我想使用Message Forwarding让任何未实现的getter方法返回0，而不是抛出无法识别的选择器异常。像
MyClass *r = [[MyClass alloc] init];
NSNumber *n = (NSNumber *)r;
NSLog(@"%d", [n integerValue]); // output 0
NSLog(@"%f", [n doubleValue]); // output 0.00000
NSLog(@"%@", [n stringValue]); // output (null)
所以我写了这个例子:
#pragma mark -
#pragma mark Application lifecycle
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
NSNumber *n = (NSNumber *)self;
NSLog(@"%d", [n integerValue]);
NSLog(@"%f", [n doubleValue]);
NSLog(@"%@", [n stringValue]);
return YES;
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
NSMethodSignature *ms = [super methodSignatureForSelector:aSelector];
if(ms)
return ms;
// Q = uint64_t, so it should also works for double which is also 64bit
return [NSMethodSignature signatureWithObjCTypes:"Q@:"];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation {
uint64_t ZERO64 = 0;
[anInvocation setReturnValue:&ZERO64];
}
在真实设备上的输出结果为0,0.00000，(null)，但在模拟器上的输出结果为0,NaN， (null)
所以double类型不能像预期的那样工作。我的第一个想法是将NSMethodSignature更改为"d@:"(d是double)
设备和模拟器上的输出结果都是正确的，但是只有模拟器上发生了一些奇怪的事情。运行这段代码，它将在第六次循环时崩溃，并出现某种CALayer异常:
#pragma mark -
#pragma mark Application lifecycle
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
for(NSInteger i = 0; i < 100; i++) {
NSInteger t = [(NSNumber *)self integerValue];
UIViewController *view = [[UIViewController alloc] init];
// it always crash on the 6th loop on this line**
UINavigationController *nc = [[UINavigationController alloc] initWithRootViewController:view];
}
return YES;
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
NSMethodSignature *ms = [super methodSignatureForSelector:aSelector];
if(ms)
return ms;
// we change to return double
return [NSMethodSignature signatureWithObjCTypes:"d@:"];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation {
uint64_t ZERO64 = 0;
[anInvocation setReturnValue:&ZERO64];
}
我对两个问题很好奇，为什么NaN在第一个示例中返回模拟器，第二个示例发生了什么?
对于您的第一个问题，这是我在模拟器上发现的
union {
double d;
uint64_t l;
} u;
NSNumber *n = (NSNumber *)self;
u.d = [n doubleValue];
NSLog(@"%f", u.d); // nan
NSLog(@"%llx",u.l); // fff8000000000000
bzero(&u, sizeof(double));
NSLog(@"%f", u.d); // 0.000000
NSLog(@"%llx",u.l); // 0
所以很明显NAN(fff8000000000000)会返回而不是0.0。
要深入了解[NSMethodSignature signatureWithObjCTypes:"d@:"]和[NSMethodSignature signatureWithObjCTypes:"Q@:"]之间的区别，请查看
NSLog(@"%@n%@", [[NSMethodSignature signatureWithObjCTypes:"Q@:"] debugDescription], [[NSMethodSignature signatureWithObjCTypes:"d@:"] debugDescription]);
<NSMethodSignature: 0x74a0950>
number of arguments = 2
frame size = 8
is special struct return? NO
return value: -------- -------- -------- --------
type encoding (Q) 'Q'
flags {}
modifiers {}
frame {offset = 0, offset adjust = 0, size = 8, size adjust = 0}
memory {offset = 0, size = 8}
argument 0: -------- -------- -------- --------
type encoding (@) '@'
flags {isObject}
modifiers {}
frame {offset = 0, offset adjust = 0, size = 4, size adjust = 0}
memory {offset = 0, size = 4}
argument 1: -------- -------- -------- --------
type encoding (:) ':'
flags {}
modifiers {}
frame {offset = 4, offset adjust = 0, size = 4, size adjust = 0}
memory {offset = 0, size = 4}
<NSMethodSignature: 0x74a1e80>
number of arguments = 2
frame size = 8
is special struct return? NO
return value: -------- -------- -------- --------
type encoding (d) 'd'
flags {isFloat} <<<<----- this flag should be set if the return value is float type
modifiers {}
frame {offset = 0, offset adjust = 0, size = 8, size adjust = 0}
memory {offset = 0, size = 8}
argument 0: -------- -------- -------- --------
type encoding (@) '@'
flags {isObject}
modifiers {}
frame {offset = 0, offset adjust = 0, size = 4, size adjust = 0}
memory {offset = 0, size = 4}
argument 1: -------- -------- -------- --------
type encoding (:) ':'
flags {}
modifiers {}
frame {offset = 4, offset adjust = 0, size = 4, size adjust = 0}
memory {offset = 0, size = 4}
你可以看到在第二个方法签名有flags {isFloat}的返回值。我不是x86和AMR以及低级ObjC运行时方面的专家。但是我认为CPU使用这个标志来识别返回值的类型。如果不在x86 CPU上设置它，预期的浮点返回值因此被解释为NAN。
对于您的第二个问题，我认为这是因为您告诉运行时它将返回64位大小的值，因此堆栈上64位大小的内存为零。然而，调用者期望一个32位的返回大小(NSInteger)。因此，发生了某种stackoverflow并导致崩溃。
我实际上实现了类似的东西，旨在使NSNull像nil一样工作。
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
if (signature)
return signature;
const Class forwardClasses[] = {[NSNumber class], [NSString class], [NSArray class], [NSOrderedSet class]}; // add new classes if you think the list is not enough
for (int i = 0; i < sizeof(forwardClasses)/sizeof(Class); i++) {
Class cls = forwardClasses[i];
signature = [cls instanceMethodSignatureForSelector:aSelector];
if (signature) {
return signature;
}
}
return signature;
}
- (void)forwardInvocation:(NSInvocation *)anInvocation {
NSUInteger len = [[anInvocation methodSignature] methodReturnLength];
char buff[len];
bzero(buff, len);
[anInvocation setReturnValue:buff];
}
如果你想使用消息转发让任何未实现的getter方法返回0，而不是抛出一个无法识别的选择器异常，也许你可以使用+resolveInstanceMethod ?
这是一个返回NSString的例子。您必须调整它以返回一个原语。如果你有困难，请告诉我。
如果你使用ARC，你也需要在void*上建立一个桥。
+ (BOOL)resolveInstanceMethod:(SEL)sel
{
NSString* name = NSStringFromSelector(sel);
IMP imp = imp_implementationWithBlock((void*) objc_unretainedPointer(^(id me, BOOL selected)
{
return @"Hello!";
}));
class_addMethod(self, sel, imp, "@"); //The type '@' is an object. For int use 'i'. Google "obj-c runtime types"
return YES;
}
当我们使用class_addMethod时，第三个参数是类型代码…解决这些问题的一个好方法是制定一个真正的方法，然后进行反思。下面是一个实用程序，用于返回类上(实际)选择器的类型代码:https://github.com/jasperblues/spring-objective-c/blob/master/Source/…- user404201 6 min ago