Activity如何将布局文件添加Windows窗口

一.首先了解几个概念

1.Window和PhoneWindow

一个顶级窗口查看和行为的一个抽象基类。这个类的实例作为一个顶级View添加到Window Manager。它提供了一套标准的UI方法，比如添加背景，标题等等。当你需要用到Window的时候，你应该使用它的唯一实现类PhoneWindow。可以看到，Window是一个抽象基类，它提供了一系列窗口的方法，比如设置背景，标题等等，而它的唯一实现类则是PhoneWindow，每一个Activity上面都有一个Window，可以通过getWindow获取；

2.DecorView

在PhoneWindow里面，出现了成员变量DecorView的而这里，DecorView则是PhoneWindow里面的一个内部类，它是继承与FrameLayout，标题栏，内容栏，顶级上看是加载在DecorView上的。而DecorView则是由PhoneWindow负责添加

二.接下来我们通过源码分析Activity是如何通过setContentView()将布局文件添加到窗口上的

首先找到Activity的setContentView()方法

/**
* Set the activity content from a layout resource.  The resource will be
* inflated, adding all top-level views to the activity.
*
* @param layoutResID Resource ID to be inflated.
*
* @see #setContentView(android.view.View)
* @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
*/
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}

通过源码可以看出这里通过getWindow()获取window对象，再调用Window的setContentView()方法

/**
* Convenience for
* {@link #setContentView(View, android.view.ViewGroup.LayoutParams)}
* to set the screen content from a layout resource.  The resource will be
* inflated, adding all top-level views to the screen.
*
* @param layoutResID Resource ID to be inflated.
* @see #setContentView(View, android.view.ViewGroup.LayoutParams)
*/
public abstract void setContentView(@LayoutRes int layoutResID);

而Window的setContentView()方法是个抽象方法，这时候我们找到Window的实现类PhoneWindow中的setContentView()方法

/**
* The ID that the main layout in the XML layout file should have.
*/
public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;
// This is the top-level view of the window, containing the window decor.
private DecorView mDecor;
// This is the view in which the window contents are placed. It is either
// mDecor itself, or a child of mDecor where the contents go.
ViewGroup mContentParent;
@Override
public void setContentView(int layoutResID) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}

if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}

其中ViewGroup mContentParent则是我们Activity布局文件的父布局，首先判断mContentParent是否存在，如果不存在则通过installDecor()方法来创建该ViewGroup

private void installDecor() {
.............
if (mDecor == null) {
mDecor = generateDecor(-1);
.........以下代码省略
} else {
...........
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);
.........以下代码省略
}
}

在创建mContentParent的时候首先判断mDecor是否已经创建，如果没有创建则通过generateDecor（）方法创建mDecor，接着判断mContentParent是否创建，没有创建则通过generateLayout(mDecor)方法获取到mContentParent（是DecorView中布局id为content的Framlayout）

protected ViewGroup generateLayout(DecorView decor) {
........................以上省略
//设置标题 状态栏 全屏等属性
if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) {
requestFeature(FEATURE_NO_TITLE);
} else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) {
// Don't allow an action bar if there is no title.
requestFeature(FEATURE_ACTION_BAR);
}
..................................
if (a.getBoolean(R.styleable.Window_windowFullscreen, false)) {
setFlags(FLAG_FULLSCREEN, FLAG_FULLSCREEN & (~getForcedWindowFlags()));
}

if (a.getBoolean(R.styleable.Window_windowTranslucentStatus,
false)) {
setFlags(FLAG_TRANSLUCENT_STATUS, FLAG_TRANSLUCENT_STATUS
& (~getForcedWindowFlags()));
}

if (a.getBoolean(R.styleable.Window_windowTranslucentNavigation,
false)) {
setFlags(FLAG_TRANSLUCENT_NAVIGATION, FLAG_TRANSLUCENT_NAVIGATION
& (~getForcedWindowFlags()));
}
//获取设置的window属性
WindowManager.LayoutParams params = getAttributes();

// Inflate the window decor.
int layoutResource;
int features = getLocalFeatures();
// System.out.println("Features: 0x" + Integer.toHexString(features));
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
layoutResource = R.layout.screen_swipe_dismiss;
setCloseOnSwipeEnabled(true);
} else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
layoutResource = R.layout.screen_simple_overlay_action_mode;

} ........
else {
// Embedded, so no decoration is needed.
layoutResource = R.layout.screen_simple;
// System.out.println("Simple!");
}

//加载资源id为layoutResource的布局并将其添加到DecorView中
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);

//找到DecorView中R.id.content的FrameLayout作为Activity中布局的父布局
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
if (contentParent == null) {
throw new RuntimeException("Window couldn't find content container view");
}

return contentParent;
}

布局screen_simple.xml的代码

<?xml version="1.0" encoding="utf-8"?>
<!--
/* //device/apps/common/assets/res/layout/screen_simple.xml
**
** Copyright 2006, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
**
**     http://www.apache.org/licenses/LICENSE-2.0 **
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
** See the License for the specific language governing permissions and
** limitations under the License.
*/

This is an optimized layout for a screen, with the minimum set of features
enabled.
-->

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
android:orientation="vertical">
<ViewStub android:id="@+id/action_mode_bar_stub"
android:inflatedId="@+id/action_mode_bar"
android:layout="@layout/action_mode_bar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:theme="?attr/actionBarTheme" />
<FrameLayout
android:id="@android:id/content"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:foregroundInsidePadding="false"
android:foregroundGravity="fill_horizontal|top"
android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>

从以上源码中可以看出requestFeature（）为什么要在setContentView（）之前调用，如果requestFeature（）在setContentView（）后调用则不能设置window属性

以上mContentParent已经创建完成 接下来看源码中如何通过LayoutInflater的inflate()方法将Activity中的布局文件添加到mContentParent中。

以下是LayoutInflater的inflate()方法

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
if (DEBUG) {
Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
+ Integer.toHexString(resource) + ")");
}

final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}

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;
//root为mContentParent
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("**************************");
}
//判断是否是marge标签
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 {
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, attrs);

ViewGroup.LayoutParams params = null;

if (root != null) {

// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);

}
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);

// 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);
}
..................
}

} catch (XmlPullParserException e) {
final InflateException ie = new InflateException(e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(parser.getPositionDescription()
+ ": " + e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;

Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}

return result;
}
}

final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}

void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {

final int depth = parser.getDepth();
int type;
boolean pendingRequestFocus = false;

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)) {
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
//判断是否是include标签
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
//判断是否是marge标签
} 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 (pendingRequestFocus) {
parent.restoreDefaultFocus();
}

if (finishInflate) {
parent.onFinishInflate();
}
}

从以上源码中可以看出通过XmlPullParser来解析xml根据不同标签来创建不同的view,同时我们知道marge为什么只能用于根布局，include则不能作为根布局