Determining equality, Writing to and reading from files – Apple WebObjects 3.5 User Manual

Page 190

Chapter 11

WebScript Programmer’s Quick Reference to Foundation Classes

190

Determining Equality

You can determine if two objects are equal using the

isEqual:

method. This

method returns YES if the receiver of the message and the speciﬁed object are
equal, and NO otherwise. The deﬁnition of equality depends on the object’s
type. For example, array objects deﬁne two arrays as equal if they contain the
same contents. For more information, see the

isEqual:

method descriptions later

in this chapter.

Writing to and Reading From Files

Strings, arrays, and dictionaries—three of the classes discussed in this chapter—
provide methods for writing to and reading from ﬁles. The method

writeToFile:atomically:

writes a textual description of the receiver’s contents to a

speciﬁed path name, and corresponding class-speciﬁc creation methods—

stringWithContentsOfFile:

arrayWithContentsOfFile:

, and

dictionaryWithContentsOfFile:

—create an

object from the contents of a speciﬁed ﬁle.

For example, the following code excerpt reads the contents of an error log stored
in a ﬁle, appends a new error to the log, and saves the updated log to the same
ﬁle:

id errorLog = [NSString stringWithContentsOfFile:errorPath];
id newErrorLog = [errorLog stringByAppendingFormat:@"%@: %@.\n",

timeStamp, @"premature end of file."];

[newErrorLog writeToFile:errorPath atomically:YES];

Writing to Files

To write to a ﬁle, use the method

writeToFile:atomically:

. It uses the

description

method

to obtain a human-readable string representation of the receiver and then writes
the string to the speciﬁed ﬁle. The resulting ﬁle is suitable for use with

classNameWithContentsOfFile:

methods. This method returns YES if the ﬁle is written

successfully, and NO otherwise.

If the argument for

atomically:

is YES, the string representation is ﬁrst written to

an auxiliary ﬁle. Then the auxiliary ﬁle is renamed to the speciﬁed ﬁlename. If
the argument is NO, the object is written directly to the speciﬁed ﬁle. The YES
option guarantees that the speciﬁed ﬁle, if it exists at all, won’t be corrupted
even if the system should crash during writing.

When

writeToFile:atomically:

fails, it returns NO. If this happens, check the

permissions on the speciﬁed ﬁle and its directory. The most common cause of
write failures is that the process owner doesn’t have the necessary permissions
to write to the ﬁle or its directory. If the argument for

atomically:

is NO, it’s

sufﬁcient to grant write permissions only on the ﬁle.