桥式转换(bridge cast)

foundation 与 coreFoundation的桥接

1. 用__bridge桥接 NSArray -> CFArrayRef

NSArray *fArray = @[@1, @2, @3, @4, @5];
// __bridge本身的意思是 ARC仍然具备这个OC对象的所有权, ARC任然会帮助我们释放
CFArrayRef cfArray = (__bridge CFArrayRef)fArray;
NSLog(@"Size of aray = %li", CFArrayGetCount(cfArray));

2. 用__bridge_retained 桥接 NSArray -> CFArrayRef

// __bridge_retained 意味着ARC交出对象的所有权
cfArray = (__bridge_retained CFArrayRef)fArray;
NSLog(@"Size of aray = %li", CFArrayGetCount(cfArray));
CFRelease(cfArray); //手动释放内存

3. 用__bridge_transfer 桥接 CFArrayRef -> NSArray

// 使ARC获得对象的所有权
fArray = (__bridge_transfer NSArray*)cfArray;

例子

const void* RetainCallback(CFAllocatorRef allocator, const void* value) {
    return CFRetain(value);
}

void ReleaseCallback(CFAllocatorRef allocator, const void* value) {
    CFRelease(value);
}

CFDictionaryKeyCallBacks keyCallbacks = {
    0,
    RetainCallback,
    ReleaseCallback,
    NULL, //copyDescription取值为NULL,采用默认实现
    //equal和hash发布采用CFEqual和CFHash这二者采用的做法与NSMutableDictionary的默认实现相同
    CFEqual, // CFEqual最终会调用isEqual方法
    CFHash,
};

CFDictionaryValueCallBacks valueCallback = {
    0,
    RetainCallback,
    ReleaseCallback,
    NULL,
    CFEqual,
};
// 用CFMutableDictionaryRef来创建NSMutableDictionary
- (void)createMutableDictionaryForCFMutableDictionaryRef {
    CFMutableDictionaryRef aCFDicionary = CFDictionaryCreateMutable(NULL, 0, &keyCallbacks, &valueCallback);
    NSMutableDictionary *anNSDictionary = (__bridge_transfer NSMutableDictionary *)aCFDicionary;
    anNSDictionary[@"a"] = @123;
    [anNSDictionary removeObjectForKey:@"a"];
}