在多线程中操作VCL对象时，需Synchronize到主线程，否则可能引起“Canvas does not allow drawing”异常。这是一个的线程同步例子，同时放了2种代码，可对照理解Synchronize的作用。-It s an example about how to operate VCL object in multithread. Use "Synchronize" To avoid "Canvas does not allow drawing" exception.

Can put your desktop as a canvas to the program