源码分析setContentView加载布局文件的过程

我们都知道通过setContentView方法将xml布局文件加载到Activity中，然后屏幕才能显示出xml中定义的view。在整个过程中，无须做太多的操作，屏幕就能显示出正常的内容。现在我们从源码的角度来看看系统到底帮我们处理了哪些操作。

首先，进入setContentView方法中查看：

Activity.java：

public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}

里面通过getWindow()获得的对象来setContentView。getWindow()返回是Window对象，它是一个抽象类。为了获取它的实现类，我们可以打印它的类信息，如下：

Log.i("yumf","getWindow():" + getWindow());

在Activity中运行后，得到打印结果信息：

getWindow():com.android.internal.policy.PhoneWindow@5c4bc31

现在我们知道真正发挥作用的是PhoneWindow类。进入该类中，查看setContentView方法：

PhoneWindow.java

@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();
}
}

首先判断mContentParent为null时，执行installDecor()方法，我们稍后分析mContentParent是什么，先分析installDecor()方法，进入该方法中：

PhoneWindow.java

private void installDecor() {
if (mDecor == null) {
mDecor = generateDecor();  //实例化窗口的顶层视图
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
}
//mContentParent 存放窗口内容的视图，可以是mDecor 或者是子mDecor 内容
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);

// Set up decor part of UI to ignore fitsSystemWindows if appropriate.
mDecor.makeOptionalFitsSystemWindows();

final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(
R.id.decor_content_parent);

if (decorContentParent != null) {
mDecorContentParent = decorContentParent;
mDecorContentParent.setWindowCallback(getCallback());
if (mDecorContentParent.getTitle() == null) {
mDecorContentParent.setWindowTitle(mTitle);
}

final int localFeatures = getLocalFeatures();
for (int i = 0; i < FEATURE_MAX; i++) {
if ((localFeatures & (1 << i)) != 0) {
mDecorContentParent.initFeature(i);
}
}

mDecorContentParent.setUiOptions(mUiOptions);

if ((mResourcesSetFlags & FLAG_RESOURCE_SET_ICON) != 0 ||
(mIconRes != 0 && !mDecorContentParent.hasIcon())) {
mDecorContentParent.setIcon(mIconRes);
} else if ((mResourcesSetFlags & FLAG_RESOURCE_SET_ICON) == 0 &&
mIconRes == 0 && !mDecorContentParent.hasIcon()) {
mDecorContentParent.setIcon(
getContext().getPackageManager().getDefaultActivityIcon());
mResourcesSetFlags |= FLAG_RESOURCE_SET_ICON_FALLBACK;
}
if ((mResourcesSetFlags & FLAG_RESOURCE_SET_LOGO) != 0 ||
(mLogoRes != 0 && !mDecorContentParent.hasLogo())) {
mDecorContentParent.setLogo(mLogoRes);
}

// Invalidate if the panel menu hasn't been created before this.
// Panel menu invalidation is deferred avoiding application onCreateOptionsMenu
// being called in the middle of onCreate or similar.
// A pending invalidation will typically be resolved before the posted message
// would run normally in order to satisfy instance state restoration.
PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
if (!isDestroyed() && (st == null || st.menu == null) && !mIsStartingWindow) {
invalidatePanelMenu(FEATURE_ACTION_BAR);
}
} else {
mTitleView = (TextView)findViewById(R.id.title);
if (mTitleView != null) {
mTitleView.setLayoutDirection(mDecor.getLayoutDirection());
if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {
View titleContainer = findViewById(
R.id.title_container);
if (titleContainer != null) {
titleContainer.setVisibility(View.GONE);
} else {
mTitleView.setVisibility(View.GONE);
}
if (mContentParent instanceof FrameLayout) {
((FrameLayout)mContentParent).setForeground(null);
}
} else {
mTitleView.setText(mTitle);
}
}
}

if (mDecor.getBackground() == null && mBackgroundFallbackResource != 0) {
mDecor.setBackgroundFallback(mBackgroundFallbackResource);
}

// Only inflate or create a new TransitionManager if the caller hasn't
// already set a custom one.
if (hasFeature(FEATURE_ACTIVITY_TRANSITIONS)) {
if (mTransitionManager == null) {
final int transitionRes = getWindowStyle().getResourceId(
R.styleable.Window_windowContentTransitionManager,
0);
if (transitionRes != 0) {
final TransitionInflater inflater = TransitionInflater.from(getContext());
mTransitionManager = inflater.inflateTransitionManager(transitionRes,
mContentParent);
} else {
mTransitionManager = new TransitionManager();
}
}

mEnterTransition = getTransition(mEnterTransition, null,
R.styleable.Window_windowEnterTransition);
mReturnTransition = getTransition(mReturnTransition, USE_DEFAULT_TRANSITION,
R.styleable.Window_windowReturnTransition);
mExitTransition = getTransition(mExitTransition, null,
R.styleable.Window_windowExitTransition);
mReenterTransition = getTransition(mReenterTransition, USE_DEFAULT_TRANSITION,
R.styleable.Window_windowReenterTransition);
mSharedElementEnterTransition = getTransition(mSharedElementEnterTransition, null,
R.styleable.Window_windowSharedElementEnterTransition);
mSharedElementReturnTransition = getTransition(mSharedElementReturnTransition,
USE_DEFAULT_TRANSITION,
R.styleable.Window_windowSharedElementReturnTransition);
mSharedElementExitTransition = getTransition(mSharedElementExitTransition, null,
R.styleable.Window_windowSharedElementExitTransition);
mSharedElementReenterTransition = getTransition(mSharedElementReenterTransition,
USE_DEFAULT_TRANSITION,
R.styleable.Window_windowSharedElementReenterTransition);
if (mAllowEnterTransitionOverlap == null) {
mAllowEnterTransitionOverlap = getWindowStyle().getBoolean(
R.styleable.Window_windowAllowEnterTransitionOverlap, true);
}
if (mAllowReturnTransitionOverlap == null) {
mAllowReturnTransitionOverlap = getWindowStyle().getBoolean(
R.styleable.Window_windowAllowReturnTransitionOverlap, true);
}
if (mBackgroundFadeDurationMillis < 0) {
mBackgroundFadeDurationMillis = getWindowStyle().getInteger(
R.styleable.Window_windowTransitionBackgroundFadeDuration,
DEFAULT_BACKGROUND_FADE_DURATION_MS);
}
if (mSharedElementsUseOverlay == null) {
mSharedElementsUseOverlay = getWindowStyle().getBoolean(
R.styleable.Window_windowSharedElementsUseOverlay, true);
}
}
}
}

这里面的代码比较多，我们逐个分析：首先是判断mDecor 对象为null时，执行了方法generateDecor()，如下：

protected DecorView generateDecor() {
return new DecorView(getContext(), -1);
}

该方法用来实例化一个DecorView 对象，PhoneWindow中对该类的解释是窗口的顶层视图。继续执行代码，判断 mContentParent 为null时，调用方法generateLayout(mDecor)，将其初始化。

PhoneWindow.java：

protected ViewGroup generateLayout(DecorView decor) {
// Apply data from current theme.

TypedArray a = getWindowStyle(); //读取当前Window的属性样式

if (false) {
System.out.println("From style:");
String s = "Attrs:";
for (int i = 0; i < R.styleable.Window.length; i++) {
s = s + " " + Integer.toHexString(R.styleable.Window[i]) + "="
+ a.getString(i);
}
System.out.println(s);
}
//窗口是否浮动，用户Dialog窗口是否浮动，是否显示在布局中间
mIsFloating = a.getBoolean(R.styleable.Window_windowIsFloating, false);
int flagsToUpdate = (FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR)
& (~getForcedWindowFlags());
if (mIsFloating) {
setLayout(WRAP_CONTENT, WRAP_CONTENT);
setFlags(0, flagsToUpdate);
} else {
setFlags(FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR, flagsToUpdate);
}
//窗口是否支持标题栏
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);
}
//ActionBar导航栏是否不占布局空间叠加显示在当前窗口之上
if (a.getBoolean(R.styleable.Window_windowActionBarOverlay, false)) {
requestFeature(FEATURE_ACTION_BAR_OVERLAY);
}

if (a.getBoolean(R.styleable.Window_windowActionModeOverlay, false)) {
requestFeature(FEATURE_ACTION_MODE_OVERLAY);
}

if (a.getBoolean(R.styleable.Window_windowSwipeToDismiss, false)) {
requestFeature(FEATURE_SWIPE_TO_DISMISS);
}
//是否支持当前Activity中全屏
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()));
}

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

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

if (a.getBoolean(R.styleable.Window_windowEnableSplitTouch,
getContext().getApplicationInfo().targetSdkVersion
>= android.os.Build.VERSION_CODES.HONEYCOMB)) {
setFlags(FLAG_SPLIT_TOUCH, FLAG_SPLIT_TOUCH&(~getForcedWindowFlags()));
}

a.getValue(R.styleable.Window_windowMinWidthMajor, mMinWidthMajor);
a.getValue(R.styleable.Window_windowMinWidthMinor, mMinWidthMinor);
if (a.hasValue(R.styleable.Window_windowFixedWidthMajor)) {
if (mFixedWidthMajor == null) mFixedWidthMajor = new TypedValue();
a.getValue(R.styleable.Window_windowFixedWidthMajor,
mFixedWidthMajor);
}
if (a.hasValue(R.styleable.Window_windowFixedWidthMinor)) {
if (mFixedWidthMinor == null) mFixedWidthMinor = new TypedValue();
a.getValue(R.styleable.Window_windowFixedWidthMinor,
mFixedWidthMinor);
}
if (a.hasValue(R.styleable.Window_windowFixedHeightMajor)) {
if (mFixedHeightMajor == null) mFixedHeightMajor = new TypedValue();
a.getValue(R.styleable.Window_windowFixedHeightMajor,
mFixedHeightMajor);
}
if (a.hasValue(R.styleable.Window_windowFixedHeightMinor)) {
if (mFixedHeightMinor == null) mFixedHeightMinor = new TypedValue();
a.getValue(R.styleable.Window_windowFixedHeightMinor,
mFixedHeightMinor);
}
if (a.getBoolean(R.styleable.Window_windowContentTransitions, false)) {
requestFeature(FEATURE_CONTENT_TRANSITIONS);
}
if (a.getBoolean(R.styleable.Window_windowActivityTransitions, false)) {
requestFeature(FEATURE_ACTIVITY_TRANSITIONS);
}

final Context context = getContext();
final int targetSdk = context.getApplicationInfo().targetSdkVersion;
final boolean targetPreHoneycomb = targetSdk < android.os.Build.VERSION_CODES.HONEYCOMB;
final boolean targetPreIcs = targetSdk < android.os.Build.VERSION_CODES.ICE_CREAM_SANDWICH;
final boolean targetPreL = targetSdk < android.os.Build.VERSION_CODES.LOLLIPOP;
final boolean targetHcNeedsOptions = context.getResources().getBoolean(
R.bool.target_honeycomb_needs_options_menu);
final boolean noActionBar = !hasFeature(FEATURE_ACTION_BAR) || hasFeature(FEATURE_NO_TITLE);

if (targetPreHoneycomb || (targetPreIcs && targetHcNeedsOptions && noActionBar)) {
setNeedsMenuKey(WindowManager.LayoutParams.NEEDS_MENU_SET_TRUE);
} else {
setNeedsMenuKey(WindowManager.LayoutParams.NEEDS_MENU_SET_FALSE);
}

// Non-floating windows on high end devices must put up decor beneath the system bars and
// therefore must know about visibility changes of those.
if (!mIsFloating && ActivityManager.isHighEndGfx()) {
if (!targetPreL && a.getBoolean(
R.styleable.Window_windowDrawsSystemBarBackgrounds,
false)) {
setFlags(FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS,
FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS & ~getForcedWindowFlags());
}
}
//是否需要这时状态栏的颜色
if (!mForcedStatusBarColor) {
mStatusBarColor = a.getColor(R.styleable.Window_statusBarColor, 0xFF000000);
}
//是否设置导航栏的颜色
if (!mForcedNavigationBarColor) {
mNavigationBarColor = a.getColor(R.styleable.Window_navigationBarColor, 0xFF000000);
}
if (a.getBoolean(R.styleable.Window_windowLightStatusBar, false)) {
decor.setSystemUiVisibility(
decor.getSystemUiVisibility() | View.SYSTEM_UI_FLAG_LIGHT_STATUS_BAR);
}

if (mAlwaysReadCloseOnTouchAttr || getContext().getApplicationInfo().targetSdkVersion
>= android.os.Build.VERSION_CODES.HONEYCOMB) {
if (a.getBoolean(
R.styleable.Window_windowCloseOnTouchOutside,
false)) {
setCloseOnTouchOutsideIfNotSet(true);
}
}

WindowManager.LayoutParams params = getAttributes();

if (!hasSoftInputMode()) {
params.softInputMode = a.getInt(
R.styleable.Window_windowSoftInputMode,
params.softInputMode);
}

if (a.getBoolean(R.styleable.Window_backgroundDimEnabled,
mIsFloating)) {
/* All dialogs should have the window dimmed */
if ((getForcedWindowFlags()&WindowManager.LayoutParams.FLAG_DIM_BEHIND) == 0) {
params.flags |= WindowManager.LayoutParams.FLAG_DIM_BEHIND;
}
if (!haveDimAmount()) {
params.dimAmount = a.getFloat(
android.R.styleable.Window_backgroundDimAmount, 0.5f);
}
}
//是否设置Activity中的动画效果
if (params.windowAnimations == 0) {
params.windowAnimations = a.getResourceId(
R.styleable.Window_windowAnimationStyle, 0);
}

// The rest are only done if this window is not embedded; otherwise,
// the values are inherited from our container.
if (getContainer() == null) {
if (mBackgroundDrawable == null) {
if (mBackgroundResource == 0) {
mBackgroundResource = a.getResourceId(
R.styleable.Window_windowBackground, 0);
}
if (mFrameResource == 0) {
mFrameResource = a.getResourceId(R.styleable.Window_windowFrame, 0);
}
mBackgroundFallbackResource = a.getResourceId(
R.styleable.Window_windowBackgroundFallback, 0);
if (false) {
System.out.println("Background: "
+ Integer.toHexString(mBackgroundResource) + " Frame: "
+ Integer.toHexString(mFrameResource));
}
}
mElevation = a.getDimension(R.styleable.Window_windowElevation, 0);
mClipToOutline = a.getBoolean(R.styleable.Window_windowClipToOutline, false);
mTextColor = a.getColor(R.styleable.Window_textColor, Color.TRANSPARENT);
}

//根据属性来初始化不同的布局资源id
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;
} else if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogTitleIconsDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = R.layout.screen_title_icons;
}
// XXX Remove this once action bar supports these features.
removeFeature(FEATURE_ACTION_BAR);
// System.out.println("Title Icons!");
} else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0
&& (features & (1 << FEATURE_ACTION_BAR)) == 0) {
// Special case for a window with only a progress bar (and title).
// XXX Need to have a no-title version of embedded windows.
layoutResource = R.layout.screen_progress;
// System.out.println("Progress!");
} else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {
// Special case for a window with a custom title.
// If the window is floating, we need a dialog layout
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogCustomTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = R.layout.screen_custom_title;
}
// XXX Remove this once action bar supports these features.
removeFeature(FEATURE_ACTION_BAR);
} else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {
// If no other features and not embedded, only need a title.
// If the window is floating, we need a dialog layout
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {
layoutResource = a.getResourceId(
R.styleable.Window_windowActionBarFullscreenDecorLayout,
R.layout.screen_action_bar);
} else {
layoutResource = R.layout.screen_title;
}
// System.out.println("Title!");
} 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!");
}

mDecor.startChanging();
//通过布局添加器获取layoutResource的布局
View in = mLayoutInflater.inflate(layoutResource, null);
//将该布局添加到decor中，这里PhoneWindow中的DecorView就添加了布局
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
mContentRoot = (ViewGroup) in;
//通过R.id.content，找到contentParent 对象
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
if (contentParent == null) {
throw new RuntimeException("Window couldn't find content container view");
}

if ((features & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {
ProgressBar progress = getCircularProgressBar(false);
if (progress != null) {
progress.setIndeterminate(true);
}
}

if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
registerSwipeCallbacks();
}
//设置窗口背景
if (getContainer() == null) {
final Drawable background;
if (mBackgroundResource != 0) {
background = getContext().getDrawable(mBackgroundResource);
} else {
background = mBackgroundDrawable;
}
mDecor.setWindowBackground(background);

final Drawable frame;
if (mFrameResource != 0) {
frame = getContext().getDrawable(mFrameResource);
} else {
frame = null;
}
mDecor.setWindowFrame(frame);
mDecor.setElevation(mElevation);
mDecor.setClipToOutline(mClipToOutline);

if (mTitle != null) {
setTitle(mTitle);
}

if (mTitleColor == 0) {
mTitleColor = mTextColor;
}
setTitleColor(mTitleColor);
}
mDecor.finishChanging();
return contentParent;
}

上面的方法中可知，首先读取的Window的基本属性，包括是否设置状态栏，是否设置标题栏，是否设置状态栏和导航栏颜色等等。然后根据不同的features 来加载系统中对应的xml布局资源。并且将该布局资源填充到view中，然后将该View添加到mDecor里面。最后将布局文件中的R.id.content的view作为contentParent并返回。这样 mContentParent就获得了初始化对象，就是R.id.content对应的View。

这里我们在回到方法setContentView中，完成了 installDecor()方法后，我们接下来将执行mLayoutInflater.inflate(layoutResID, mContentParent)，通过mLayoutInflater将传进来的布局Id加载到父容器mContentParent中，到这里就将布局添加到Activity中了。

总结：

1.Activity中执行setContentView时，内部调用的是PhoneWindow中的setContentView方法。而PhoneWindow中的setContentView方法里，有分别调用了installDecor方法，mLayoutInflater.inflate(layoutResID, mContentParent)，和回调方法cb.onContentChanged()。

2.PhoneWindow中的installDecor方法，主要是初始化了DecorView对象和mContentParent父容器，让后设置窗口背景颜色和进入/退出是的动画属性。

3.installDecor方法中，执行的generateLayout(mDecor)方法，主要是设置系统默认属性，如：状态栏，设置标题栏，设置状态栏和导航栏颜色等等。然后选择加载系统中的布局文件到DecorView容器中，其中R.id.content的view作为mContentParent的父容器，然后返回mContentParent。

通过上面可以知道，如果在Activity中想要执行requestWindowFeature(Window.FEATURE_NO_TITLE)的方法，那么则需要在setContentView方法之前使用，否则无效。

那么在Activity加载完布局之后，所有显示的层级结构可以用下图表示（图片来自网络）：