Simple .NET/ASP.NET PDF document editor web control SDK

We don t just have to use streaming APIs for reading. We can write to the stream, too. One very common programming task is to copy data from one stream to another. We use this kind of thing all the time copying data, or concatenating the content of several files into another, for example. (If you want to copy an entire file, you d use File.Copy, but streams give you the flexibility to concatenate or modify data, or to work with nonfile sources.) Example 11-46 shows how to read data from one stream and write it into another. This is just for illustrative purposes .NET 4 added a new CopyTo method to Stream which does this for you. In practice you d need Example 11-46 only if you were targeting an older version of the .NET Framework, but it s a good way to see how to write to a stream.

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private static void WriteTo(Stream source, Stream target, int bufferLength) { bufferLength = Math.Max(100, bufferLength); var buffer = new byte[bufferLength]; int bytesRead; do {

private: Ui::HttpDialog ui;

}

bytesRead = source.Read(buffer, 0, buffer.Length); if (bytesRead != 0) { target.Write(buffer, 0, bytesRead); } } while (bytesRead > 0);

We create a buffer which is at least 100 bytes long. We then Read from the source and Write to the target, using the buffer as the intermediary. Notice that the Write method takes the same parameters as the read: the buffer, an offset into that buffer, and the number of bytes to write (which in this case is the number of bytes read from the source buffer, hence the slightly confusing variable name). As with Read, it steadily advances the current position in the stream as it writes, just like that ticker tape. Unlike Read, Write will always process as many bytes as we ask it to, so with Write, there s no need to keep looping round until it has written all the data. Obviously, we need to keep looping until we ve read everything from the source stream. Notice that we keep going until Read returns 0. This is how streams indicate that we ve reached the end. (Some streams don t know in advance how large they are, so you can rely on the Length property for only certain kinds of streams such as FileStream. Testing for a return value of 0 is the most general way to know that we ve reached the end.)

So, we ve seen how to read and write data to and from streams, and how we can move the current position in the stream by seeking to some offset from a known position. Up until now, we ve been using the File.OpenRead and File.OpenWrite methods to create our file streams. There is another method, File.Open, which gives us access to some extra features. The simplest overload takes two parameters: a string which is the path for the file, and a value from the FileMode enumeration. What s the FileMode Well, it lets us specify exactly what we want done to the file when we open it. Table 11-6 shows the values available.

Listing 9-13. The constructor of the wizard Wizard::Wizard() : QDialog() { QGridLayout *layout = new QGridLayout( this ); QPushButton *cancel = new QPushButton( tr("Cancel") ); next = new QPushButton( tr("Next") ); previous = new QPushButton( tr("Previous" ) ); pages = new QStackedWidget; layout->addWidget( pages, 0, 0, 1, 5 ); layout->setColumnMinimumWidth( 0, 50 ); layout->addWidget( previous, 1, 1 ); layout->addWidget( next, 1, 2 ); layout->setColumnMinimumWidth( 3, 5 ); layout->addWidget( cancel, 1, 4 ); previous->setEnabled( false ); next->setEnabled( false ); connect( next, SIGNAL(clicked()), this, SLOT(doNext()) ); connect( previous, SIGNAL(clicked()), this, SLOT(doPrev()) ); connect( cancel, SIGNAL(clicked()), this, SLOT(reject()) ); pages->addWidget( pageOne = new PageOne( pages ) ); pages->addWidget( pageTwo = new PageTwo( pages ) ); pages->addWidget( pageThree = new PageThree( pages ) ); connect( pageOne->acceptDeal, SIGNAL(toggled(bool)), next, SLOT(setEnabled(bool)) ); } When the user has checked the box and clicked the Next button, the dialog shown in Figure 9-14 is displayed. There are a number of things to deal with when the next button is clicked: the enabled property of the Next button is no longer depending on the state of the checkbox, the Previous button needs to be enabled, and you mustn t forget to show the next page. All this is managed in the doNext slot.

Purpose Creates a brand new file. Throws an exception if it already existed. Creates a new file, deleting any existing file and overwriting it if necessary. Opens an existing file, seeking to the beginning by default. Throws an exception if the file does not exist. Opens an existing file, or creates a new file if it doesn t exist. Opens an existing file, and deletes all its contents. The file is automatically opened for writing only. Opens an existing file and seeks to the end of the file. The file is automatically opened for writing only. You can seek in the file, but only within any information you ve appended you can t touch the existing content.

improve the user experience. In this case, the map you are viewing consists of a number of tiles. As you are viewing the map surface, the tiles for the surrounding areas are downloaded and cached. If you drag the map around, another download for these tiles isn t necessary. However, if you drag really fast to see areas that are far away, you ll see the Atlas Script working to catch up, caching the tiles as it goes. See Figure 10-3 for an example of this.