1. Fragment状态保存和恢复

可以参考ViewPager的FragmentStatePagerAdapter适配器中的处理

// 保存
FragmentManager.saveFragmentInstanceState(fragment)
// 还原
Fragment.setInitialSavedState(SavedState)

2. App 黑白化引申

父 View 的 Canvas 和 Paint 是往下分发的.所以修改父View即可(DecorView)

//新建一支画笔
val paint = Paint()
//通过 ColorMatrix 将饱和度设置为 0
val cm = ColorMatrix()
cm.setSaturation(0f)
//将画笔的色彩饱和度设置为 0
paint.colorFilter = ColorMatrixColorFilter(cm)

• window.decorView.setLayerType(View.LAYER_TYPE_HARDWARE,paint)
• Application 中注册 registerActivityLifecycleCallbacks 回调，回调中通过 activity 实例同样可以拿到 DecorView

2.1 替换系统的 R.id.content 容器Fragment

public class GrayFrameLayout extends FrameLayout {
    private Paint mPaint = new Paint();

    public GrayFrameLayout(Context context, AttributeSet attrs) {
        super(context, attrs);

        ColorMatrix cm = new ColorMatrix();
        cm.setSaturation(0);
        mPaint.setColorFilter(new ColorMatrixColorFilter(cm));
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {
        canvas.saveLayer(null, mPaint, Canvas.ALL_SAVE_FLAG);
        super.dispatchDraw(canvas);
        canvas.restore();
    }


    @Override
    public void draw(Canvas canvas) {
        canvas.saveLayer(null, mPaint, Canvas.ALL_SAVE_FLAG);
        super.draw(canvas);
        canvas.restore();
    }
}
// BaseActivity
@Override
public View onCreateView(String name, Context context, AttributeSet attrs) {
    try {
        if ("FrameLayout".equals(name)) {
            int count = attrs.getAttributeCount();
            for (int i = 0; i < count; i++) {
                String attributeName = attrs.getAttributeName(i);
                String attributeValue = attrs.getAttributeValue(i);
                if (attributeName.equals("id")) {
                    int id = Integer.parseInt(attributeValue.substring(1));
                    String idVal = getResources().getResourceName(id);
                    if ("android:id/content".equals(idVal)) {
                        GrayFrameLayout grayFrameLayout = new GrayFrameLayout(context, attrs);
//                            grayFrameLayout.setWindow(getWindow());
                        return grayFrameLayout;
                    }
                }
            }
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return super.onCreateView(name, context, attrs);
}

2.2 Hook反射替换WindowManagerGlobal内的mViews容器

// 转载自U2tzJTNE
// https://juejin.cn/post/6892277675012915207
public static void enable(boolean enable) {
    try {
        //灰色调Paint
        final Paint mPaint = new Paint();
        ColorMatrix mColorMatrix = new ColorMatrix();
        mColorMatrix.setSaturation(enable ? 0 : 1);
        mPaint.setColorFilter(new ColorMatrixColorFilter(mColorMatrix));

        //反射获取windowManagerGlobal
        @SuppressLint("PrivateApi")
        Class<?> windowManagerGlobal = Class.forName("android.view.WindowManagerGlobal");
        @SuppressLint("DiscouragedPrivateApi")
        java.lang.reflect.Method getInstanceMethod = windowManagerGlobal.getDeclaredMethod("getInstance");
        getInstanceMethod.setAccessible(true);
        Object windowManagerGlobalInstance = getInstanceMethod.invoke(windowManagerGlobal);

        //反射获取mViews
        Field mViewsField = windowManagerGlobal.getDeclaredField("mViews");
        mViewsField.setAccessible(true);
        Object mViewsObject = mViewsField.get(windowManagerGlobalInstance);

        //创建具有数据感知能力的ObservableArrayList
        ObservableArrayList<View> observerArrayList = new ObservableArrayList<>();
        observerArrayList.addOnListChangedListener(new ObservableArrayList.OnListChangeListener() {
            @Override
            public void onChange(ArrayList list, int index, int count) {
            }

            @Override
            public void onAdd(ArrayList list, int start, int count) {
                // 拿到DecorView触发重绘
                View view = (View) list.get(start);
                if (view != null) {
                    view.setLayerType(View.LAYER_TYPE_HARDWARE, mPaint);
                }
            }

            @Override
            public void onRemove(ArrayList list, int start, int count) {
            }
        });
        //将原有的数据添加到新创建的list
        observerArrayList.addAll((ArrayList<View>) mViewsObject);
        //替换掉原有的mViews
        mViewsField.set(windowManagerGlobalInstance, observerArrayList);
    } catch (Exception e) {
        e.printStackTrace();
    }
}