问题
Consider this ARC code:
- (void)main {
NSString *s = [[NSString alloc] initWithString:@"s"];
[NSApp beginSheet:sheet
modalForWindow:window
modalDelegate:self
didEndSelector:@selector(sheetDidEnd:returnCode:context:)
contextInfo:(__bridge void *)s
];
}
- (void)sheetDidEnd:(NSWindow *)sheet returnCode:(NSInteger)returnCode context:(void *)context {
NSString *s = (__bridge_transfer NSString *)context;
}
Question: on line 7, should __bridge
be used, or __bridge_retained
, or does it not matter, or does the choice depend on the string's retain count (that is, whether the string is explicitly alloced vs being autoreleased through a class initializer like +[NSString stringWithString:]
?
回答1:
Generally, it is either
// Object to void *:
contextInfo:(__bridge void *)s
// void * to object:
NSString *s = (__bridge NSString *)context;
or
// Object to void *, retaining the object:
contextInfo:(__bridge_retained void *)s
// void * to object, transferring ownership.
// The object is released when s goes out of scope:
NSString *s = (__bridge_transfer NSString *)context;
In the first case, there is no transfer of ownership, therefore the main program must hold a strong reference to the object as long as the sheet is active.
In the second case, the object is retained when creating the sheet, and released
in the sheetDidEnd:
method. It is not required that the main program holds
a strong reference, so this is the safe method.
来源:https://stackoverflow.com/questions/20842310/arc-bridge-versus-bridge-retained-using-contextinfo-test-case