The problem I refer to someone else's blog: https://www.cnblogs.com/jdemarryme/p/8806752.html
View class handler added message, recognizes mouse clicks, loosening and movement.
line objects in the view class party, the line is used to draw the line.
(Here's message handler, calls the method of the object we created the line, a line drawing.)
Cline class created will deal with how to draw the line, save the starting and ending points, determine the slope, draw lines, all in one.