PS一句：当初你所逃避的问题终会在未来的某一天重新出现在你面前，因此，当你第一次遇到它时，请不要逃避。
相信很多初学者对XML布局怎么加载到Activity上并且显示在手机屏幕上很好奇吧？今天我们就从经常使用的方法
setContentView来从源码分析一下XML布局是怎么加载到当前Activity上的。
Activity#setContentView
我们知道，Activity是在onCreate方法中使用setContentView方法来加载布局的，那么它内部的源码是怎么实现的呢？
Setp 1
处于好奇，我们进入了Activity的源码，找到setContentView方法如下：
public void setContentView(int layoutResID) { getWindow().setContentView(layoutResID); initWindowDecorActionBar(); }setContentView方法实现很简单，里面调用两个方法，我们这里分析第一个方法：
getWindow().setContentView(layoutResID);getWindow()得到一个Window对象 mWindow ，Window是一个抽象类，用来描述Activity视图最顶端的窗口显示和行为操作。Window是
一个抽象类，那么里面的setContentView(layoutResID)是一个抽象方法，并没有具体的实现，既然Window是一个抽象类，那么
在Activity里面就有一个Window抽象类的实现，我们查找代码发现 mWindow对象赋值方法如下：
mWindow = PolicyManager.makeNewWindow(this);Step 2
Look the fuck resoure code，继续看看 PolicyManager类
public final class More PolicyManager {30 private static final String POLICY_IMPL_CLASS_NAME =31 "com.android.internal.policy.impl.Policy";3233 private static final IPolicy sPolicy;3435 static {36 // Pull in the actual implementation of the policy at run-time37 try {38 Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);39 sPolicy = (IPolicy)policyClass.newInstance();40 } catch (ClassNotFoundException ex) {41 throw new RuntimeException(42 POLICY_IMPL_CLASS_NAME + " could not be loaded", ex);43 } catch (InstantiationException ex) {44 throw new RuntimeException(45 POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);46 } catch (IllegalAccessException ex) {47 throw new RuntimeException(48 POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);49 }50 }5152 // Cannot instantiate this class53 private More PolicyManager() {}5455 // The static methods to spawn new policy-specific objects56 public static Window More makeNewWindow(Context context) {57 return sPolicy.makeNewWindow(context);58 }地 57 行 sPolicy对象是有第 38，39行通过类路径”com.android.internal.policy.impl.Policy“生成的，那么我们在源码中找到 Policy类，在此类中找到了如下方法：
public Window makeNewWindow(Context context) { return new PhoneWindow(context); }由此可见，我们终于找到Activity类中的 mWindow对象的实现类了，就是PhoneWindow类。
Setp 3
从源码发现，PhoneWindow类就是抽象类Window的实现类。那么 setContentView方法的实现也在这个类里面了，
到PhoneWindow类发现，setContentView方法实现如下：
@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); } final Callback cb = getCallback(); if (cb != null && !isDestroyed()) { cb.onContentChanged(); } }首先判断mContentParent是否为null，mContentParent是什么呢？接下来会分析，一开始条件成立，进入installDecor()方法。
Setp 4
到installDecor()方法里，实现如下：
private void installDecor() { if (mDecor == null) { mDecor = generateDecor(); mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS); mDecor.setIsRootNamespace(true); if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) { mDecor.postOnAnimation(mInvalidatePanelMenuRunnable); } } 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)) { 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); } } } }在代码的第 3 行我们看到 mDecor = generateDecor();方法调用，继续跳进 generateDecor()方法：
protected DecorView generateDecor() { return new DecorView(getContext(), -1); }这里生成一个DecorView对象，DecorView是PhoneWindow类的内部类，继承自FrameLayout。到目前为止，setContentView
方法里生成一个FrameLayout类型的DecorView组件。
Step 5
继续分析代码，看第 11 行：
mContentParent = generateLayout(mDecor);把 DecorView 对象 mDecor 作为参数传递给 generateLayout方法得到 mContentParent。generateLayout()方法中的代码实现如下：
protected ViewGroup generateLayout(DecorView decor) { // Apply data from current theme. TypedArray a = getWindowStyle(); ......... /**以下这些是Activity 窗口属性特征的设置*/ //窗口是否浮动，一般用于Dialog窗口是否浮动：是否显示在布局的正中间。 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); } //设置窗口是否支持标题栏，隐藏显示标题栏操作在此处。 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); } //当前Activity是否支持全屏 if (a.getBoolean(R.styleable.Window_windowFullscreen, false)) { setFlags(FLAG_FULLSCREEN, FLAG_FULLSCREEN & (~getForcedWindowFlags())); } ................ //设置状态栏的颜色 if (!mForcedStatusBarColor) { mStatusBarColor = a.getColor(R.styleable.Window_statusBarColor, 0xFF000000); } //设置导航栏的颜色 if (!mForcedNavigationBarColor) { mNavigationBarColor = a.getColor(R.styleable.Window_navigationBarColor, 0xFF000000); } 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(); //设置输入法的状态 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); } } //设置当前Activity的出现动画效果 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); } //以下代码为当前Activity窗口添加 decor根布局。 // 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; } 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LayoutInflater获取layoutResource布局， View in = mLayoutInflater.inflate(layoutResource, null); //将XML资源为layoutResource的布局添加到decor容器里面，至此PhoneWindow 内部类DecorView就添加了之布局 decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT)); mContentRoot = (ViewGroup) in; //此处很重要，通过findViewById找到 contentParent容器，也是该方法的返回值。 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(); } // Remaining setup -- of background and title -- that only applies // to top-level windows. //以下代码设置Activity窗口的背景，标题等 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; }以上代码代比较复杂，主要做了以下几件事情。
- 从第 8 行到第110行，主要是初始化窗口的特征，是否显示标题栏，是否全屏，是否支持ActionBar浮动等等。
- 第112-187行，主要是给容器DecorView添加id为layoutResource布局的根布局。待会分析layoutResource布局。
- 第178行，通过LayoutInflater 将xml布局转换成VIEW.
- 第180行将找到的View添加到DecorView根布局当中。
- 第184行，从根布局中找到id为R.id.content的 contentParent 容器。也就是当前方法的返回值。
接下来，看看 id为layoutResource的布局到底实现了什么？我们来看看171行的R.layout.screen_simple;资源
<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>原来我们的DecorView根布局里面添加了类似上面的布局，线性布局LinearLaout里包含两个组件，ViewStub是懒加载，默认不显示，FrameLayout是什么呢？看看id=content，就是我们184行找到的父容器 contentParent。那么这个父容器 contentParent有什么作用呢？
我们回到 Step4 的第 11行，mContentParent = generateLayout(mDecor); 获得 父容器 mContentParent。我们再次回到 Step3步的第17行， mLayoutInflater.inflate(layoutResID, mContentParent); 这里通过LayoutInflater将 setContentView（layoutResID）传进来的布局id加载到 父容器mContentParent中，至此，setContentView就将布局添加到Activity里面了。
现在我们来梳理一下流程：
Activity setContentView—>Window setContentView—>PhoneWindow setContentView—->PhoneWindow installDecor—–>PhoneWindow generateLayout——>PhoneWindow mLayoutInflater.inflate(layoutResID, mContentParent);
Activity 类中有一个Window抽象类的实现PhoneWindow类，该类中有个内部类DecorView，继承自FrameLayout，在DecorView容器中添加了根布局，根布局中包含了一个id为 contnet的FrameLayout 内容布局，我们的Activity加载的布局xml最后添加到 id为content的FrameLayout布局当中了。用一个图来描述，如下：
总结：
1.关于requestWindowFeature(Window.FEATURE_NO_TITLE); 去除标题栏的疑问，如果你自己的xxxActivity是继承自Activity，那么恭喜你使用以上方法可以去除标题栏，如果你自己的xxxActivity是继承自AppCompatActivity或者ActionBarActivity，那么很遗憾告诉你，此次系统默认的标题栏已经在主题中去除，此时显示的标题栏是ActionBar导航栏，如果需要去除导航栏，你可以通过如下代码：getSupportActionBar().hide();来隐藏导航栏。
2.requestWindowFeature(Window.FEATURE_NO_TITLE);方法需要在 setContentView方法之前使用，由上面 Step5分析可得，设置Activity Window 特征是在setContentView方法中设置的，因此，如果需要改变Activity Window窗口特征，需要在setContentView方法之前。其实这里有疑问？？？为什么设置全屏的方法
getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);可以在setContentView之后呢？？？求解。
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