一、Dialog 构造方法

    Dialog(@NonNull Context context, @StyleRes int themeResId, boolean createContextThemeWrapper) {
        if (createContextThemeWrapper) {
            if (themeResId == Resources.ID_NULL) {
                final TypedValue outValue = new TypedValue();
                context.getTheme().resolveAttribute(R.attr.dialogTheme, outValue, true);
                themeResId = outValue.resourceId;
            }
            mContext = new ContextThemeWrapper(context, themeResId);
        } else {
            mContext = context;
        }
        // 标注1️⃣
        mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
        // 标注2️⃣
        final Window w = new PhoneWindow(mContext);
        mWindow = w;
        w.setCallback(this);
        w.setOnWindowDismissedCallback(this);
        w.setOnWindowSwipeDismissedCallback(() -> {
            if (mCancelable) {
                cancel();
            }
        });
        w.setWindowManager(mWindowManager, null, null);
        w.setGravity(Gravity.CENTER);

        mListenersHandler = new ListenersHandler(this);
    }

标注1️⃣：从SystemService里面拿到WindowManager，实则是WindowManagerImpl;
标注2️⃣：初始PhoneWindow对象

二、Dialog 的show方法

    public void show() {
        if (mShowing) {
            if (mDecor != null) {
                if (mWindow.hasFeature(Window.FEATURE_ACTION_BAR)) {
                    mWindow.invalidatePanelMenu(Window.FEATURE_ACTION_BAR);
                }
                mDecor.setVisibility(View.VISIBLE);
            }
            return;
        }

        mCanceled = false;

        if (!mCreated) {
            dispatchOnCreate(null);
        } else {
            // Fill the DecorView in on any configuration changes that
            // may have occured while it was removed from the WindowManager.
            final Configuration config = mContext.getResources().getConfiguration();
            mWindow.getDecorView().dispatchConfigurationChanged(config);
        }

        onStart();
        // 标注1️⃣
        mDecor = mWindow.getDecorView();

        if (mActionBar == null && mWindow.hasFeature(Window.FEATURE_ACTION_BAR)) {
            final ApplicationInfo info = mContext.getApplicationInfo();
            mWindow.setDefaultIcon(info.icon);
            mWindow.setDefaultLogo(info.logo);
            mActionBar = new WindowDecorActionBar(this);
        }

        WindowManager.LayoutParams l = mWindow.getAttributes();
        boolean restoreSoftInputMode = false;
        if ((l.softInputMode
                & WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION) == 0) {
            l.softInputMode |=
                    WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION;
            restoreSoftInputMode = true;
        }
         // 标注2️⃣
        mWindowManager.addView(mDecor, l);
        if (restoreSoftInputMode) {
            l.softInputMode &=
                    ~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION;
        }

        mShowing = true;

        sendShowMessage();
    }

标注1️⃣： 从PhoneWindow中拿到DecorView；
标注2️⃣： 然后WindowManagerImpl把DecorView添加到addView()方法中。
最后会调用ViewRootImpl的SetView()方法把DecorView添加进去，然后进行绘制。

想要继续了解ViewRootImpl源码，往这里：ViewRootImpl源码解读