.NET/ASP.NET/C#/VB.NET PDF Document SDK

So we finally spotted the problem, and stopped it from causing trouble. How do we find out what is wrong Well, first we should take a look at the error message. That tells us that it has something to do with rotating the turtle, which gives us a bit of a clue. The easiest way to see what is really going on, though, might be to set a breakpoint in our error handler and see what state the system is in when the error occurs. To set a breakpoint, we can put the cursor on the line where we want to break into the debugger, and press F9. Figure 6-1 shows the code with a breakpoint set.

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Figure 9-4. The same widget shows different tooltips for different parts. Let s get started with intercepting the right event and set the tooltip text for each of the four squares. All events are passed through the event method before some of them are distributed to the different handlers, such as the paintEvent, mouseMoveEvent, and keyPressEvent methods. Because there is no toolTipEvent method, you have to intercept the event in the event method. The source code for the interception is shown in Listing 9-5. Because the event method receives a QEvent object, you must use the type property to determine whether a ToolTip event was received. The QEvent class is the base class for all specialized event classes, so as soon as you can tell that you are dealing with a tooltip, you can cast the QEvent object into a QHelpEvent object.

If we run this now, the application will break into the debugger when we hit our error handler. If we press Ctrl-Alt-C, we can inspect the call stack to see where we went wrong, as shown in Figure 6-2.

Fill these out with details for a wiki user. This will add the user to the database, but it will not give them any permissions. You ll see a dialog box like the one in Figure 7-15 when you create a user.

As you can see, there s not an awful lot to help us; we lost context in which the error occurred because we returned out of the method that had the actual problem, and wound back up to our calling function. It isn t completely useless we now know which call had the problem (this time), so we can put a breakpoint on the relevant line and run again; but what if this was a hardto-reproduce, intermittent error We may have lost our one chance this week to identify and fix the problem! These are not the only problems with a return-value-based approach to error handling. What if we already need to use the return value on the method We re heading into the realm of magic values that mean an error has occurred, or we could add out or ref parameters to allow our method to return both a useful output and an error code. And what about property setters; we don t have the option of a return value, but we might well like to return an error of some kind if the value is out of range. If you re thinking surely there has to be a better way, you re right. C# (like most modern languages) supports an alternative means of signaling errors: exceptions.

Rather than return an error code from a method, we can instead throw an instance of any type derived from Exception. Let s rewrite our Rotate method to do that (see Example 6-11).

Image readers and writers work with QIODevice objects so an image can be read or written from, Tip and to network streams, memory buffers, files you name it because QIODevice is the base class of the classes that manage these interfaces.

private void Rotate(double duration) { if (PlatformWidth <= 0.0) { throw new InvalidOperationException( "The PlatformWidth must be initialized to a value > 0.0"); } // This is the total circumference of turning circle double circum = Math.PI * PlatformWidth; // This is the total distance traveled double d = duration * MotorSpeed; if (LeftMotorState == MotorState.Reversed)

{

}

} // So we've driven it this proportion of the way round double proportionOfWholeCircle = d / circum; // Once round is 360 degrees (or 2pi radians), so we have traveled CurrentOrientation = CurrentOrientation + (Math.PI * 2.0 * proportionOfWholeCircle); // return true; (This is now redunant, so you can delete it)

Both the reading and writing cases are shown in Figure 11-2. The figure also shows what part is Qt and what part is the plugin. The shown scenario is commonly used with Qt plugins. One is queried for what the plugin has to offer and then returns instances that perform the actual tasks. In the case of image plugins, the QImageIOPlugin is queried and returns a QImageIOHandler.

// And we're going backwards if the motors are reversed d *= 1.0;

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Notice that we changed the return specification back to void, and removed the unnecessary return at the end. The interesting bit, though, is in our test at the beginning of the method.