ARC:__bridge与__bridge_retained使用contextInfo测试用例

Question

考虑这个ARC代码:

- (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;
}

问题:在第7行,应该__bridge使用,或者__bridge_retained,或者无关紧要,或者选择是否依赖于字符串的保留计数(即,是否显式分配字符串与是否通过类初始化程序自动释放+[NSString stringWithString:]？

Answer 1

一般来说,它是

// Object to void *:
contextInfo:(__bridge void *)s

// void * to object:
NSString *s = (__bridge NSString *)context;

要么

// 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;

在第一种情况下,没有所有权转移,因此只要工作表处于活动状态,主程序 就必须对该对象进行强有力的引用.

在第二种情况下,在创建工作表时保留对象,并在sheetDidEnd:方法中释放.主程序不需要强有力的参考,因此这是安全的方法.