深入解析Android中的setContentView加载布局原理

前言

对于Android的开发者来说，setContentView大家再熟悉不过了，在我们的Activity中首先就是要用它加载我们的布局，但是应该有一部分人是不知道加载布局的原理，也包括我，今天就从源码的角度分析setContentView加载布局原理。

准备工作

由于我们使用的Android API部分源码是隐藏的，当我们在AndroidStudio中是不能找到源码的，我们可以去官网下载相应源码去查看，当然在GitHub下载相应版本的API替换我们sdk下platforms相应api的android.jar。这样我们就可以在AndroidStudio查看到隐藏的api了，可以断点调试帮助我们阅读源码。

本篇文章分析源码是Android7.1（API25）。

Activiy setContentView源码分析

/**
 * Set the activity content from a layout resource. The resource will be
 * inflated, adding all top-level views to the activity.
 */
 public void setContentView(@LayoutRes int layoutResID) {
 getWindow().setContentView(layoutResID);
 initWindowDecorActionBar();
 }

在Activity中setContentView最终调用了getWindow()的setContentView·方法，getWindow()返回的是一个Window类，它表示一个窗口的概念，我们的Activity就是一个Window，Dialog和Toast也都是通过Window来展示的，这很好理解，它是一个抽象类，具体的实现是PhoneWindow，加载布局的相关逻辑都几乎都是它处理的。

@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;
 }

先判断mContentParent 是否为空，当然第一次启动时mContentParent 时为空的，然后执行installDecor();方法。

mContentParent不为空是通过hasFeature(FEATURE_CONTENT_TRANSITIONS)判断是否有转场动画，当没有的时候就把通过mContentParent.removeAllViews();移除mContentParent节点下的所有View.再通过inflate将我们的把布局填充到mContentParent，最后就是内容变化的回调。至于mContentParent 是什么东东，先留个悬念，稍后再说。

 private void installDecor() {
 mForceDecorInstall = false;
 if (mDecor == null) {
 mDecor = generateDecor(-1);
 mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
 mDecor.setIsRootNamespace(true);
 if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
 mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
 }
 } else {
 mDecor.setWindow(this);
 }
 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 {
 //设置标题
 mTitleView = (TextView) findViewById(R.id.title);
 if (mTitleView != null) {
  if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {
  final View titleContainer = findViewById(R.id.title_container);
  if (titleContainer != null) {
  titleContainer.setVisibility(View.GONE);
  } else {
  mTitleView.setVisibility(View.GONE);
  }
  mContentParent.setForeground(null);
  } else {
  mTitleView.setText(mTitle);
  }
 }
 }
 //......初始化属性变量
 }
 }

在上面的方法中主要工作就是初始化mDecor和mContentParent ，以及一些属性的初始化

 protected DecorView generateDecor(int featureId) {
 // System process doesn't have application context and in that case we need to directly use
 // the context we have. Otherwise we want the application context, so we don't cling to the
 // activity.
 Context context;
 if (mUseDecorContext) {
 Context applicationContext = getContext().getApplicationContext();
 if (applicationContext == null) {
 context = getContext();
 } else {
 context = new DecorContext(applicationContext, getContext().getResources());
 if (mTheme != -1) {
  context.setTheme(mTheme);
 }
 }
 } else {
 context = getContext();
 }
 return new DecorView(context, featureId, this, getAttributes());
 }

generateDecor初始化一个DecorView对象，DecorView继承了FrameLayout，是我们要显示布局的顶级View，我们看到的布局，标题栏都是它里面。

然后将mDecor作为参数调用generateLayout初始化mContetParent

 protected ViewGroup generateLayout(DecorView decor) {
 // Apply data from current theme.
 //获取主题样式
 TypedArray a = getWindowStyle();
 //......省略样式的设置
 // Inflate the window decor.
 int layoutResource;
 //获取feature并根据其来加载对应的xml布局文件
 int features = getLocalFeatures();
 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();
 mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);

 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();
 }

 // 给顶层窗口设置标题和背景
 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;
 }

代码较多，先通过getWindowStyle获取主题样式进行初始化，然后通过getLocalFeatures获取设置的不同features加载不同的布局，例如我们通常在Activity 加入requestWindowFeature(Window.FEATURE_NO_TITLE);来隐藏标题栏，不管根据Feature最终使用的是哪一种布局，里面都有一个android:id="@android:id/content"的FrameLayout，我们的布局文件就添加到这个FrameLayout中了。我们看一下一个简单的布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:orientation="vertical"
 android:fitsSystemWindows="true">
 <!-- Popout bar for action modes -->
 <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:layout_width="match_parent"
 android:layout_height="?android:attr/windowTitleSize"
 style="?android:attr/windowTitleBackgroundStyle">
 <TextView android:id="@android:id/title"
 style="?android:attr/windowTitleStyle"
 android:background="@null"
 android:fadingEdge="horizontal"
 android:gravity="center_vertical"
 android:layout_width="match_parent"
 android:layout_height="match_parent" />
 </FrameLayout>
 <FrameLayout android:id="@android:id/content"
 android:layout_width="match_parent"
 android:layout_height="0dip"
 android:layout_weight="1"
 android:foregroundGravity="fill_horizontal|top"
 android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>

通过上面的分析，你应该明白了requestWindowFeature为什么必须在setContentView之前设置了，如果在之后设置，那么通过上面的分析在setContentView执行时已经从本地读取features,而此时还没有设置，当然就无效了。

 ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
 public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;

通过上面findViewById获取该对象。不过在获取ViewGroup之前还有一个重要的方法

 void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
 mStackId = getStackId();

 if (mBackdropFrameRenderer != null) {
 loadBackgroundDrawablesIfNeeded();
 mBackdropFrameRenderer.onResourcesLoaded(
  this, mResizingBackgroundDrawable, mCaptionBackgroundDrawable,
  mUserCaptionBackgroundDrawable, getCurrentColor(mStatusColorViewState),
  getCurrentColor(mNavigationColorViewState));
 }

 mDecorCaptionView = createDecorCaptionView(inflater);
 final View root = inflater.inflate(layoutResource, null);
 if (mDecorCaptionView != null) {
 if (mDecorCaptionView.getParent() == null) {
 addView(mDecorCaptionView,
  new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
 }
 mDecorCaptionView.addView(root,
  new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));
 } else {

 // Put it below the color views.
 addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
 }
 mContentRoot = (ViewGroup) root;
 initializeElevation();
 }

这个比较好理解，root就是在上面判断的根据不同的features，加载的布局，然后将该布局通过addView添加到DecorView.到这里初始都成功了.

 mLayoutInflater.inflate(layoutResID, mContentParent);

在回到最初setContentView中的一句代码，如上，我们也就好理解了，它就是将我们的布局文件inflate到mContentParent中。到这里Activity的加载布局文件就完毕了。

AppCompatActivity的setContentView分析

由于AppCompatActivity的setContentView加载布局的与Activity有很多不同的地方，而且相对Activity稍微复杂点，在这里也简单分析一下。

 @Override
 public void setContentView(@LayoutRes int layoutResID) {
 getDelegate().setContentView(layoutResID);
 }

通过名字也就知道把加载布局交给了一个委托对象。

 @NonNull
 public AppCompatDelegate getDelegate() {
 if (mDelegate == null) {
 mDelegate = AppCompatDelegate.create(this, this);
 }
 return mDelegate;
 }

AppCompatDelegate时一个抽象类，如下图他有几个子类实现

为啥有那么多子类呢，其实通过名字我们也能猜到，是为了兼容。为了证明这点，我们看看create方法

 private static AppCompatDelegate create(Context context, Window window,
 AppCompatCallback callback) {
 final int sdk = Build.VERSION.SDK_INT;
 if (BuildCompat.isAtLeastN()) {
 return new AppCompatDelegateImplN(context, window, callback);
 } else if (sdk >= 23) {
 return new AppCompatDelegateImplV23(context, window, callback);
 } else if (sdk >= 14) {
 return new AppCompatDelegateImplV14(context, window, callback);
 } else if (sdk >= 11) {
 return new AppCompatDelegateImplV11(context, window, callback);
 } else {
 return new AppCompatDelegateImplV9(context, window, callback);
 }
 }

这里就很明显了，根据不同的API版本初始化不同的delegate。通过查看代码setContentView方法的实现是在AppCompatDelegateImplV9中

 @Override
 public void setContentView(int resId) {
 ensureSubDecor();
 ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
 contentParent.removeAllViews();
 LayoutInflater.from(mContext).inflate(resId, contentParent);
 mOriginalWindowCallback.onContentChanged();
 }

有了分析Activity的加载经验，我们就很容易明白contentParent和Activity中的mContentParent是一个东东，ensureSubDecor就是初始mSubDecor，然后removeAllViews，再将我们的布局填充到contentParent中。最后执行回调。

 private void ensureSubDecor() {
 if (!mSubDecorInstalled) {
  mSubDecor = createSubDecor();
  //省略部分代码
  onSubDecorInstalled(mSubDecor);
 }
 }
 private ViewGroup createSubDecor() {
 TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);

 //如果哦们不设置置AppCompat主题会报错，就是在这个地方
 if (!a.hasValue(R.styleable.AppCompatTheme_windowActionBar)) {
  a.recycle();
  throw new IllegalStateException(
   "You need to use a Theme.AppCompat theme (or descendant) with this activity.");
 }

 //省略..... 初始化一下属性
 ViewGroup subDecor = null;
 //PhtoWindowgetDecorView会调用installDecor，在Activity已经介绍过，主要工作就是初始化mDecor,mContentParent。
 mWindow.getDecorView();
 //省略
//根据设置加载不同的布局
 if (!mWindowNoTitle) {
  if (mIsFloating) {
  // If we're floating, inflate the dialog title decor
  subDecor = (ViewGroup) inflater.inflate(
   R.layout.abc_dialog_title_material, null);

  // Floating windows can never have an action bar, reset the flags
  mHasActionBar = mOverlayActionBar = false;
  } else if (mHasActionBar) {
  /**
   * This needs some explanation. As we can not use the android:theme attribute
   * pre-L, we emulate it by manually creating a LayoutInflater using a
   * ContextThemeWrapper pointing to actionBarTheme.
   */
  TypedValue outValue = new TypedValue();
  mContext.getTheme().resolveAttribute(R.attr.actionBarTheme, outValue, true);

  Context themedContext;
  if (outValue.resourceId != 0) {
   themedContext = new ContextThemeWrapper(mContext, outValue.resourceId);
  } else {
   themedContext = mContext;
  }

  // Now inflate the view using the themed context and set it as the content view
  subDecor = (ViewGroup) LayoutInflater.from(themedContext)
   .inflate(R.layout.abc_screen_toolbar, null);

  mDecorContentParent = (DecorContentParent) subDecor
   .findViewById(R.id.decor_content_parent);
  mDecorContentParent.setWindowCallback(getWindowCallback());

  /**
   * Propagate features to DecorContentParent
   */
  if (mOverlayActionBar) {
   mDecorContentParent.initFeature(FEATURE_SUPPORT_ACTION_BAR_OVERLAY);
  }
  if (mFeatureProgress) {
   mDecorContentParent.initFeature(Window.FEATURE_PROGRESS);
  }
  if (mFeatureIndeterminateProgress) {
   mDecorContentParent.initFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
  }
  }
 } else {
  if (mOverlayActionMode) {
  subDecor = (ViewGroup) inflater.inflate(
   R.layout.abc_screen_simple_overlay_action_mode, null);
  } else {
  subDecor = (ViewGroup) inflater.inflate(R.layout.abc_screen_simple, null);
  }

  if (Build.VERSION.SDK_INT >= 21) {
  // If we're running on L or above, we can rely on ViewCompat's
  // setOnApplyWindowInsetsListener
  ViewCompat.setOnApplyWindowInsetsListener(subDecor,
   new OnApplyWindowInsetsListener() {
    @Override
    public WindowInsetsCompat onApplyWindowInsets(View v,
     WindowInsetsCompat insets) {
    final int top = insets.getSystemWindowInsetTop();
    final int newTop = updateStatusGuard(top);

    if (top != newTop) {
     insets = insets.replaceSystemWindowInsets(
      insets.getSystemWindowInsetLeft(),
      newTop,
      insets.getSystemWindowInsetRight(),
      insets.getSystemWindowInsetBottom());
    }

    // Now apply the insets on our view
    return ViewCompat.onApplyWindowInsets(v, insets);
    }
   });
  } else {
  // Else, we need to use our own FitWindowsViewGroup handling
  ((FitWindowsViewGroup) subDecor).setOnFitSystemWindowsListener(
   new FitWindowsViewGroup.OnFitSystemWindowsListener() {
    @Override
    public void onFitSystemWindows(Rect insets) {
    insets.top = updateStatusGuard(insets.top);
    }
   });
  }
 }

 if (subDecor == null) {
  throw new IllegalArgumentException(
   "AppCompat does not support the current theme features: { "
    + "windowActionBar: " + mHasActionBar
    + ", windowActionBarOverlay: "+ mOverlayActionBar
    + ", android:windowIsFloating: " + mIsFloating
    + ", windowActionModeOverlay: " + mOverlayActionMode
    + ", windowNoTitle: " + mWindowNoTitle
    + " }");
 }

 if (mDecorContentParent == null) {
  mTitleView = (TextView) subDecor.findViewById(R.id.title);
 }

 // Make the decor optionally fit system windows, like the window's decor
 ViewUtils.makeOptionalFitsSystemWindows(subDecor);
 //contentView 是我们布局填充的地方
 final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
  R.id.action_bar_activity_content);
 //这个就是和我们Activity中的介绍的mDecor层级中的mContentParent是一个东西，
 final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);
 if (windowContentView != null) {
  // There might be Views already added to the Window's content view so we need to
  // migrate them to our content view
  while (windowContentView.getChildCount() > 0) {
  final View child = windowContentView.getChildAt(0);
  windowContentView.removeViewAt(0);
  contentView.addView(child);
  }

  // Change our content FrameLayout to use the android.R.id.content id.
  // Useful for fragments.
  //清除windowContentView的id
  windowContentView.setId(View.NO_ID);
  //将contentView的id设置成android.R.id.content，在此我们应该明白了，contentView 就成为了Activity中的mContentParent，我们的布局加载到这个view中。
  contentView.setId(android.R.id.content);

  // The decorContent may have a foreground drawable set (windowContentOverlay).
  // Remove this as we handle it ourselves
  if (windowContentView instanceof FrameLayout) {
  ((FrameLayout) windowContentView).setForeground(null);
  }
 }

 // Now set the Window's content view with the decor
 //将subDecor 填充到DecorView中
 mWindow.setContentView(subDecor);

 //省略部分代码
 return subDecor;
 }

上面的处理逻辑就是先初始化一些主题样式，然后通过mWindow.getDecorView()初始化DecorView.和布局，然后createSubDecor根据主题加载不同的布局subDecor，通过findViewById获取contentView（ AppCompat根据不同主题加载的布局中的View R.id.action_bar_activity_content）和windowContentView （
DecorView中的View android.R.id.content）控件。获取控件后将windowContentView 的id清空，并将 contentView的id由R.id.action_bar_activity_content更改为android.R.id.content。最后通过 mWindow.setContentView(subDecor);将subDecor添加到DecorView中。

//调用两个参数方法
 @Override
 public void setContentView(View view) {
 setContentView(view, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
 }
//此处处理和在Activity中分析的setContentView传资源ID进行加载布局是一样的，不同的是此时mContentParent 不为空，先removeAllViews（无转场动画情况）后再直接执行mContentParent.addView(view, params);即将subDecor添加到mContentParent
 @Override
 public void setContentView(View view, ViewGroup.LayoutParams params) {
 // 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)) {
  view.setLayoutParams(params);
  final Scene newScene = new Scene(mContentParent, view);
  transitionTo(newScene);
 } else {
  mContentParent.addView(view, params);
 }
 mContentParent.requestApplyInsets();
 final Callback cb = getCallback();
 if (cb != null && !isDestroyed()) {
  cb.onContentChanged();
 }
 mContentParentExplicitlySet = true;
 }

关于subDecor到底是什么布局，我们随便看一个布局R.layout.abc_screen_toolbar,有标题（mWindowNoTitle为false）并且有ActionBar(mHasActionBar 为true)的情况加载的布局。

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.ActionBarOverlayLayout
 xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:app="http://schemas.android.com/apk/res-auto"
 android:id="@+id/decor_content_parent"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:fitsSystemWindows="true">

 <include layout="@layout/abc_screen_content_include"/>

 <android.support.v7.widget.ActionBarContainer
  android:id="@+id/action_bar_container"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_alignParentTop="true"
  style="?attr/actionBarStyle"
  android:touchscreenBlocksFocus="true"
  android:gravity="top">

 <android.support.v7.widget.Toolbar
  android:id="@+id/action_bar"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  app:navigationContentDescription="@string/abc_action_bar_up_description"
  style="?attr/toolbarStyle"/>

 <android.support.v7.widget.ActionBarContextView
  android:id="@+id/action_context_bar"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:visibility="gone"
  android:theme="?attr/actionBarTheme"
  style="?attr/actionModeStyle"/>

 </android.support.v7.widget.ActionBarContainer>

</android.support.v7.widget.ActionBarOverlayLayout>

不管哪个主题下的布局，都会有一个id 为 abc_screen_content_include最好将id更改为androd.R,content，然后添加到mDecor中的mContentParent中。我们可以同SDK中tools下hierarchyviewer工具查看我们的布局层级结构。例如我们AppCompatActivity中setContentView传入的布局文件，是一个线程布局，该布局下有一个Button，则查看到层级结构

参考链接：http://www.weyye.me/detail/framework-appcompatactivity-setcontentview/

总结

以上就是这篇文章的全部内容了，到这里setContentView已经分析完毕，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，由于水平有限，难免有错误，若在阅读时发现不妥或者错误的地方留言指正，谢谢大家对我们的支持。