上一篇講了 setContentView 到所有布局的加載： 傳送門

要想了解DecorView 如何添加到Window里面的，需要先了解Activity如何運(yùn)行的
這里就要提到一個類ActivityThread，這里面的一個方法

  public Activity handleLaunchActivity(ActivityClientRecord r,
        PendingTransactionActions pendingActions, Intent customIntent) {
     // ... 
    // 這里可以看到啟動Activity
    final Activity a = performLaunchActivity(r, customIntent);
    // ...

    return a;
}


private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ActivityInfo aInfo = r.activityInfo;
   

    ContextImpl appContext = createBaseContextForActivity(r);
    Activity activity = null;
    try {
    // 這一步 創(chuàng)建了一個activity
        java.lang.ClassLoader cl = appContext.getClassLoader();
        activity = mInstrumentation.newActivity(
                cl, component.getClassName(), r.intent);
        StrictMode.incrementExpectedActivityCount(activity.getClass());
        r.intent.setExtrasClassLoader(cl);
        r.intent.prepareToEnterProcess();
        if (r.state != null) {
            r.state.setClassLoader(cl);
        }
    } catch (Exception e) {
        
    }

    try {
        Application app = r.packageInfo.makeApplication(false, mInstrumentation);
        if (activity != null) {
            // ....
            appContext.setOuterContext(activity);
            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,
                    r.assistToken);

            if (customIntent != null) {
                activity.mIntent = customIntent;
            }
            r.lastNonConfigurationInstances = null;
            checkAndBlockForNetworkAccess();
            activity.mStartedActivity = false;
            int theme = r.activityInfo.getThemeResource();
            if (theme != 0) {
                activity.setTheme(theme);
            }

            activity.mCalled = false;
            if (r.isPersistable()) {
             // 這里調(diào)用 activity的OnCreate方法
                mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
            } else {
                mInstrumentation.callActivityOnCreate(activity, r.state);
            }
            if (!activity.mCalled) {
                throw new SuperNotCalledException(
                    "Activity " + r.intent.getComponent().toShortString() +
                    " did not call through to super.onCreate()");
            }
            r.activity = activity;
        }
        r.setState(ON_CREATE);

        // updatePendingActivityConfiguration() reads from mActivities to update
        // ActivityClientRecord which runs in a different thread. Protect modifications to
        // mActivities to avoid race.
        synchronized (mResourcesManager) {
            mActivities.put(r.token, r);
        }

    } catch (SuperNotCalledException e) {
        throw e;

    } catch (Exception e) {
        if (!mInstrumentation.onException(activity, e)) {
            throw new RuntimeException(
                "Unable to start activity " + component
                + ": " + e.toString(), e);
        }
    }

    return activity;
}

其中比較關(guān)鍵的方法是調(diào)用是：
activity.attach(...)方法，這個方法內(nèi)容如下：

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, IBinder assistToken) {
    attachBaseContext(context);

    mFragments.attachHost(null /*parent*/);
    //  實例化PhoneWindow對象
    mWindow = new PhoneWindow(this, window, activityConfigCallback);
    mWindow.setWindowControllerCallback(this);
    mWindow.setCallback(this);
    mWindow.setOnWindowDismissedCallback(this);
    mWindow.getLayoutInflater().setPrivateFactory(this);
    if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) {
        // 設(shè)置輸入法
        mWindow.setSoftInputMode(info.softInputMode);
    }
    if (info.uiOptions != 0) {
        mWindow.setUiOptions(info.uiOptions);
    }
    mUiThread = Thread.currentThread();

    mMainThread = aThread;
    mInstrumentation = instr;
    mToken = token;
    mAssistToken = assistToken;
    mIdent = ident;
    mApplication = application;
    mIntent = intent;
    mReferrer = referrer;
    mComponent = intent.getComponent();
    mActivityInfo = info;
    mTitle = title;
    mParent = parent;
    mEmbeddedID = id;
    mLastNonConfigurationInstances = lastNonConfigurationInstances;
    if (voiceInteractor != null) {
        if (lastNonConfigurationInstances != null) {
            mVoiceInteractor = lastNonConfigurationInstances.voiceInteractor;
        } else {
            mVoiceInteractor = new VoiceInteractor(voiceInteractor, this, this,
                    Looper.myLooper());
        }
    }

     // 這里比較關(guān)鍵， 設(shè)置了WindowManager對象，實際上就是創(chuàng)建了WindowManagerImpl對象
    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);

    setAutofillOptions(application.getAutofillOptions());
    setContentCaptureOptions(application.getContentCaptureOptions());
}

隨后再來看看 ActivityThread類中的handleResumeActivity方法，方法內(nèi)容如下：

@Override
public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
        String reason) {
     // ...
    /// 這里調(diào)用了activity.performResume()--->mInstrumentation.callActivityOnResume(this)--->activity.onResume();
    final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
    if (r == null) {
        // We didn't actually resume the activity, so skipping any follow-up actions.
        return;
    }
     // ...

    final Activity a = r.activity;

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

    if (r.window == null && !a.mFinished && willBeVisible) {
        // 這里就是PhoneWindow對象，可以會看attach方法賦值過程
        r.window = r.activity.getWindow();
        // 拿到decorView對象
        View decor = r.window.getDecorView();
        // 設(shè)置隱藏
        decor.setVisibility(View.INVISIBLE);
        // 拿到WindowManagerImpl對象
        ViewManager wm = a.getWindowManager();
        WindowManager.LayoutParams l = r.window.getAttributes();
        a.mDecor = decor;
        l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
        l.softInputMode |= forwardBit;
        if (r.mPreserveWindow) {
            a.mWindowAdded = true;
            r.mPreserveWindow = false;
            ViewRootImpl impl = decor.getViewRootImpl();
            if (impl != null) {
                impl.notifyChildRebuilt();
            }
        }
        if (a.mVisibleFromClient) {
            if (!a.mWindowAdded) {
                a.mWindowAdded = true;
                // 這里就把decorView添加到WindowManagerImpl里面了
                wm.addView(decor, l);
            } else {
                a.onWindowAttributesChanged(l);
            }
        }

    } else if (!willBeVisible) {
        if (localLOGV) Slog.v(TAG, "Launch " + r + " mStartedActivity set");
        r.hideForNow = true;
    }

    if (!r.activity.mFinished && willBeVisible && r.activity.mDecor != null && !r.hideForNow) {
       
        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();
                // 這里是更新decor的view布局
                wm.updateViewLayout(decor, l);
            }
        }

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

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

繼續(xù)查看 WindowManagerImpl里面的addView方法

@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
    applyDefaultToken(params);
    mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}

public void addView(View view, ViewGroup.LayoutParams params,
        Display display, Window parentWindow) {
    //...
    final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
    if (parentWindow != null) {
        parentWindow.adjustLayoutParamsForSubWindow(wparams);
    } else {
        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) {
        // ...
        // 這里實例化了一個非常重要的類，所有的View 真正繪制都是在這個類里面執(zhí)行的
        root = new ViewRootImpl(view.getContext(), display);

        view.setLayoutParams(wparams);
        // 這里通過三個三個集合分別把 decorView 、root 、 params 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 {
            // 給ViewRootImpl 設(shè)置一個View，即decorView
            root.setView(view, wparams, panelParentView);
        } catch (RuntimeException e) {
            // BadTokenException or InvalidDisplayException, clean up.
            if (index >= 0) {
                removeViewLocked(index, true);
            }
            throw e;
        }
    }
}

繼續(xù)查看 ViewRootImpl類里面的setView方法，如下：

/**
 * We have one child
 *  root 只有一個子View
 */
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
    synchronized (this) {
    // 這里只有一個view
        if (mView == null) {
            mView = view;

            // .... 

            // 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.
            // 這個方法很眼熟，這里就是調(diào)用具體繪制的方法
            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();
                // 這里通過方法名稱可以看出是 添加顯示到設(shè)備的方法
                res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
                        getHostVisibility(), mDisplay.getDisplayId(), mTmpFrame,
                        mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
                        mAttachInfo.mOutsets, mAttachInfo.mDisplayCutout, mInputChannel,
                        mTempInsets);
                setFrame(mTmpFrame);
            } 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();
                }
            }
            // ...
            // 這個就是給decorView設(shè)置父級為 ViewRootImpl
            view.assignParent(this);
            // ...
        }
    }
}

到這里，decorView 添加到PhoneWindow 基本結(jié)束了。下面我們繼續(xù)查看，view 是如何在RootViewImpl類中繪制出來的。

查看ViewRootImpl類里面的 requestLayout()方法

@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {

        // 這個就是檢測當(dāng)前線程是否在主線程
        checkThread();
        mLayoutRequested = true;
        // 重點(diǎn)是這個方法
        scheduleTraversals();
    }
}

@UnsupportedAppUsage
void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        // 這里執(zhí)行了一個任務(wù)，mTraversalRunnable
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        if (!mUnbufferedInputDispatch) {
            // 計劃使用批處理輸入
            scheduleConsumeBatchedInput();
        }
        // 通知渲染器幀掛起
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

繼續(xù)查看 mTraversalRunnable

final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
        doTraversal();
    }
}

void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
        // 重點(diǎn)是這個方法
        performTraversals();
    }
}

 private void performTraversals() {
    // 這個就是我們的DecorView
    final View host = mView;
    //... 
    //頂層視圖DecorView所需要窗口的寬度和高度，因為我們必須要知道父容器的寬高才能測量子View的寬高。 
    int desiredWindowWidth;
    int desiredWindowHeight;
    mDisplay.getRealSize(size);
    desiredWindowWidth = size.x;
    desiredWindowHeight = size.y;
    // ... 
    else {
    // 這這里的寬高是是手機(jī)屏幕的寬高，見下面代碼
            DisplayMetrics packageMetrics =
                mView.getContext().getResources().getDisplayMetrics();
            desiredWindowWidth = packageMetrics.widthPixels;
            desiredWindowHeight = packageMetrics.heightPixels;
        }
     // .. 省略，大部分為初始化操作
     // Ask host how big it wants to be
     // 重點(diǎn)方法，通過注釋可以猜測，這里是測量布局所占用空間。
    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
    //  這個就是比較常見的 View 布局方法
    performLayout(lp, desiredWindowWidth, desiredWindowHeight);
    // 開始繪制方法
    performDraw();

}

繼續(xù)查看 performMeasure

 private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
    try {
        // 這里的mView 就是decorView (參看ViewRootImpl的setView() 方法 )
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

decorView 實際上繼承了 ViewGroup 、 View ，所以實際上也是調(diào)用的View的measure方法。所以這些 measure 、 layout 、 draw都是再ViewRootImpl發(fā)起的。

到這里，Activity 啟動過程中生成PhoneWindow , 并加載DecorView ，之后調(diào)起繪制流程。

總結(jié)：ActivityThread.handleResumeActivity中，wm.addView(decor, l);
把decorView 添加到windowManager , 然后在WindowManagerImpl 里面初始化了 ViewRootImpl ，然后調(diào)用 ViewRootImpl.setView()，在setview方法里調(diào)用了view.assignParent(this);,將Decorview的mParent設(shè)置成ViewRootImpl

image.png