Smalltalk and Object Orientation: an Introduction - Free

You can also copy, move and delete files, get the contents of directories and files as well as setting
file permissions and printing files all from within Smal ltalk. The messages used for these functions are
all fairly intuitive and readily available in the Smalltalk documentation.
13.3.3 Handling file IO errors
You should always wrap your file access code up in a block and use the valueNowOrOnUnwindDo:
message to evaluate it. This is a special version of value, which will execute the block passed to it as a
parameter if an exception is raised, while the block it was sent to is executing. For example, if you try
and read to a file which does not exist, then an exception will be raised and your system will be left in
an unstable state with streams still open. However, if you use the valueNowOrOnUnwindDo:
message, then you can define what should happen if such an event occurs. For example, you could
specify that the stream should be closed. This is a very good way of ensuring that your file accesses are
handled tidily.
For example, consider the load: method defined below. This method accesses a file, reading each
line at a time. Once all the lines in the file have been accessed it terminates.
load: filename
|newFile newLineChar stream readingBlock item student |
newFile := filename value asFilename.
newFile exists
ifTrue:
[newLineChar := Character cr.
stream := newFile readStream.
contents := OrderedCollection new: 250.
readingBlock :=
[[stream atEnd]
whileFalse:
[item := stream upTo: newLineChar.
contents add: item.]].
Cursor read showWhile:
[readingBlock valueNowOrOnUnwindDo:
[stream close]]].
ifFalse: [Dialog warn: 'File does not exist!!']
There are three things to note about this method from a Smalltalk point of view.
• The first is that we read in one line of the file at a time by telling the (read) stream that we wanted to
read up to a carriage return. To do this we send the upTo: message to the stream with the
newLineChar as the parameter. You can use this to read in text up to any character in the input
stream.
• If the file exists, a block is created which co ntains the code which will read in the data in the file.
This block is assigned to a variable readingBlock. This block is evaluated by sending it the
valueNowOrOnUnwindDo: message. Notice that if anything happens to raise an error during
these operations ( e.g. the file system becomes full), the block passed as a parameter to the
valueNowOrOnUnwindDo: message will be executed. In this case, it will close the stream (i.e. it
will close the file). Thus, if anything goes wrong during the processing of the file, the file will be
closed and the system left in a stable state. Indeed when the last statement has been executed in the
write block, the block passed as a parameter will be executed. Thus the stream will also be closed.
This is a very good way of handling the process of closing the link to files.
• The other point is that the whole of the read expression (i.e.
readingBlock
valueNowOrOnUnwindDo: [stream close]) is passed in a block to the showWhile:
message. This message is part of the cursor protocol and is used to specify the type of cursor to be
displayed while a particular operation is being performed. In this case, the read cursor will be
displayed while the text file is being read in. This provides some visual feedback to the user.
13.4 The Binary Object Streaming Service
The Binary Object Streaming Service (or BOSS for short) is a very useful and effective way of storing
information in the form of objects (instances). If you were to save information to an ordinary text file,
then all you could save would be the ASCII version of the data held by the object. You would then need
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