Capture Screen For Multi Screens On Mac

There are many implementation about capture screen for mac.
A sample is in xcode sample code.
And mogutt also show code in https://github.com/mogutt/TTMacClient.
But on dual monitors mode, they are all not work,
even weixin's mac client and http://snip.qq.com/.
Only QQ is powerful and perfect.
And it's not open source.

So, I try to do the capture screen sample to support dual monitors.