I am using a third party library (https://github.com/zhang28602/ZZYQRCode) for my project. When I indergrated it into my project it started throwing the following error: Typedef redefinition with different types ('void (^)(NSString *__strong)' vs 'void (^)(__strong id, NSURLResponse *__strong)')
for the following line of code: typedef void(^SuccessBlock)(NSString *reuslt);
Please help me resolve this.
That project has defined a global typedef in its public header using a very generic name (SuccessBlock). You likely have defined SuccessBlock in another way somewhere else in your program.
If possible, have the developer use prefixes on their names. This should have been ZZYSuccessBlock. Alternately, rename the other instance of SuccessBlock.
If that's not possible, try to ensure that no part of your system imports both ZZYQRCode.h and whatever .h file defines the other version of it.
In my class, I have a reference on an UIViewController and want to implement a delegate on this ViewController during runtime. The delegate has only one method (with two parameters) and when the delegate-method on the ViewController is invoked, my class should handle the call.
I am quite sure this is possible with some kind of method swizzling, etc. but I don't know how to accomplish this.
What you want is possible, but it's not method swizzling, since you don't want to switch to methods but add a new one. It can be done, thanks to Objective-C's dynamic nature, but it's still a dirty hack so also file a feature request with the library vendor.
What you want is class_addMethod() and a C function with the actual implementation for that. One more thing, Objective-C methods are C methods, but with two implicit parameters, self and _cmd, which have to keep in mind (both when creating your C method and when telling class_addMethod your methods signature. And here is an SSCE of how to pull something like that off:
#import <Foundation/Foundation.h>
#import <objc/runtime.h> // Required for class_addMethod()
#interface MyClass : NSObject
#end
#implementation MyClass
#end
#protocol MyProtocol <NSObject>
- (void)printString:(NSString *)string;
#end
// Note the method signature containing the
// two implicit parameters self and _cmd!
void MyClassPrinStringIMP(id self, SEL _cmd, NSString *string)
{
NSLog(#"Hi I'm %#:%s and this is the string: %#", self, sel_getName(_cmd), string);
}
void PimpMyClass()
{
// The last argument is the signature. First character is the return type, in our case void
// Then comes self and _cmd, followed by the NSString. You can use #encode() to find out how your
// type is encoded. Best is to build this string at runtime, since the encoding can change with architectures
class_addMethod([MyClass class], #selector(printString:), (IMP)MyClassPrinStringIMP, "v#:#");
}
int main(int argc, const char * argv[])
{
#autoreleasepool
{
PimpMyClass();
id foo = [[MyClass alloc] init]; // id, to silence the compiler!
[foo printString:#"Hello World"];
}
return 0;
}
Example output:
Hi I'm <MyClass: 0x100101810>:printString: and this is the string: Hello World
Edit: Something that you may find is that the passed object is checked at runtime wether it conforms to a protocol or not using conformsToProtocol:. Since this code just adds the method implementation, it would still fail, but you can tell the runtime that you totally do implement that protocol with this one function call:
class_addProtocol([MyClass class], #protocol(MyProtocol));
Alternative: proxies
Objective-Cs dynamism and message forwarding is already praised by #JasperBlues, however, there is one particular class in Objective-C that is designed to do just that: NSProxy. It is designed to intercept sent messages and dispatching them dynamically to the relevant target, and does use the high-level NSInvocation approach. If you can pass a proxied object in some way as the delegate (depending on what your code allows for and what not), creating a NSProxy subclass might be the cleanest way to go.
However, note though that you then end up with a shim object that wraps over your other object, which comes with its own bag of pain and will break when you try to directly access variables via -> syntax. It's not a perfectly invisible proxy, but good enough for most cases.
Firstly, some comments indicate that what you're asking is instantly "a bad thing to do" or a "dirty hack". I disagree here. Most modern Object Oriented languages support these features, and they are used to good effect by numerous system-level frameworks. Of course it is human-nature to perhaps use these dynamic features where they're not really required (for fun or practice), even when a simpler approach would work fine. Beware of this.
Objective-C is admirable in that its somewhat of a legacy language and close to the "bare metal", and yet features a surprising level of dynamism, making it relatively easy to support these requirements without any external libraries or frameworks.
Besides using the class_addMethod guide that another answer correctly indicates, some other approaches are:
Message Forwarding: (recommended)
All NSObject sub-classes have the ability to forward a method that they're not able to respond to, to another target object. This is similar to the lower-level concept of trampolines. Apple publishes a guide on using this approach.
The advantages of using forward invocation is that it uses the NSInvocation level of abstraction, instead of directly calling the C ObjC runtime API. This abstracts the following details away:
Structs and primitives will be box/unboxed automatically
Dispatching to methods with a dynamic/unknown number of arguments becomes easy. Until arm64, this could be done using va_args, however on arm64 va_args can be copied directly to registers, and not popped off the stack.
Resolve Instance Method:
Instance methods are created by by registering a C function as the implementation to respond to a given message. This can be done neatly with blocks, using IMP_ImplementationWithBlock:
+ (BOOL)resolveInstanceMethod:(SEL)sel
{
IMP imp = imp_implementationWithBlock((__bridge id) objc_unretainedPointer(
^(id me, BOOL firstParam, NSString* secondParam)
{
//Implementation goes in here
return something; //something of type 'id'
}));
class_addMethod(self, sel, imp, "##:");
return YES;
}
return NO;
}
Using libffi:
Libffi can also do this kind of thing, though it should not be necessary if you're using pure Objective-C, as the runtime already has these features baked in.
NSURLConnection has some missing features (and some small bugs!), so I've been re-implementing it via CFNetwork. I'm aiming for a drop-in replacement - identical input, identical output.
Everything works great, except ... I can't find a way to map from CFNetwork error codes ( http://developer.apple.com/library/ios/#documentation/Networking/Reference/CFNetworkErrors/Reference/reference.html ) to the NSURLError codes ( https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Miscellaneous/Foundation_Constants/Reference/reference.html )
NB: please do not tell me to use ASIHttpRequest, AFNetworking, etc - I love them, I use them frequently, but ... for this specific project it's not possible to use any 3rd party code unless it's provided by Apple.
I'm trying to assign a function to the AURenderCallback inputProc
int setupRemoteIO(audio unit etc){
inProc.inputProc = playerCallback
}
but it says that playerCallback is not declared in this scope although playerCallback is present in the same file and class as setupRemoteIO.
The player callback is like this
static OSStatus playerCallback(void *inRefCon etc)
What could be the problem?
In C, you need to declare a function before its first use, i.e. higher up in the file than the point where you try to use the function. That's why include files are usually clustered at the top of a file; all of the symbols declared in the headers will be available throughout the code in the including file.
In this case, that means the declaration of your callback:
static OSStatus playerCallback(void *inRefCon etc);
must appear before your setupRemoteIO() function so that the compiler knows the function exists when you come to use it.
As you're on iOS, I'll also make the point that in recent compilers this restriction doesn't apply to Objective-C methods. It used to: you could only use method selectors that had already been seen. But in newer versions of Clang an Objective-C method can make use of a selector declared later in the same file without error.
For the life of me I can't figure out how to resolve the declared NTQueryKey value in my device driver. I looked for a device driver forum, but didn't find one.
Can someone point me to the right place? OSR isn't very responsive with dumb questions like how to link to NTQueryKey.
Here is my prototype:
NTSYSAPI NTSTATUS NTAPI NtQueryKey(HANDLE, KEY_INFORMATION_CLASS, PVOID, ULONG, ULONG *);
and it compiles fine, but the linker doesn't like it.
Thanks
NtXXXX functions should not be called from kernel mode. Use the ZwXXXX functions instead. In your case, you want ZwQueryKey. It has the same signature as NtQueryKey, but it performs actions on the x86 required for talking with kernel mode, and it's provided by ntoskrnl.exe rather than by ntdll.dll.
In kernel mode you link to the Zw.... equivalent functions. See Here. NT.... functions are called from user mode (for example the Win32 subsystem would call the NT... functions).