OpenCV is the most mature computer vision library in the world. Xcode is the best IDE on macOS. OpenCV is written in C++ and various bindings (first- and third-party) exist for most popular language. I prefer to work in C++. Although Xcode has excellent support for C++ projects, getting it to play nicely with OpenCV is rather tricky.
A window is an instance of the UIWindow class and it is the topmost element of any application UI's hierarchy. It doesn't draw any visual object and can be considered as a blank container for the UI elements called views. An application must have at least one window that normally fills the entire screen.
Applications on iOS and macOS are packaged differently from applications on other platform, most notably Windows.