ASP.NET PDF Viewer using C#, VB/NET

Listing 11-1. The class declaration of the image IO plugin class TextImagePlugin : public QImageIOPlugin { public: TextImagePlugin(); ~TextImagePlugin(); QStringList keys() const; Capabilities capabilities( QIODevice *device, const QByteArray &format ) const; QImageIOHandler *create( QIODevice *device, const QByteArray &format = QByteArray() ) const; }; The most interesting of the methods is capabilities (shown in Listing 11-2), which determines what the plugin can do for a given format or device. This means that the format QByteArray must either contain the string ti or be empty for the plugin to be able to do anything with it. If the format QByteArray is empty, you must peek at the QIODevice. If it is open and writeable, you can always write to it. If it is readable, and the plugin can read from it (more about the static canRead method later on), you can read from it. It is important not to affect the device in any way (ensure that you are just peeking; not actually reading, writing, or seeking).

create barcodes in excel 2010, excel barcode generator add in free, barcode fonts for excel 2007, random barcode generator excel, how to create barcodes in excel 2016, barcode for excel 2010, no active barcode in excel 2007, barcode excel 2003 free download, 2d barcode font for excel, download barcode font for excel 2010,

}

Then, we can update the calling program, and strip out the code that deals with the return value (see Example 6-13).

static void Main(string[] args) { Turtle arthurTheTurtle = new Turtle { PlatformWidth = 0.0, PlatformHeight = 10.0, MotorSpeed = 5.0 }; ShowPosition(arthurTheTurtle); // We want to proceed forwards arthurTheTurtle.LeftMotorState = MotorState.Running; arthurTheTurtle.RightMotorState = MotorState.Running; // For two seconds arthurTheTurtle.RunFor(2.0); ShowPosition(arthurTheTurtle); // Now, let's rotate clockwise for a bit arthurTheTurtle.RightMotorState = MotorState.Reversed; // PI / 2 seconds should do the trick arthurTheTurtle.RunFor(Math.PI / 2.0); ShowPosition(arthurTheTurtle); // And let's go into reverse arthurTheTurtle.RightMotorState = MotorState.Reversed; arthurTheTurtle.LeftMotorState = MotorState.Reversed; // And run for 5 seconds arthurTheTurtle.RunFor(5); ShowPosition(arthurTheTurtle); // Then rotate back the other way arthurTheTurtle.RightMotorState = MotorState.Running; // And run for PI/4 seconds to give us 45 degrees arthurTheTurtle.RunFor(Math.PI / 4.0); ShowPosition(arthurTheTurtle); // And finally drive backwards for a bit arthurTheTurtle.RightMotorState = MotorState.Reversed; arthurTheTurtle.LeftMotorState = MotorState.Reversed; arthurTheTurtle.RunFor(Math.Cos(Math.PI / 4.0)); ShowPosition(arthurTheTurtle); } Console.ReadKey();

Note A QByteArray can be treated as Qt s controlled version of char*. You can use it to carry text just like a plain C string. Never use QString to do that (as you might have been doing with std::string) because it internally converts to Unicode, which might corrupt your binary data.

OK, what happens if you compile and run (make sure you press F5 or choose Debug Start Debugging so that you run in the debugger) Well, you drop very rapidly into the debugger, as you can see in Figure 6-3.

As the debugger implies, we ve broken in here because the exception is unhandled; but notice that we ve broken right at the point at which the exception actually occurred. Even if we hadn t provided some nice descriptive error text, we can clearly see why we failed, unlike with error codes, where by the time the debugger got involved, we had already lost track of the root cause of the failure. If we want an even closer look, we can click the View Detail link on the callout. This produces a dialog containing a property grid view of the exception object that was thrown. We can examine this to help us debug the problem. (You can see the Message and Data properties that we previously looked at, and I ve popped open the StackTrace for the exception, in the example in Figure 6-4.) That s already a huge improvement over the return value approach; but are there any obvious downsides to throwing an exception

Clicking the Security link on this screen will take you into the user configuration and security tool (see Figure 7-17).

Well, the first downside is that throwing an exception is way more expensive than simply returning a value. You don t really want to throw an exception just to manage your normal flow of control. Passing parameters and looking at internal state is always going to be a better choice for anything you might call the success path. That being said, expense is, of course, relative, and as usual, you should use the best tool for the job. Plus, exceptions are actually lower cost than return values if you don t actually throw them. In our previous example, we allocated and copied return values even if everything was OK. With the exception model, the success path is basically free.

Listing 11-2. Determining what the plugin can do with the given format and device QImageIOPlugin::Capabilities TextImagePlugin::capabilities( QIODevice *device, const QByteArray &format ) const { if( format == "ti" ) return (QImageIOPlugin::CanRead | QImageIOPlugin::CanWrite); if( !format.isEmpty() ) return 0;

The debate about when to use exceptions versus return values continues to rage in our industry. I don t expect it to let up anytime soon. As I said at the beginning of the chapter, it is almost like we re not really on top of the whole error-handling situation.

We ve seen what happens if we don t handle an exception that we throw (i.e., it percolates up until eventually we crash); and while that behavior is far more satisfactory than the situation when we ignored a return value, we would like to do much better. We want to handle the error and exit gracefully, as we did before.