Android中setContentView加载布局的原理是什么？相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。
Activiy setContentView源码分析
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 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，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" > <TextView android:id="@android:id/title" 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) { 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()); 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（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" 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" /> <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" /> </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
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