Android中将xml布局文件转化为View树的过程分析（上）

    /**
     * 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(int layoutResID) { // 实际上其内部都是delegate给了getWindow()方法
        getWindow().setContentView(layoutResID);
        initActionBar();
    }

    /**
     * Set the activity content to an explicit view.  This view is placed
     * directly into the activity‘s view hierarchy.  It can itself be a complex
     * view hierarchy.  When calling this method, the layout parameters of the
     * specified view are ignored.  Both the width and the height of the view are
     * set by default to {@link ViewGroup.LayoutParams#MATCH_PARENT}. To use
     * your own layout parameters, invoke
     * {@link #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)}
     * instead.
     * 
     * @param view The desired content to display.
     *
     * @see #setContentView(int)
     * @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
     */
    public void setContentView(View view) {
        getWindow().setContentView(view);
        initActionBar();
    }

    /**
     * Set the activity content to an explicit view.  This view is placed
     * directly into the activity‘s view hierarchy.  It can itself be a complex
     * view hierarchy.
     * 
     * @param view The desired content to display.
     * @param params Layout parameters for the view.
     *
     * @see #setContentView(android.view.View)
     * @see #setContentView(int)
     */
    public void setContentView(View view, ViewGroup.LayoutParams params) {
        getWindow().setContentView(view, params);
        initActionBar();
    }

    /**
     * Add an additional content view to the activity.  Added after any existing
     * ones in the activity -- existing views are NOT removed.
     * 
     * @param view The desired content to display.
     * @param params Layout parameters for the view.
     */
    public void addContentView(View view, ViewGroup.LayoutParams params) {
        getWindow().addContentView(view, params);
        initActionBar();
    }

    private Window mWindow; // Activity的一个字段

    /**
     * Retrieve the current {@link android.view.Window} for the activity.
     * This can be used to directly access parts of the Window API that
     * are not available through Activity/Screen.
     * 
     * @return Window The current window, or null if the activity is not
     *         visual.
     */
    public Window getWindow() {
        return mWindow;
    }

    final void attach(Context context, ActivityThread aThread, Instrumentation instr, IBinder token,
            Application application, Intent intent, ActivityInfo info, CharSequence title, 
            Activity parent, String id, NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config) {
        attach(context, aThread, instr, token, 0, application, intent, info, title, parent, id,
            lastNonConfigurationInstances, config);
    }
    
    final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config) {
        attachBaseContext(context);

        mFragments.attachActivity(this, mContainer, null);
        
        mWindow = PolicyManager.makeNewWindow(this); // 注意这行代码，这里实际上创建了一个PhoneWindow的实例
        mWindow.setCallback(this); // window对象里的Callback接口的实现是Activity
        mWindow.getLayoutInflater().setPrivateFactory(this);
        if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) {
            mWindow.setSoftInputMode(info.softInputMode);
        }
        if (info.uiOptions != 0) {
            mWindow.setUiOptions(info.uiOptions);
        }
        mUiThread = Thread.currentThread();
        
        mMainThread = aThread;
        mInstrumentation = instr;
        mToken = token;
        mIdent = ident;
        mApplication = application;
        mIntent = intent;
        mComponent = intent.getComponent();
        mActivityInfo = info;
        mTitle = title;
        mParent = parent;
        mEmbeddedID = id;
        mLastNonConfigurationInstances = lastNonConfigurationInstances;

        mWindow.setWindowManager(
                (WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
                mToken, mComponent.flattenToString(),
                (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
        if (mParent != null) {
            mWindow.setContainer(mParent.getWindow());
        }
        mWindowManager = mWindow.getWindowManager();
        mCurrentConfig = config;
    }

    public final class PolicyManager {
    private static final String POLICY_IMPL_CLASS_NAME =
        "com.android.internal.policy.impl.Policy";

    private static final IPolicy sPolicy;

    static {
        // Pull in the actual implementation of the policy at run-time
        try {
            Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME); // 加载class文件
            sPolicy = (IPolicy)policyClass.newInstance(); // 根据Class对象，创建个实例
        } catch (ClassNotFoundException ex) {
            throw new RuntimeException(
                    POLICY_IMPL_CLASS_NAME + " could not be loaded", ex);
        } catch (InstantiationException ex) {
            throw new RuntimeException(
                    POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
        } catch (IllegalAccessException ex) {
            throw new RuntimeException(
                    POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
        }
    }

    // Cannot instantiate this class
    private PolicyManager() {}

    // The static methods to spawn new policy-specific objects
    public static Window makeNewWindow(Context context) {
        return sPolicy.makeNewWindow(context);
    }

    public static LayoutInflater makeNewLayoutInflater(Context context) {
        return sPolicy.makeNewLayoutInflater(context);
    }

    public static WindowManagerPolicy makeNewWindowManager() {
        return sPolicy.makeNewWindowManager();
    }

    public static FallbackEventHandler makeNewFallbackEventHandler(Context context) {
        return sPolicy.makeNewFallbackEventHandler(context);
    }
}

public class Policy implements IPolicy {
    private static final String TAG = "PhonePolicy";

    private static final String[] preload_classes = {
        "com.android.internal.policy.impl.PhoneLayoutInflater",
        "com.android.internal.policy.impl.PhoneWindow",
        "com.android.internal.policy.impl.PhoneWindow$1",
        "com.android.internal.policy.impl.PhoneWindow$DialogMenuCallback",
        "com.android.internal.policy.impl.PhoneWindow$DecorView",
        "com.android.internal.policy.impl.PhoneWindow$PanelFeatureState",
        "com.android.internal.policy.impl.PhoneWindow$PanelFeatureState$SavedState",
    };

    static {
        // For performance reasons, preload some policy specific classes when
        // the policy gets loaded.
        for (String s : preload_classes) {
            try {
                Class.forName(s); // 预加载这些类的class文件，以便后面new他们的对象
            } catch (ClassNotFoundException ex) {
                Log.e(TAG, "Could not preload class for phone policy: " + s);
            }
        }
    }

    public Window makeNewWindow(Context context) {
        return new PhoneWindow(context); // 至此我们看到了Android系统里真正且唯一的Window类型，PhoneWindow
    }

    public LayoutInflater makeNewLayoutInflater(Context context) {
        return new PhoneLayoutInflater(context); // LayoutInflater接口的实际实现者，以后我们的代码里出现的类似
    }                               // (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)
                                    // 这样的代码，返回的都是此对象。
    public WindowManagerPolicy makeNewWindowManager() {
        return new PhoneWindowManager();
    }

    public FallbackEventHandler makeNewFallbackEventHandler(Context context) {
        return new PhoneFallbackEventHandler(context);
    }
}

    // This is the top-level view of the window, containing the window decor.
    private DecorView mDecor; // window中的顶层view

    // 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.
    private ViewGroup mContentParent; // Android为我们提供的window布局文件中id=@android:id/content的view
                                      // 后面我们会看几个典型的window布局文件
    private LayoutInflater mLayoutInflater;

    public PhoneWindow(Context context) {
        super(context);
        mLayoutInflater = LayoutInflater.from(context); // 我们在上文中提到的通过context.getSystemService实现
    }

    @Override
    public void setContentView(int layoutResID) {
        if (mContentParent == null) { // 第一次调用的时候执行
            installDecor();           
        } else { // 可以看出setContentView支持多次调用，只是相当于把之前的view层次结构扔掉，从头再来而已
            mContentParent.removeAllViews();
        }
        mLayoutInflater.inflate(layoutResID, mContentParent); // 将我们dev提供的顶层layout文件加到mContentParent里面
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged(); // 调用回调函数，一般是Activity或Dialog
        }
    }

    @Override
    public void setContentView(View view) { // 默认的LayoutParams是MATCH_PARENT，当然你也可以指定
        setContentView(view, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
    }

    @Override
    public void setContentView(View view, ViewGroup.LayoutParams params) {
        if (mContentParent == null) {
            installDecor();
        } else {
            mContentParent.removeAllViews();
        }
        mContentParent.addView(view, params);
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }

    @Override
    public void addContentView(View view, ViewGroup.LayoutParams params) {
        if (mContentParent == null) { 
            installDecor();
        } // 注意相比setContentView来说，少了mContentParent.removeAllViews()调用，
          // 所以效果就是之前的view层次结构还在，只是新增了一个view
        mContentParent.addView(view, params);
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }

    private void installDecor() {
        if (mDecor == null) {
            mDecor = generateDecor(); // new一个DecorView（一种特殊的FrameLayout）
            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
            mDecor.setIsRootNamespace(true);
            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
            }
        }
        if (mContentParent == null) {
            mContentParent = generateLayout(mDecor); // 初始化mContentParent

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

            mTitleView = (TextView)findViewById(com.android.internal.R.id.title);
            if (mTitleView != null) {
                mTitleView.setLayoutDirection(mDecor.getLayoutDirection());
                if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {
                    View titleContainer = findViewById(com.android.internal.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);
                }
            } else {
                mActionBar = (ActionBarView) findViewById(com.android.internal.R.id.action_bar);
                if (mActionBar != null) {
                    mActionBar.setWindowCallback(getCallback());
                    if (mActionBar.getTitle() == null) {
                        mActionBar.setWindowTitle(mTitle);
                    }
                    final int localFeatures = getLocalFeatures();
                    if ((localFeatures & (1 << FEATURE_PROGRESS)) != 0) {
                        mActionBar.initProgress();
                    }
                    if ((localFeatures & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {
                        mActionBar.initIndeterminateProgress();
                    }

                    final ActionBarOverlayLayout abol = (ActionBarOverlayLayout) findViewById(
                            com.android.internal.R.id.action_bar_overlay_layout);
                    if (abol != null) {
                        abol.setOverlayMode(
                                (localFeatures & (1 << FEATURE_ACTION_BAR_OVERLAY)) != 0);
                    }

                    boolean splitActionBar = false;
                    final boolean splitWhenNarrow =
                            (mUiOptions & ActivityInfo.UIOPTION_SPLIT_ACTION_BAR_WHEN_NARROW) != 0;
                    if (splitWhenNarrow) {
                        splitActionBar = getContext().getResources().getBoolean(
                                com.android.internal.R.bool.split_action_bar_is_narrow);
                    } else {
                        splitActionBar = getWindowStyle().getBoolean(
                                com.android.internal.R.styleable.Window_windowSplitActionBar, false);
                    }
                    final ActionBarContainer splitView = (ActionBarContainer) findViewById(
                            com.android.internal.R.id.split_action_bar);
                    if (splitView != null) {
                        mActionBar.setSplitView(splitView);
                        mActionBar.setSplitActionBar(splitActionBar);
                        mActionBar.setSplitWhenNarrow(splitWhenNarrow);

                        final ActionBarContextView cab = (ActionBarContextView) findViewById(
                                com.android.internal.R.id.action_context_bar);
                        cab.setSplitView(splitView);
                        cab.setSplitActionBar(splitActionBar);
                        cab.setSplitWhenNarrow(splitWhenNarrow);
                    } else if (splitActionBar) {
                        Log.e(TAG, "Requested split action bar with " +
                                "incompatible window decor! Ignoring request.");
                    }

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

                    // Post the panel invalidate for later; avoid application onCreateOptionsMenu
                    // being called in the middle of onCreate or similar.
                    mDecor.post(new Runnable() {
                        public void run() {
                            // Invalidate if the panel menu hasn‘t been created before this.
                            PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
                            if (!isDestroyed() && (st == null || st.menu == null)) {
                                invalidatePanelMenu(FEATURE_ACTION_BAR);
                            }
                        }
                    });
                }
            }
        }
    }

    protected DecorView generateDecor() { // 我会在合适的时候专门分析下DecorView
        return new DecorView(getContext(), -1);
    }

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

        TypedArray a = getWindowStyle();

        if (false) {
            System.out.println("From style:");
            String s = "Attrs:";
            for (int i = 0; i < com.android.internal.R.styleable.Window.length; i++) {
                s = s + " " + Integer.toHexString(com.android.internal.R.styleable.Window[i]) + "="
                        + a.getString(i);
            }
            System.out.println(s);
        }
        // 接下来的一大堆代码都是从window的theme中获取属性值，然后调用相应的requestFeature或setFlags方法
        mIsFloating = a.getBoolean(com.android.internal.R.styleable.Window_windowIsFloating, false);
        int flagsToUpdate = (FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR)
                & (~getForcedWindowFlags());
        if (mIsFloating) { // 比如这里，如果是floating的（如dialog），则设置layout为WRAP_CONTENT，即非全屏
            setLayout(WRAP_CONTENT, WRAP_CONTENT);
            setFlags(0, flagsToUpdate);
        } else {
            setFlags(FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR, flagsToUpdate);
        }

        if (a.getBoolean(com.android.internal.R.styleable.Window_windowNoTitle, false)) {
            requestFeature(FEATURE_NO_TITLE);
        } else if (a.getBoolean(com.android.internal.R.styleable.Window_windowActionBar, false)) {
            // Don‘t allow an action bar if there is no title.
            requestFeature(FEATURE_ACTION_BAR);
        }

        if (a.getBoolean(com.android.internal.R.styleable.Window_windowActionBarOverlay, false)) {
            requestFeature(FEATURE_ACTION_BAR_OVERLAY);
        }

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

        if (a.getBoolean(com.android.internal.R.styleable.Window_windowFullscreen, false)) {
            setFlags(FLAG_FULLSCREEN, FLAG_FULLSCREEN & (~getForcedWindowFlags()));
        }

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

        if (a.getBoolean(com.android.internal.R.styleable.Window_windowTranslucentNavigation,
                false)) {
            setFlags(FLAG_TRANSLUCENT_NAVIGATION, FLAG_TRANSLUCENT_NAVIGATION
                    & (~getForcedWindowFlags()));
        }

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

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

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

        a.getValue(com.android.internal.R.styleable.Window_windowMinWidthMajor, mMinWidthMajor);
        a.getValue(com.android.internal.R.styleable.Window_windowMinWidthMinor, mMinWidthMinor);
        if (a.hasValue(com.android.internal.R.styleable.Window_windowFixedWidthMajor)) {
            if (mFixedWidthMajor == null) mFixedWidthMajor = new TypedValue();
            a.getValue(com.android.internal.R.styleable.Window_windowFixedWidthMajor,
                    mFixedWidthMajor);
        }
        if (a.hasValue(com.android.internal.R.styleable.Window_windowFixedWidthMinor)) {
            if (mFixedWidthMinor == null) mFixedWidthMinor = new TypedValue();
            a.getValue(com.android.internal.R.styleable.Window_windowFixedWidthMinor,
                    mFixedWidthMinor);
        }
        if (a.hasValue(com.android.internal.R.styleable.Window_windowFixedHeightMajor)) {
            if (mFixedHeightMajor == null) mFixedHeightMajor = new TypedValue();
            a.getValue(com.android.internal.R.styleable.Window_windowFixedHeightMajor,
                    mFixedHeightMajor);
        }
        if (a.hasValue(com.android.internal.R.styleable.Window_windowFixedHeightMinor)) {
            if (mFixedHeightMinor == null) mFixedHeightMinor = new TypedValue();
            a.getValue(com.android.internal.R.styleable.Window_windowFixedHeightMinor,
                    mFixedHeightMinor);
        }

        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 targetHcNeedsOptions = context.getResources().getBoolean(
                com.android.internal.R.bool.target_honeycomb_needs_options_menu);
        final boolean noActionBar = !hasFeature(FEATURE_ACTION_BAR) || hasFeature(FEATURE_NO_TITLE);

        if (targetPreHoneycomb || (targetPreIcs && targetHcNeedsOptions && noActionBar)) {
            addFlags(WindowManager.LayoutParams.FLAG_NEEDS_MENU_KEY);
        } else {
            clearFlags(WindowManager.LayoutParams.FLAG_NEEDS_MENU_KEY);
        }
        
        if (mAlwaysReadCloseOnTouchAttr || getContext().getApplicationInfo().targetSdkVersion
                >= android.os.Build.VERSION_CODES.HONEYCOMB) {
            if (a.getBoolean(
                    com.android.internal.R.styleable.Window_windowCloseOnTouchOutside,
                    false)) {
                setCloseOnTouchOutsideIfNotSet(true);
            }
        }
        
        WindowManager.LayoutParams params = getAttributes();

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

        if (a.getBoolean(com.android.internal.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);
            }
        }

        if (params.windowAnimations == 0) {
            params.windowAnimations = a.getResourceId(
                    com.android.internal.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(
                            com.android.internal.R.styleable.Window_windowBackground, 0);
                }
                if (mFrameResource == 0) {
                    mFrameResource = a.getResourceId(com.android.internal.R.styleable.Window_windowFrame, 0);
                }
                if (false) {
                    System.out.println("Background: "
                            + Integer.toHexString(mBackgroundResource) + " Frame: "
                            + Integer.toHexString(mFrameResource));
                }
            }
            mTextColor = a.getColor(com.android.internal.R.styleable.Window_textColor, 0xFF000000);
        }

        // Inflate the window decor.
        // 接下来就是根据设定好的features（即窗口风格属性）选择对应的xml文件
        int layoutResource;
        int features = getLocalFeatures();
        // System.out.println("Features: 0x" + Integer.toHexString(features));
        if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {
            if (mIsFloating) {
                TypedValue res = new TypedValue();
                getContext().getTheme().resolveAttribute(
                        com.android.internal.R.attr.dialogTitleIconsDecorLayout, res, true);
                layoutResource = res.resourceId;
            } else {
                layoutResource = com.android.internal.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 = com.android.internal.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(
                        com.android.internal.R.attr.dialogCustomTitleDecorLayout, res, true);
                layoutResource = res.resourceId;
            } else {
                layoutResource = com.android.internal.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(
                        com.android.internal.R.attr.dialogTitleDecorLayout, res, true);
                layoutResource = res.resourceId;
            } else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {
                layoutResource = com.android.internal.R.layout.screen_action_bar;
            } else {
                layoutResource = com.android.internal.R.layout.screen_title;
            }
            // System.out.println("Title!");
        } else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
            layoutResource = com.android.internal.R.layout.screen_simple_overlay_action_mode;
        } else {
            // Embedded, so no decoration is needed.
            layoutResource = com.android.internal.R.layout.screen_simple;
            // System.out.println("Simple!");
        }

        mDecor.startChanging(); // 回调点，表示开始。。。

        View in = mLayoutInflater.inflate(layoutResource, null); // 将选定的layout文件inflate成view
        decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT)); // 将其添加到decor中

        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT); // 找到系统layout文件中为我们客户端布局预留
        if (contentParent == null) {                                     // 的placeholder，我们的Activity布局将从这里开始。
            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);
            }
        }

        // Remaining setup -- of background and title -- that only applies
        // to top-level windows.
        if (getContainer() == null) {
            Drawable drawable = mBackgroundDrawable;
            if (mBackgroundResource != 0) {
                drawable = getContext().getResources().getDrawable(mBackgroundResource);
            }
            mDecor.setWindowBackground(drawable);
            drawable = null;
            if (mFrameResource != 0) {
                drawable = getContext().getResources().getDrawable(mFrameResource);
            }
            mDecor.setWindowFrame(drawable);

            // System.out.println("Text=" + Integer.toHexString(mTextColor) +
            // " Sel=" + Integer.toHexString(mTextSelectedColor) +
            // " Title=" + Integer.toHexString(mTitleColor));

            if (mTitleColor == 0) {
                mTitleColor = mTextColor;
            }

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

        mDecor.finishChanging(); // 回调点，表示结束了。。。

        return contentParent; // 返回客户端（Activity）布局的parent view
    }

<!-- screen_title.xml -->
<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" />
    <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" // 注意这里的共同点，这就是给客户端程序预留的placeholder，mContentParent view
        android:layout_width="match_parent" 
        android:layout_height="0dip"
        android:layout_weight="1"
        android:foregroundGravity="fill_horizontal|top"
        android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>

<!-- screen_simple.xml -->
<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" />
    <FrameLayout
         android:id="@android:id/content" // 共同点，id都是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>