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square of the distance for the initial velocity. I experimented with some numbers and, as I mentioned
earlier, 700 seemed to work.
Building the application and making it your own
Lets now take a look at the code for the basic firing of a cannonball, without a cannon, based on
horizontal and initial vertical speeds; the firing of a cannonball from a cannon, based on angle and initial
speed out of the cannon; and the slingshot, based on angle and initial speed determined from the position
of the mouse. As in previous chapters, Ill present the functions and what they call or are called by for
each application. In this case, the tables are similar, though not identical, for all three applications. The
calling is more varied than previous examples in that there are situations in which functions are invoked
because they are named as methods of a programmer-defined object or as part of a declaration ( var )
statement. This is a characteristic of object-oriented, event-driven programming . Ill also present the
complete code for each application in its own table, along with an explanation of what each line does.
Table 4-1 shows the functions for the basic cannonball application.
Table 4-1. Functions in the Simplest Cannonball Application
Function
Invoked By / Called By
Calls
init
Action of the onLoad in body tag
drawall
drawall
Invoked directly by init ,
fire ,
change
Calls the draw method of all objects in the
everything array. These are the functions
drawball , drawrects .
fire
Invoked by action of the onSubmit attribute
in form
drawall
change
Invoked by action of the setInterval
function called in fire
drawall , calls the moveit method of
cball , which is moveball
Ball
Invoked directly by code in a var statement
Myrectangle Invoked directly by code in a var statement
drawball
Invoked by call of the draw method for the
one Ball object
drawrects
Invoked by call of the draw method for the
target object
moveball
Invoked by call of the moveit method for the
one Ball object
 
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