.NET/Java PDF, Tiff, Barcode SDK Library

* In fact, it is slightly more constrained than that. The .NET Framework limits arrays to 2 GB, and will throw an exception if you try to load a larger file into memory all at once.

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Once upon a time, we used to store programs and data in exactly this way, on a stream of paper tape with holes punched in it; the basic technology for this was invented in the 19th century. Later, we got magnetic tape, although that was less than useful in machine shops full of electric motors generating magnetic fields, so paper systems (both tape and punched cards) lasted well into the 1980s (when disk systems and other storage technologies became more robust, and much faster). The concept of a machine that reads data items one at a time, and can step forward or backward through that stream, goes back to the very foundations of modern computing. It is one of those highly resilient metaphors that only really falls down in the face of highly parallelized algorithms: a single input stream is often the choke point for scalability in that case.

Listing 14-10. Starting a download and showing progress void FtpDialog::getClicked() { QString fileName = QFileDialog::getSaveFileName( this, tr("Get File"), ui.dirList->selectedItems()[0]->text() ); if( fileName.isEmpty() ) return; file = new QFile( fileName, this ); if( !file->open( QIODevice::WriteOnly|QIODevice::Truncate ) ) { QMessageBox::warning( this, tr("Error"), tr("Failed to open file %1 for writing.").arg( fileName ) ); delete file; file = 0; return; }

To illustrate this, let s write a method that s equivalent to File.ReadAllBytes using a stream (see Example 11-40).

private static byte[] ReadAllBytes(string filename) { using (FileStream stream = File.OpenRead(filename)) { long streamLength = stream.Length; if (streamLength > 0x7fffffffL) { throw new InvalidOperationException( "Unable to allocate more than 0x7fffffffL bytes" + "of memory to read the file"); } // Safe to cast to an int, because // we checked for overflow above int bytesToRead = (int) stream.Length; // This could be a big buffer! byte[] bufferToReturn = new byte[bytesToRead]; // We're going to start at the beginning int offsetIntoBuffer = 0; while (bytesToRead > 0) { int bytesRead = stream.Read(bufferToReturn, offsetIntoBuffer, bytesToRead); if (bytesRead == 0) { throw new InvalidOperationException( "We reached the end of file before we expected..." + "Has someone changed the file while we weren't looking "); } // Read may return fewer bytes than we asked for, so be // ready to go round again. bytesToRead -= bytesRead; offsetIntoBuffer += bytesRead;

erhaps it is natural that a framework called Atlas has a built-in set of libraries for handling mapping functionality! In this chapter, you will look at this functionality and how you can use and leverage it in your own applications.

} } return bufferToReturn;

ui.disconnectButton->setEnabled( false ); ui.cdButton->setEnabled( false ); ui.upButton->setEnabled( false ); ui.getButton->setEnabled( false ); ftp.get( ui.dirList->selectedItems()[0]->text(), file ); ui.statusLabel->setText( tr("Downloading file...") ); } void FtpDialog::ftpProgress( qint64 done, qint64 total ) { if( total == 0 ) return; ui.statusLabel->setText( tr("Downloading file... (%1%)") .arg( QString::number( done*100.0/total, 'f', 1 ) ) ); } When the get command finishes, it is handled by the ftpFinished slot (the code is shown in Listing 14-11). When the download fails (and even when it succeeds), the QFile object is closed and deleted, the buttons are re-enabled, and the status label is updated. The call to selectionUpdated ensures that the buttons are enabled according to the current selection in the directory contents list. This means that either Get File or Change Directory is enabled, or neither is enabled (but not both). The difference between a failed and a successful download is that when the download fails, you call the remove method on the QFile object before deleting it. This removes the file from the disk so that you don t leave a half-finished file for the user. Listing 14-11. Taking care of the file when the download has completed void FtpDialog::ftpFinished( int request, bool error ) { if( error ) { switch( ftp.currentCommand() ) { ... case QFtp::Get: QMessageBox::warning( this, tr("Error"), tr("Failed to get file ") ); file->close(); file->remove(); delete file; file = 0;

}

The call to File.OpenRead creates us an instance of a FileStream. This class derives from the base Stream class, which defines most of the methods and properties we re going to use. First, we inspect the stream s Length property to determine how many bytes we need to allocate in our result. This is a long, so it can support truly enormous files, even if we can allocate only 2 GB of memory.

If you try using the stream.Length argument as the array size without checking it for size first, it will compile, so you might wonder why we re doing this check. In fact, C# converts the argument to an int first, and if it s too big, you ll get an OverflowException at runtime. By checking the size explicitly, we can provide our own error message.