在哪里以及如何 __bridge [英] Where and how to __bridge

问题描述

我需要一些关于 __bridge 在 iOS 中的建议.

I need some advice on __bridge-ing in iOS.

希望下面的 SSCCE¹ 能比我用文字更好地解释这个问题，但我需要知道如何转换 void* 到 NSMutableArray*;应该使用哪个 __bridge 变体(请参阅代码中的注释).

Hopefully the SSCCE¹ below will explain the problem better than I can in words, but I need to know how I can convert a void* to an NSMutableArray*; which __bridge variation should be used (See comment in code).

阅读不同的桥接器后，我推断我需要 __bridge_transfer 但随后我在 addObject:

Reading about the different bridges, I deduced that I would need __bridge_transfer but then I receive an EXC_BAD_ACCESS on addObject:

最终，在调用 CGPathApply 之后，我希望在 CGPath 中有一个 CGPoints 数组.

Ultimately, I'd like to have an array of the CGPoints in the CGPath after CGPathApply has been called.

#import <Foundation/Foundation.h>

void _processPathElement(void* info, const CGPathElement* element)
{
    NSMutableArray *array = (/* WHAT BRIDGE HERE */ NSMutableArray*) info;
    switch (element->type)
    {
        case kCGPathElementMoveToPoint:
        case kCGPathElementAddLineToPoint:
        {
            CGPoint point = element->points[0];
            [array addObject:[NSValue valueWithCGPoint:point]];
            break;
        }
        default:
            break;
    }
}

int main(int argc, char *argv[])
{
    @autoreleasepool
    {
        //Create path
        CGMutablePathRef path = CGPathCreateMutable();
        CGPathMoveToPoint(   path, NULL, 0, 0);
        CGPathAddLineToPoint(path, NULL, 1, 0);
        CGPathAddLineToPoint(path, NULL, 1, 1);
        CGPathAddLineToPoint(path, NULL, 0, 1);
        CGPathCloseSubpath(path);

        NSMutableArray *pathPoints = [NSMutableArray array];
        CGPathApply(path, &pathPoints, _processPathElement);

        NSLog(@"Points:%@", pathPoints);
    }
}

<子>1:SSCCE

推荐答案

关于使用桥接关键字的文档 可以在这里找到.具体来说，我想指出 §3.2.4:

The documentation on the use of the bridge keyword can be found here. Specifically, I want to point out §3.2.4:

(__bridge T) op 将操作数强制转换为目标类型 T.如果 T 是可保留的对象指针类型，则 op 必须具有不可保留的指针类型.如果 T 是不可保留的指针类型，则 op 必须具有可保留的对象指针类型.否则演员阵容是不合格的.没有所有权转移，ARC 插入没有保留操作.

(__bridge T) op casts the operand to the destination type T. If T is a retainable object pointer type, then op must have a non-retainable pointer type. If T is a non-retainable pointer type, then op must have a retainable object pointer type. Otherwise the cast is ill-formed. There is no transfer of ownership, and ARC inserts no retain operations.

(__bridge_retained T) op 将必须具有可保留对象指针类型的操作数强制转换为必须是不可保留指针类型的目标类型.ARC 保留该值，受制于对本地值的通常优化，并且接收者负责平衡 +1.

(__bridge_retained T) op casts the operand, which must have retainable object pointer type, to the destination type, which must be a non-retainable pointer type. ARC retains the value, subject to the usual optimizations on local values, and the recipient is responsible for balancing that +1.

(__bridge_transfer T) op 将必须具有不可保留指针类型的操作数强制转换为必须是可保留对象指针类型的目标类型.ARC 将在封闭的完整表达式的末尾释放值，这取决于对局部值的通常优化.

(__bridge_transfer T) op casts the operand, which must have non-retainable pointer type, to the destination type, which must be a retainable object pointer type. ARC will release the value at the end of the enclosing full-expression, subject to the usual optimizations on local values.

传入的指针 (void*) 是不可保留的指针类型，而您的 NSMutableArray 是可保留的指针类型.这立即排除了 __bridge_retained.所以问题是，要__bridge 还是__bridge_transfer?

The pointer you're being passed in (void*) is a non retainable pointer type, whereas your NSMutableArray is a retainable pointer type. This rules out __bridge_retained straight away. So the question is, to __bridge or to __bridge_transfer?

__bridge_transfer 通常用于从返回已保留的 CF 对象的方法中获取 Objective-C 指针的情况.例如，CFStringCreateWithFormat 将返回一个保留的 CFString，但如果你想要一个 NSString，你需要在它们之间 __bridge_transfer.这将使ARC在适当的时候释放CF保留的对象.例如，NSString* str = (__bridge_transfer NSString*) CFStringCreateWithFormat(...);

__bridge_transfer is typically used when you want the Objective-C pointer from a method that returns a CF Object that has been retained. For example, CFStringCreateWithFormat will return a retained CFString, but if you want an NSString from it, you need to __bridge_transfer between them. This will make ARC release the object that CF retained when appropriate. For example, NSString* str = (__bridge_transfer NSString*) CFStringCreateWithFormat(...);

您的代码没有这样做，您不需要干预所有权.您的 main 方法控制其内存管理，并且只是将引用传递给它调用的方法(尽管是间接的，但都在 main 的范围内).因此，您将使用 __bridge.

Your code isn't doing that, you don't need to meddle with the ownership. Your main method is in control of its memory management, and is simply passing a reference to a method it calls (albeit indirectly, but it's all within the scope of main). As such, you would use __bridge.

但是等等，当我使用 __bridge 时，我的代码会出现内存访问错误！?

啊，这是您发布的代码的问题，与整个桥接讨论无关.您需要将 void* 传递给 CGApplyPath，用于处理函数 _processPathElement.你传递的是NSMutableArray**.

Ah, this is an issue with the code you posted, and isn't in relation to the whole bridging discussion. You need to pass a void* to CGApplyPath, for your processing function _processPathElement. What you're passing is NSMutableArray**.

当您重新转换为 NSMutableArray* 时，您实际上是在转换一个 NSMutableArray**.这将导致臭名昭著的 EXC_BAD_ACCESS.您需要传递指针本身，而不是指向指针的指针.但是，CGPathApply(path, pathPoints, _processPathElement) 将不起作用，您不能将 NSMutableArray* 作为 void*.您需要(具有讽刺意味的是)是一座桥梁.出于与以前相同的原因，您只需要__bridge.请参阅下面的代码，使用正确的桥接器，并按预期工作:

When you recast to the NSMutableArray*, you're actually casting a NSMutableArray**. This will cause the infamous EXC_BAD_ACCESS. You need to pass the pointer itself, not a pointer to a pointer. But, CGPathApply(path, pathPoints, _processPathElement) will not work, you cannot pass off a NSMutableArray* as a void*. What you need (ironically), is a bridge. For the same reasons as before, all you need is __bridge. See below the code, with the correct bridges in place, and working as expected:

#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>

void _processPathElement(void* info, const CGPathElement* element)
{
    NSMutableArray *array = (__bridge NSMutableArray*) info;
    switch (element->type)
    {
        case kCGPathElementMoveToPoint:
        case kCGPathElementAddLineToPoint:
        {
            CGPoint point = element->points[0];
            [array addObject:[NSValue valueWithCGPoint:point]];
            break;
        }
        default:
            break;
    }
}

int main(int argc, char *argv[])
{
    @autoreleasepool
    {
        //Create path
        CGMutablePathRef path = CGPathCreateMutable();
        CGPathMoveToPoint(   path, NULL, 0, 0);
        CGPathAddLineToPoint(path, NULL, 1, 0);
        CGPathAddLineToPoint(path, NULL, 1, 1);
        CGPathAddLineToPoint(path, NULL, 0, 1);
        CGPathCloseSubpath(path);
        
        NSMutableArray *pathPoints = [[NSMutableArray alloc] init];
        CGPathApply(path, (__bridge void*)pathPoints, _processPathElement);
        
        NSLog(@"Points:%@", pathPoints);
    }
}

这将打印出来:

Points:(
    "NSPoint: {0, 0}",
    "NSPoint: {1, 0}",
    "NSPoint: {1, 1}",
    "NSPoint: {0, 1}"
)

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