Setting breakpoints usually isn’t an exact science. While tracking down
a bug, you may observe that problems occur after clicking on a certain command
button. That doesn’t necessarily mean that the Click event code in the command
button is at fault, of course. You don’t have to wait to pin down the problem
more closely before setting a breakpoint, however. Because you know that the problem
occurs at some point after that procedure executes, go ahead and set a breakpoint
there so that you are at least that much closer to the problem when you are in
Break mode.
Because program execution is suspended during Break mode, you may wonder how
this helps you get closer to the bug. After all, you haven’t reached the
bug yet, and the program has stopped. How do you get to the problem?
Besides enabling you to watch variable and property values during Break mode,
VB also enables you to continue to execute the program on an incremental basis.
The process is typically called stepping through your code, and it enables you
to execute a program a line at a time. You may not be right at the point at which
a problem occurs, but you can step through your code to sneak up on the bug and
catch it in the act.
The following four stepping commands are available in Break mode:
The stepping commands are available in several ways: from the Debug menu, from
the keyboard via the F8 key (by itself, or in conjunction with the Shift, Control,
and Alt keys), or from the Debug toolbar—as shown in Figure 18.8. If the
Debug toolbar is not already visible, you can pull down the View menu and choose
Toolbars, where you can toggle it off and on. The Debug toolbar can be docked
or permitted to float.
FIGURE 18.8 The Debug toolbar.
Here is a simple example to illustrate how the step commands behave after a
program is in Break mode. Consider a project containing one form with a single
command button. The code looks like this:
Sub cmdCommand_Click()
Dim strName as String
Msgbox "After this message, break mode begins."
Stop
strName = MyFunction()
End Sub
Function MyFunction() as String
Dim strPrompt as String
strPrompt = "Please enter your name"
MyFunction = InputBox (strPrompt)
End Function
When you run this project and click on the command button, the program enters
Break mode when it reaches the Stop command. The code editor will appear with
the Stop command highlighted in the cmdCommand_Click function. Program execution
is suspended, and nothing else happens until you issue a step command.
Step Into
If you select Step Into, the highlight moves to the next line of code, where
strName is assigned the value returned by MyFunction. If you select Step Into
again, the code editor will display the code for MyFunction with the highlight
in its first line (recall that the highlighted line hasn’t executed yet;
it runs after you perform the next stepping action). If you continue to select
Step Into, you will see where the command gets its name: Step Into enables you
to step into any code that is called in your project, executing every procedure
a line at a time.
Step Over
Step Over is similar to Step Into. Within the current procedure, Step Over
continues to execute the code one line at a time. The difference is how it handles
calls to other procedures. If you run this project again and select Step Over
when you reach the strName = MyFunction() line in the Click event, you won’t
step into the MyFunction code. Instead, it runs all at once and you are returned
to the function that called it. When you aren’t interested in observing
the behavior of another procedure called from within the one you are debugging,
choose Step Over.
Step Out
What if you selected Step Into, only to realize that you really don’t
need to watch the execution of every step of the procedure you just stepped into?
No, you don’t have to lean on the F8 key to force the execution of every
line. That’s what Step Out is for; it causes the rest of the current procedure
to run and returns to the calling procedure. That is, you could select Step Out
as soon as you entered MyFunction, and you would find yourself back in the Click
event just after MyFunction had completed its work.
Step to Cursor
You may step into a procedure that only has a few lines that you want to watch
execute. If that happens, and there are a lot of lines from your point of entry
to the lines that interest you, you don’t need to wade through the boring
parts with Step Into or Step Over. Instead, move the cursor to the first line
you want to watch, and then select Step to Cursor. All the code will execute between
the line at which you originally entered Break mode and the line where you placed
the cursor, then you can begin to step through the interesting parts again.
Setting Stepping Options
You will also notice a couple of other options at the bottom of the Debug menu.
If you want to prevent a few lines of code from executing, but otherwise let your
program continue to execute normally, use Set Next Statement
While in Break mode, this enables you to move the cursor to the next line that
you want your program to execute. Because this skips the intervening lines, use
this feature carefully. If you inadvertently skip some lines that modify values
in your program, you may not catch the bug you are after. If you aren’t
sure what line is supposed to execute next, use Show Next Statement.
The Set Next statement option only works with lines of code that are inside
the currently running procedure. In other words, you can’t set the next
statement to a line of code in a different procedure from the currently running
procedure.
To exit Break mode and resume the normal operation of your program, choose
Continue from the Debug menu or toolbar. (Its keyboard equivalent is F5.) If you
have seen all that you need to see, you can also halt the program by choosing
End, which is available on the Debug menu or on the toolbar.
