LinearLayout
class android.widget.FrameLayout
class com.android.internal.widget.ActionBarOverlayLayout
class com.android.internal.policy.impl.PhoneWindow$DecorView
从代码中可以看到DecorView为所有组件的父容器
下面进入mLayoutInflater.inflate(layoutResID, mContentParent)方法中
// 为节省篇幅，删除一些调试代码
public View inflate(int resource, ViewGroup root) {
return inflate(resource, root, root != null);
}
public View inflate(int resource, ViewGroup root, boolean attachToRoot) {
XmlResourceParser parser = getContext().getResources().getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
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();
// 根标签为merge时，root不能为空
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, attrs, false);
} else {
// Temp is the root view that was found in the xml
View temp;
if (TAG_1995.equals(name)) {
temp = new BlinkLayout(mContext, attrs);
} else {
temp = createViewFromTag(root, name, attrs);
}
ViewGroup.LayoutParams params = null;
if (root != null) {
// 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);
}
}
// 解析temp的所有子View
rInflate(parser, temp, attrs, true);
// 将temp添加到root中
if (root != null && attachToRoot) {
root.addView(temp, params);
}
// 如果未指定root或者不附加到root，则返回xml所代表的view;
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
InflateException ex = new InflateException(e.getMessage());
ex.initCause(e);
throw ex;
} catch (IOException 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;
}
return result;
}
}
可以看到最终都调用了rInflate
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 {
final View view = createViewFromTag(parent, name, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflate(parser, view, attrs, true);
viewGroup.addView(view, params);
}
}
if (finishInflate) parent.onFinishInflate();
}
rInflate通过XmlPullParser解析出XML里所有的组件，并使用递归的方式逐层去获取子组件。看到这里我们就知道为什么不建议在View中做过多的嵌套了吧，因为递归调用会占用大量的栈帧，导致内存使用飙高，这样程序会栈溢出和内存溢出的风险。
最终 rInflate 调用 createViewFromTag 方法生成了view 并加载到 DecorView中。
最后通知Callback，ContentView发生改变
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
在Activity方法attach中可以发现setCallback就是Activity本身
final void attach(...) {
...
mWindow = PolicyManager.makeNewWindow(this);
mWindow.setCallback(this);
...
}
public void onContentChanged() {
}
界面发生变化后会调用Activity重写的onContentChanged，通知界面已经发生了改变。
网友评论