__bridge的位置和方式 [英] Where and how to __bridge
问题描述
我需要对iOS中的 __ bridge
-ing提出一些建议。
I need some advice on __bridge
-ing in iOS.
希望SSCCE 1 会比我说的更好地解释问题,但我需要知道如何将 void *
转换为的NSMutableArray *
;应该使用 __ bridge
变体(参见代码中的注释)。
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).
阅读不同的桥梁,我推断出我需要 __ bridge_transfer
但是我在 addObject上收到一个EXC_BAD_ACCESS:
Reading about the different bridges, I deduced that I would need __bridge_transfer
but then I receive an EXC_BAD_ACCESS on addObject:
最后,我想在<$ c后的 CGPath
中有一个 CGPoints
的数组已调用$ c> CGPathApply 。
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);
}
}
1: SSCCE
推荐答案
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:
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。否则演员阵容不合理。没有所有权转让,ARC没有保留任何保留操作。
(__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
强制转换操作数必须具有可保留对象指针类型的目标类型,它必须是不可保留的指针类型。 ARC保留该值,取决于对本地值的通常优化,并且收件人负责平衡+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
将操作数(必须具有不可保留的指针类型)强制转换为目标类型,该目标类型必须是可保留的对象指针类型。 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。
(__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.
指针你传入( void *
)是一种不可保留的指针类型,而NSMutableArray是一种可保留的指针类型。这将立即排除 __ bridge_retained
。所以问题是, __ bridge
或 __ 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
通常在需要返回已保留的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,你需要在它们之间 __ bridge_transfer
。这将使ARC在适当时释放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(...);
您的代码不是要做到这一点,你不需要干涉所有权。您的主要方法是控制其内存管理,并且只是将引用传递给它调用的方法(尽管是间接的,但它都在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
.
但等等,当我使用__bridge时,我的代码获取内存访问错误!
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,以处理函数 _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**
.
当你重铸 NSMutableArray时*
,你实际上正在施放 NSMutableArray **
。这将导致臭名昭着的EXC_BAD_ACCESS。您需要传递指针本身,而不是指针指针。 但, CGPathApply(path,pathPoints,_processPathElement)
将无效,您无法传递 NSMutableArray *
作为 void *
。你需要什么(具有讽刺意味),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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