如何在拍摄照片后立即在预览中裁剪图像？

I am having a headache of croping users' photos after they have picked up an image as profile pic. Do anyone has ideas how I can make it, just like what Facebook does, that the image is dimmed except a pre-determined square which users can move it to crop/select which part they want. I am having a headache of croping users' photos