ios 在哪里以及如何 __bridge
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Where and how to __bridge
提问by James Webster
I need some advice on __bridge-ing in iOS.
我需要一些关于__bridge-ing 在 iOS 中的建议。
Hopefully the SSCCE1below 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 __bridgevariation should be used (See comment in code).
希望下面的 SSCCE 1能比我用文字更好地解释这个问题，但我需要知道如何将 a 转换void*为 an NSMutableArray*；__bridge应该使用哪种变体（请参阅代码中的注释）。
Reading about the different bridges, I deduced that I would need __bridge_transferbut then I receive an EXC_BAD_ACCESS on addObject:
阅读不同的桥梁，我推断我需要，__bridge_transfer但后来我收到了一个 EXC_BAD_ACCESSaddObject:
Ultimately, I'd like to have an array of the CGPointsin the CGPathafter CGPathApplyhas been called.
最终，我想CGPoints在调用CGPath之后有一个数组CGPathApply。
#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);
}
}
1: SSCCE
1：SSCCE
回答by WDUK
The documentation on the use of the bridge keyword can be found here. Specifically, I want to point out §3.2.4:
可以在此处找到有关使用 bridge 关键字的文档。具体来说，我想指出§3.2.4：
(__bridge T) opcasts 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) opcasts 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) opcasts 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.
(__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_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目标类型，该类型必须是不可保留指针类型。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_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目标类型，该类型必须是可保留对象指针类型。ARC 将在封闭的完整表达式的末尾释放值，这取决于对本地值的通常优化。
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_retainedstraight away. So the question is, to __bridgeor to __bridge_transfer?
您传入 ( void*) 的指针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__bridge_retained了。所以问题是，去__bridge还是去__bridge_transfer？
__bridge_transferis 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_transferbetween them. This will make ARC release the object that CF retained when appropriate. For example, NSString* str = (__bridge_transfer NSString*) CFStringCreateWithFormat(...);
__bridge_transfer通常在您希望来自返回已保留的 CF 对象的方法的 Objective-C 指针时使用。例如，CFStringCreateWithFormat 将返回一个保留的 CFString，但是如果你想要一个 NSString，你需要__bridge_transfer在它们之间。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(...);
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.
你的代码没有这样做，你不需要干预所有权。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.
But wait, when I use __bridge, my code gets memory access errors!?
但是等等，当我使用 __bridge 时，我的代码会出现内存访问错误！？
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**.
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 a 传递void*给 CGApplyPath，用于您的处理功能_processPathElement。你正在传递的是NSMutableArray**.
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:
当你重铸到 时NSMutableArray*，你实际上是在投射一个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)不起作用，您不能将 aNSMutableArray*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：
#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);
}
}
This will print out:
这将打印出：
Points:(
"NSPoint: {0, 0}",
"NSPoint: {1, 0}",
"NSPoint: {1, 1}",
"NSPoint: {0, 1}"
)
回答by James Webster
I'm not actually sure why this works, but I've found the solution to be:
我实际上不确定为什么会这样，但我发现解决方案是：
NSMutableArray *array = (__bridge NSMutableArray*) info;
//AND
CGPathApply(path, (__bridge void*)pathPoints, _processPathElement);
If anyone can explain why this works and confirm that there aren't (/are) any memory leaks I'd be grateful
如果有人可以解释为什么这样做并确认没有（/是）任何内存泄漏，我将不胜感激
