// LetterboxUiController.java
void updateLetterboxSurface(WindowState winHint) {
    final WindowState w = mActivityRecord.findMainWindow();
    if (w != winHint && winHint != null && w != null) {
        return;
    }
    // 对界面四周需要显示的 Layer 进行位置计算
    layoutLetterbox(winHint);
    if (mLetterbox != null && mLetterbox.needsApplySurfaceChanges()) {
        // 对 Surface 执行创建、参数设置等操作
        mLetterbox.applySurfaceChanges(mActivityRecord.getSyncTransaction());
    }
}

void layoutLetterbox(WindowState winHint) {
    final WindowState w = mActivityRecord.findMainWindow();
    if (w == null || winHint != null && w != winHint) {
        return;
    }
    updateRoundedCorners(w);
    updateWallpaperForLetterbox(w);
    // 是否进入 Letterbox 模式的关键判断
    if (shouldShowLetterboxUi(w)) {
        if (mLetterbox == null) {
            // 把具体逻辑委托给 Letterbox 
            mLetterbox = new Letterbox(() -> mActivityRecord.makeChildSurface(null),
                    mActivityRecord.mWmService.mTransactionFactory,
                    mLetterboxConfiguration::isLetterboxActivityCornersRounded,
                    this::getLetterboxBackgroundColor,
                    this::hasWallpaperBackgroudForLetterbox,
                    this::getLetterboxWallpaperBlurRadius,
                    this::getLetterboxWallpaperDarkScrimAlpha);
            mLetterbox.attachInput(w);
        }
        mActivityRecord.getPosition(mTmpPoint);
        // Get the bounds of the "space-to-fill". The transformed bounds have the highest
        // priority because the activity is launched in a rotated environment. In multi-window
        // mode, the task-level represents this. In fullscreen-mode, the task container does
        // (since the orientation letterbox is also applied to the task).
        final Rect transformedBounds = mActivityRecord.getFixedRotationTransformDisplayBounds();
        final Rect spaceToFill = transformedBounds != null
                ? transformedBounds
                : mActivityRecord.inMultiWindowMode()
                        ? mActivityRecord.getRootTask().getBounds()
                        : mActivityRecord.getRootTask().getParent().getBounds();
        // 位置计算
        mLetterbox.layout(spaceToFill, w.getFrame(), mTmpPoint);
    } else if (mLetterbox != null) {
        mLetterbox.hide();
    }
}

// Letterbox.LetterboxSurface.java
public void applySurfaceChanges(SurfaceControl.Transaction t) {
    if (!needsApplySurfaceChanges()) {
        // Nothing changed.
        return;
    }
    mSurfaceFrameRelative.set(mLayoutFrameRelative);
    if (!mSurfaceFrameRelative.isEmpty()) {
        if (mSurface == null) {
            // 创建挂在 ActivityRecord 节点下的 Surface， 设置为 ColorLayer 类型
            createSurface(t);
        }
        // 设置颜色、位置、裁剪
        mColor = mColorSupplier.get();
        t.setColor(mSurface, getRgbColorArray());
        t.setPosition(mSurface, mSurfaceFrameRelative.left, mSurfaceFrameRelative.top);
        t.setWindowCrop(mSurface, mSurfaceFrameRelative.width(),
                mSurfaceFrameRelative.height());

// 对壁纸背景设置透明度和模糊度
        mHasWallpaperBackground = mHasWallpaperBackgroundSupplier.get();
        updateAlphaAndBlur(t);

t.show(mSurface);
    } else if (mSurface != null) {
        t.hide(mSurface);
    }
    if (mSurface != null && mInputInterceptor != null) {
        mInputInterceptor.updateTouchableRegion(mSurfaceFrameRelative);
        t.setInputWindowInfo(mSurface, mInputInterceptor.mWindowHandle);
    }
}