前面我們說了Activity的啟動過程招狸，接著我們說一下view的繪制過程
我們都知道設(shè)置一個布局到屏幕是通過setContentView()來實現(xiàn)冯丙，當(dāng)然這一步只是在當(dāng)前屏幕添加一個子布局席赂，一些初始化的操作杯活，更早的是在創(chuàng)建Activity時設(shè)置的。
我們先來回顧一下當(dāng)時創(chuàng)建Activity的代碼谴咸，
ActivityThread#handleLaunchActivity

//   創(chuàng)建一個Activity對象
Activity a = performLaunchActivity(r, customIntent);
...
...
//  調(diào)用Activity的onResume方法   
handleResumeActivity(r.token, false, r.isForward,
                    !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);

在performLaunchActivity中，我們通過反射創(chuàng)建了一個Activity對象骗露，并調(diào)用了其attach（）和onCreate（）

  public void performLaunchActivity(){
                activity.attach(appContext, this, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstances, config,
                        r.referrer, r.voiceInteractor, window, r.configCallback);
  }
  window 參數(shù)是ActivityClientRecord中的岭佳，但是此時的 window == null；如下
  ActivityClientRecord r = new ActivityClientRecord();

            r.token = token;
            r.ident = ident;
            r.intent = intent;
            r.referrer = referrer;
            r.voiceInteractor = voiceInteractor;
            r.activityInfo = info;
            r.compatInfo = compatInfo;
            r.state = state;
            r.persistentState = persistentState;

            r.pendingResults = pendingResults;
            r.pendingIntents = pendingNewIntents;

            r.startsNotResumed = notResumed;
            r.isForward = isForward;

            r.profilerInfo = profilerInfo;

            r.overrideConfig = overrideConfig;
            updatePendingConfiguration(curConfig);

            sendMessage(H.LAUNCH_ACTIVITY, r);

1）window的初始化
我們來看下attach：

    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, String referrer, IVoiceInteractor voiceInteractor,
            Window window, ActivityConfigCallback activityConfigCallback) {
        attachBaseContext(context);

        mFragments.attachHost(null /*parent*/);
       //  創(chuàng)建了一個PhoneWindow對象萧锉，并將Activity與傳進來的window傳入了進去 
        mWindow = new PhoneWindow(this, window, activityConfigCallback);
       //  設(shè)置一些監(jiān)聽 
        mWindow.setWindowControllerCallback(this);
        mWindow.setCallback(this);
        mWindow.setOnWindowDismissedCallback(this);
        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);
        }
       // ....代碼省略
       //  為window設(shè)置WMS
        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;

        mWindow.setColorMode(info.colorMode);
    }

上面是在創(chuàng)建Activity的時候做的一些初始化的操作珊随，主要是創(chuàng)建一個Window對象，并為其關(guān)聯(lián)WMS柿隙。
我們接著來看onCreate()叶洞；
我們自己在重寫onCreate（）的時候會通過setContentView（）來設(shè)置一個布局，我們來看下

//  Activity#setContentView 
    public void setContentView(@LayoutRes int layoutResID) {
        //  getWindow返回的是我們上面創(chuàng)建PhoneWindow
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }
//  PhoneWindow#setContentView（）
   @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.
        //  我們在Activity#attach中創(chuàng)建PhoneWindow的時候禀崖，并沒有設(shè)置mContentParent衩辟，所以這里為空；
        //  當(dāng)Activity被異常銷毀而重新啟動的時候 此時的mContentParent  != null
        if (mContentParent == null) {
            installDecor();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

       //  這里跟動畫有關(guān)系波附，暫不多說
        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                    getContext());
            transitionTo(newScene);
        } else {
            //  將傳進來的布局id 加載到`內(nèi)容布局`中
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
        mContentParentExplicitlySet = true;
    }

FEATURE_CONTENT_TRANSITIONS其實我也不熟艺晴，但是一般都是為null的，因為這個是需要手動設(shè)置的
getWindow().requestFeature(Window.FEATURE_CONTENT_TRANSITIONS);
getWindow().setEnterTransition( new Explode() );
getWindow().setExitTransition( new Explode() );
setContentView(R.layout.layout_id);

我們先來看installDecor（）：

    private void installDecor() {
        mForceDecorInstall = false;
        if (mDecor == null) {
           //  這里 創(chuàng)建了一個DecorView對象
            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) {
            //  這里獲取到DecorView的content子view
            mContentParent = generateLayout(mDecor);

            // Set up decor part of UI to ignore fitsSystemWindows if appropriate.
            mDecor.makeOptionalFitsSystemWindows();
           //  這里是當(dāng)DecorView加載了ActionBar懸浮布局的時候 才會拿到這個對象掸屡，其他情況為null封寞，if就不考慮了
            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 {
               //  拿到titile對象
                mTitleView = findViewById(R.id.title);
                if (mTitleView != null) {
                    //  如果設(shè)置主題設(shè)置了NO_TITLE則將Title對象或是Title的父View設(shè)置為Gone，否則為其設(shè)置內(nèi)容
                    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 {
                          //  為title設(shè)置內(nèi)容 默認為項目名稱  
                        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.
            //  這是也是判斷是否設(shè)置了轉(zhuǎn)場動畫  
            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();
                    }
                }
              //  這里是解析轉(zhuǎn)場動畫  包括 進入 退出  共享元素等 
                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);
                }
            }
        }
    }

兩個方法generateDecor和generateLayout仅财，generateDecor主要是獲取一個DecorView對象钥星，generateLayout則是獲取DecorView中用來展示界面內(nèi)容的布局View，我們先來看generateDecor

//   PhoneWindow#generateDecor
    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();
        }
       // 主要是創(chuàng)建了DecorView 對象满着，DecorView本身繼承于FrameLayout
        return new DecorView(context, featureId, this, getAttributes());
    }

我們再來看下generateLayout：

    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 < R.styleable.Window.length; i++) {
                s = s + " " + Integer.toHexString(R.styleable.Window[i]) + "="
                        + a.getString(i);
            }
            System.out.println(s);
        }

        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);
        }
       // ....  代碼省略  1）解析主題的一些設(shè)置   比如懸浮谦炒、透明、全屏风喇、有無標題欄等等
        int layoutResource;
        int features = getLocalFeatures();
        // System.out.println("Features: 0x" + Integer.toHexString(features));
       //  下面就是根據(jù)前面的設(shè)置選擇相應(yīng)的根布局：有無標題欄宁改、標題欄是否懸浮等等
        if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
            //  如果設(shè)置了getWindow().requestFeature(Window.FEATURE_SWIPE_TO_DISMISS)會進入到這里，主要是實現(xiàn)右滑退出功能
            layoutResource = R.layout.screen_swipe_dismiss;
            setCloseOnSwipeEnabled(true);
        } 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 {
               //  自帶進度條以及標題圖標的layout
                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.
           //  帶進度條的layout
            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 {
           //   可以定制title的layout魂莫，主要是為title添加了一個父布局还蹲，可在父布局里面自加元素
                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,
                        //  帶ActionBar的布局
                        R.layout.screen_action_bar);
            } else {
                  //  也是為title添加了一個父布局
                layoutResource = R.layout.screen_title;
            }
            // System.out.println("Title!");
        } else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
              //ActionBar懸浮布局（其實也就是根布局是個FrameLayout，ActionBar懸浮在內(nèi)容布局之上）
            layoutResource = R.layout.screen_simple_overlay_action_mode;
        } else {
            // Embedded, so no decoration is needed.
            //  一般我們都是進入的這個判斷 
            layoutResource = R.layout.screen_simple;
            // System.out.println("Simple!");
        }

        mDecor.startChanging();
        //  這里通過addView將上面選擇布局添加到當(dāng)前DecorView之中
        mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
        //  獲取到內(nèi)容View
        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(contentParent);
        }

        // Remaining setup -- of background and title -- that only applies
        // to top-level windows.
        //  為DecorView初始化一些參數(shù)
        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();
        //  最終返回內(nèi)容布局
        return contentParent;
    }

//  screen_simple布局，內(nèi)容布局為 id = content的FrameLayout
<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>

2）上面就是對將要啟動的Activity的Window進行的一些初始化的操作谜喊，此時的界面還沒有顯示出來潭兽，下面我們來看調(diào)用Activity#onResume的時候做的操作

//  ActivityThread#handleResumeActivity:
    final void handleResumeActivity(IBinder token,
            boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
        ActivityClientRecord r = mActivities.get(token);
        if (!checkAndUpdateLifecycleSeq(seq, r, "resumeActivity")) {
            return;
        }

        // If we are getting ready to gc after going to the background, well
        // we are back active so skip it.
        unscheduleGcIdler();
        mSomeActivitiesChanged = true;

        // TODO Push resumeArgs into the activity for consideration
        //  這里調(diào)用Activity的onResume（）  但是過程是先從Activity集合中通過token獲取到一個ActivityClientRecord 對象，
        //  然后調(diào)用該對象里面的Activity類型的參數(shù)斗遏，使之進入onResume,此時的ActivityClientRecord 還沒有window對象（下面會用到）
        r = performResumeActivity(token, clearHide, reason);

        if (r != null) {
            final Activity a = r.activity;

            if (localLOGV) Slog.v(
                TAG, "Resume " + r + " started activity: " +
                a.mStartedActivity + ", hideForNow: " + r.hideForNow
                + ", finished: " + a.mFinished);

            final int forwardBit = isForward ?
                    WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0;

            // If the window hasn't yet been added to the window manager,
            // and this guy didn't finish itself or start another activity,
            // then go ahead and add the window.
            boolean willBeVisible = !a.mStartedActivity;
            if (!willBeVisible) {
                try {
                    willBeVisible = ActivityManager.getService().willActivityBeVisible(
                            a.getActivityToken());
                } catch (RemoteException e) {
                    throw e.rethrowFromSystemServer();
                }
            }
           //  上面說了 r.window == null
            if (r.window == null && !a.mFinished && willBeVisible) {
               //  為r.window進行賦值 
                r.window = r.activity.getWindow();
                View decor = r.window.getDecorView();
                decor.setVisibility(View.INVISIBLE);
                //  得到ViewManager對象（實際是WindowManagerImpl）
                ViewManager wm = a.getWindowManager();
                WindowManager.LayoutParams l = r.window.getAttributes();
                a.mDecor = decor;
                l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
                l.softInputMode |= forwardBit;
                //  r是否保存了window
                if (r.mPreserveWindow) {
                    a.mWindowAdded = true;
                    r.mPreserveWindow = false;
                    // Normally the ViewRoot sets up callbacks with the Activity
                    // in addView->ViewRootImpl#setView. If we are instead reusing
                    // the decor view we have to notify the view root that the
                    // callbacks may have changed.
                    ViewRootImpl impl = decor.getViewRootImpl();
                    if (impl != null) {
                        impl.notifyChildRebuilt();
                    }
                }
               //  activity是否準備顯示了
                if (a.mVisibleFromClient) {
                    //  是否與ViewManager進行了管理
                    if (!a.mWindowAdded) {
                        a.mWindowAdded = true;
                        //  將ViewManager與deocorView進行關(guān)聯(lián)
                        wm.addView(decor, l);
                    } else {
                        // The activity will get a callback for this {@link LayoutParams} change
                        // earlier. However, at that time the decor will not be set (this is set
                        // in this method), so no action will be taken. This call ensures the
                        // callback occurs with the decor set.
                        a.onWindowAttributesChanged(l);
                    }
                }

            // If the window has already been added, but during resume
            // we started another activity, then don't yet make the
            // window visible.
            } else if (!willBeVisible) {
                if (localLOGV) Slog.v(
                    TAG, "Launch " + r + " mStartedActivity set");
                r.hideForNow = true;
            }

            // Get rid of anything left hanging around.
            cleanUpPendingRemoveWindows(r, false /* force */);

            // The window is now visible if it has been added, we are not
            // simply finishing, and we are not starting another activity.
            if (!r.activity.mFinished && willBeVisible
                    && r.activity.mDecor != null && !r.hideForNow) {
                if (r.newConfig != null) {
                    performConfigurationChangedForActivity(r, r.newConfig);
                    if (DEBUG_CONFIGURATION) Slog.v(TAG, "Resuming activity "
                            + r.activityInfo.name + " with newConfig " + r.activity.mCurrentConfig);
                    r.newConfig = null;
                }
                if (localLOGV) Slog.v(TAG, "Resuming " + r + " with isForward="
                        + isForward);
                WindowManager.LayoutParams l = r.window.getAttributes();
                if ((l.softInputMode
                        & WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)
                        != forwardBit) {
                    l.softInputMode = (l.softInputMode
                            & (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION))
                            | forwardBit;
                    if (r.activity.mVisibleFromClient) {
                        ViewManager wm = a.getWindowManager();
                        View decor = r.window.getDecorView();
                        wm.updateViewLayout(decor, l);
                    }
                }

                r.activity.mVisibleFromServer = true;
                mNumVisibleActivities++;
                if (r.activity.mVisibleFromClient) {
                    r.activity.makeVisible();
                }
            }

            if (!r.onlyLocalRequest) {
                r.nextIdle = mNewActivities;
                mNewActivities = r;
                if (localLOGV) Slog.v(
                    TAG, "Scheduling idle handler for " + r);
                Looper.myQueue().addIdleHandler(new Idler());
            }
            r.onlyLocalRequest = false;

            // Tell the activity manager we have resumed.
            if (reallyResume) {
                try {
                    ActivityManager.getService().activityResumed(token);
                } catch (RemoteException ex) {
                    throw ex.rethrowFromSystemServer();
                }
            }

        } else {
            // If an exception was thrown when trying to resume, then
            // just end this activity.
            try {
                ActivityManager.getService()
                    .finishActivity(token, Activity.RESULT_CANCELED, null,
                            Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }
    }

3）
上面我們說到了創(chuàng)建了一個WindowManagerImpl對象山卦，調(diào)用了其addView（）將DecorView傳入了進去

private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
// WindowManagerImpl
   @Override
    public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
        applyDefaultToken(params);
        //mContext.getDisplay獲取屏幕信息  這里就不在展開說了
        mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
    }
// WindowManagerGlobal #addView
    public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
        if (view == null) {
            throw new IllegalArgumentException("view must not be null");
        }
        if (display == null) {
            throw new IllegalArgumentException("display must not be null");
        }
        if (!(params instanceof WindowManager.LayoutParams)) {
            throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
        }

        final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
        if (parentWindow != null) {
            parentWindow.adjustLayoutParamsForSubWindow(wparams);
        } else {
            // If there's no parent, then hardware acceleration for this view is
            // set from the application's hardware acceleration setting.
            final Context context = view.getContext();
            if (context != null
                    && (context.getApplicationInfo().flags
                            & ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
                wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
            }
        }

        ViewRootImpl root;
        View panelParentView = null;

        synchronized (mLock) {
            // Start watching for system property changes.
            if (mSystemPropertyUpdater == null) {
                mSystemPropertyUpdater = new Runnable() {
                    @Override public void run() {
                        synchronized (mLock) {
                            for (int i = mRoots.size() - 1; i >= 0; --i) {
                                mRoots.get(i).loadSystemProperties();
                            }
                        }
                    }
                };
                SystemProperties.addChangeCallback(mSystemPropertyUpdater);
            }

            int index = findViewLocked(view, false);
            if (index >= 0) {
                if (mDyingViews.contains(view)) {
                    // Don't wait for MSG_DIE to make it's way through root's queue.
                    mRoots.get(index).doDie();
                } else {
                    throw new IllegalStateException("View " + view
                            + " has already been added to the window manager.");
                }
                // The previous removeView() had not completed executing. Now it has.
            }

            // If this is a panel window, then find the window it is being
            // attached to for future reference.
            if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&
                    wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {
                final int count = mViews.size();
                for (int i = 0; i < count; i++) {
                    if (mRoots.get(i).mWindow.asBinder() == wparams.token) {
                        panelParentView = mViews.get(i);
                    }
                }
            }
            //  創(chuàng)建一個ViewRootImpl對象，view表示DecorView;
            root = new ViewRootImpl(view.getContext(), display);
           // wparams表示DecorView的LayoutParams诵次；
            view.setLayoutParams(wparams);
           //  下面的三局代碼账蓉，大致意思就是將布局對象、該布局的根節(jié)點以及LayoutParams保存起來
            mViews.add(view);
            mRoots.add(root);
            mParams.add(wparams);

            // do this last because it fires off messages to start doing things
            try {
                 // 最后調(diào)用這里
                root.setView(view, wparams, panelParentView);
            } catch (RuntimeException e) {
                // BadTokenException or InvalidDisplayException, clean up.
                if (index >= 0) {
                    removeViewLocked(index, true);
                }
                throw e;
            }
        }
    }

// ViewRootImpl#setView

   /**
     * We have one child
     */
    public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
        synchronized (this) {
            if (mView == null) {
                mView = view;
             //....  代碼省略
                mAdded = true;
                int res; /* = WindowManagerImpl.ADD_OKAY; */

                // Schedule the first layout -before- adding to the window
                // manager, to make sure we do the relayout before receiving
                // any other events from the system.
                //  這里很重要 
                requestLayout();
                if ((mWindowAttributes.inputFeatures
                        & WindowManager.LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) {
                    mInputChannel = new InputChannel();
                }
                mForceDecorViewVisibility = (mWindowAttributes.privateFlags
                        & PRIVATE_FLAG_FORCE_DECOR_VIEW_VISIBILITY) != 0;
                try {
                    mOrigWindowType = mWindowAttributes.type;
                    mAttachInfo.mRecomputeGlobalAttributes = true;
                    collectViewAttributes();
                    //  這里注意下
                    res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
                            getHostVisibility(), mDisplay.getDisplayId(),
                            mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
                            mAttachInfo.mOutsets, mInputChannel);
                } catch (RemoteException e) {
                    mAdded = false;
                    mView = null;
                    mAttachInfo.mRootView = null;
                    mInputChannel = null;
                    mFallbackEventHandler.setView(null);
                    unscheduleTraversals();
                    setAccessibilityFocus(null, null);
                    throw new RuntimeException("Adding window failed", e);
                } finally {
                    if (restore) {
                        attrs.restore();
                    }
                }
             //...  代碼省略 
             // 這里很重要逾一，這里是為view設(shè)置父view铸本，也就是為DecorView設(shè)置父view，也就是ViewRootImpl
              view.assignParent(this);
            //...  代碼省略  
            }
        }
    }

先說下mWindowSession遵堵，mWindowSessio是在ViewRootImpl的構(gòu)造器的創(chuàng)建的mWindowSession = WindowManagerGlobal.getWindowSession()箱玷，在getWindowSession中是通過WMS#openSession獲取到的一個IWindowSession的子類對象Session，
本篇就到這里了陌宿。
小結(jié)：
當(dāng)Activity被創(chuàng)建成果之后汪茧，在attach中獲取PhoneWindow對象以及WMS，然后在onCreate中設(shè)置我們自己的布局限番，并將此布局添加到DecorView中的內(nèi)容view中，之后在Activity的onResume之后呀舔，創(chuàng)建一個ViewRootImpl作為此布局的根布局弥虐，并將布局、跟布局以及LayoutParams保存下來媚赖，然后一方面執(zhí)行刷新界面一方面對界面進行展示霜瘪，也就是最后遺留的兩個問題（或者說是兩個方法）requestLayout和mWindowSession.addToDisplay