LooK this method:
beginAnimations:context:
This is a method of class UIView. The context need parameter which is a type of void-pointer,and I need to send a UIImageView to context.
I get a warning,which says void* has been forbidden when I use ARC. So how can I send UIImageView to context except not use ARC.
The comments above provided the correct answer for this particular case (use the block-based animation methods) but in general if you have an API which takes a context void * and you'd like to pass an object, I find it best to convert your id to a CFTypeRef so you can get manual memory management semantics on the pointer (CFTypeRef is a typedef for void *), etc. Note however that this requires that the callback must be called in order to get your object released (i.e. converted back to ARC's built-in management).
Here's an example for some imaginary API I just dreamt up:
- (void) doSomethingWithObject: (id) object {
// use CFBridgingRetain() to turn object into a manually-retained CFTypeRef/void*
[someObject performBackgroundTaskWithTarget: self
selector: #selector(bgTask:)
context: CFBridgingRetain(object)];
}
- (void) bgTask: (void *) context
{
// use CFBridgingRelease() to turn our void*/CFTypeRef into an ARC-managed id
id object = CFBridgingRelease((CFTypeRef)context);
...
}
Related
Trying to make sense of MTAudioProcessingTap sample code from WWDC 2012.
This structure is used by the Tap to pass data around.
typedef struct AVAudioTapProcessorContext {
...
void *self; //Note use of word self
} AVAudioTapProcessorContext;
The word self here is used apparently as a standard variable name even though Xcode highlights it in pink. Is this just a code parsing oversight in Xcode?
Later, the Tap management class passes a reference to itself to this variable.
//Setting up Tap callbacks:
callbacks.clientInfo = (__bridge void *)self,
//And then storing it in the above struct when it's passed by the callback:
static void tap_InitCallback(MTAudioProcessingTapRef tap, void *clientInfo, void **tapStorageOut)
{
AVAudioTapProcessorContext *context = calloc(1, sizeof(AVAudioTapProcessorContext));
...
context->self = clientInfo;
*tapStorageOut = context;
}
And the confusion starts again in a later function (another Tap callback). Once again, the word self is used to refer to the Tap management class reference. Note that this occurs in a function defined after #end of the Tap management class implementation.
AudioFFTTapProcessor *self = ((__bridge AudioFFTTapProcessor *)context->self);
self has a special meaning only in the context of an Objective-C method. In all other contexts, it's just an ordinary identifier. You can think of Objective-C methods as being rewritten to take a hidden parameter with the name self (and a second hidden parameter named _cmd that's not relevant for this discussion), e.g.:
#implementation MyClass
- (void)doSomething:(int)param1 withThis:(NSString *)param2
{
// 'self' refers to the MyClass instance in this function
...
}
#end
could be imagined as if it were rewritten like this:
void MyClass_doSomething(id self, SEL _cmd, int param1, NSString *param2)
{
// 'self' refers to the MyClass instance in this function
...
}
Outside of Objective-C methods, you can use self as any other kind of identifier, whether it be a local variable, a struct member, a global function, or type name, etc., though the latter uses should be highly discouraged due to high potential for confusion.
So yes, XCode highlighting the self instance in pink in the struct definition is just a parsing anomaly.
self is not a keyword in C. It's a keyword for Objective-C, which is probably why it's getting highlighted.
self does not mean anything special as a C struct field name. Objective-C uses it as the equivalent of most other OO languages' this, which is why Xcode highlights it specially.
I'm trying to get some code going that lets me display raw trackpad data from my macbook pro, like the app FingerMgmt. Unfortunately, no one seems to have the source for FingerMgmt. I did find some other source code that kind of works, however. I was able to NSLog the data I wanted to see like this:
int callback(int device, Finger *data, int nFingers, double timestamp, int frame) {
for (int i=0; i<nFingers; i++) {
Finger *f = &data[i];
NSLog(#"Frame %7d: Angle %6.2f, ellipse %6.3f x%6.3f; "
"position (%6.3f,%6.3f) vel (%6.3f,%6.3f) "
"ID %d, state %d [%d %d?] size %6.3f, %6.3f?\n",
f->frame,
f->angle * 90 / atan2(1,0),
f->majorAxis,
f->minorAxis,
f->normalized.pos.x,
f->normalized.pos.y,
f->normalized.vel.x,
f->normalized.vel.y,
f->identifier, f->state, f->foo3, f->foo4,
f->size, f->unk2);
//todo-get data from raw C to obj-C variable
}
return 0;
}
But whenever I try to store any of the data to an Obj-c string or variable, the C code does not see the variable as having been declared. Because of this, I cannot write to any text fields or graphical displays in Obj-C, and I cannot store the data to a variable that Obj-c can access.
Basically, I need a way to write to an Obj-C variable or object from within the callback.
On a side note, I had a very similar problem with an iPhone app a while back, and I ended up fixing it by somehow declaring the app delegate within the C code and writing to or reading from the variable like this-
me.delegate=(id <UIApplicationDelegate,UITabBarControllerDelegate>)[[UIApplication sharedApplication] delegate];//allows access to the delegate within C function
me.delegate.number0=5;//writes to this variable in the delegate
For some reason, I can not seem to adapt this to my current situation. I always get the error that "me" is undeclared.
A Objective-C method can access instance variables because it is automagically passed a hidden parameter with the public name self - any reference to an instance variable, say fred, is translated by the compiler into a field reference, say self->fred (and a similar translation for property references).
For your C function callback to access the fields of any object (or call an object's methods) you need to pass the function a reference to the object. Two simple ways:
Add an argument to the function. Many C callback protocols include a general "user defined" values which is passed around as void *, if you are calling one of these pass your object reference as this value and cast it within the C function back to the correct Objective-C type.
Pass the object via a global (or file static) variable, e.g. static NSSomeType *objectForCallback;. This method works when you're stuck with an existing C callback protocol which doesn't support a user defined value. However it is not thread or re-entrant safe as you are sharing a single static variable.
In both cases make sure the objected is retain'ed if you're not using garbage collection.
In response to comment
Case 1: You will see C functions declared which (a) take a callback function and (b) a user-defined value to pass to that function on every call. For example:
typedef T ...;
T findMatching(T *buffer, // an array of T to search
size_t count, // number of items in array
int (*matcher)(T item, void *user), // user function for match, 1st arg is item, 2nd user-supplied value
void *userValue); // user-supplied value to pass to matcher
If you are faced with C function like this you can pass a (retain'ed if needed) Objective-C object as userValue and cast it back to its Objective-C type inside matcher. For example:
int myMatcher(T item, void *user)
{
NSMutableDictionary *myDictionary = (NSMutableDictionary *)user;
...
}
- (void) someMethod
{
NSMutableDictionary *sharedWithC = ...;
...
T found = findMatching(buffer, count, myMatcher, (void *)sharedWithC);
...
}
Case 2: Objective-C is (a superset of) C. You declare a global just as you would in C. For example (little checking, not thread safe):
static NSMutableDictionary *myGlobalDictionary = nil; // "static" makes the variable only visible to code in the same file
- (void) setupTheSharedDictionary
{
myGlobalDictionary = [[[NSMutableDictionary alloc] init] retain];
}
- (void) releaseTheSharedDictionary
{
if(myGlobalDictionary != nil)
{
[myGlobalDictionary release];
myGlobalDictionary = nil;
}
}
In response to second comment
I'm guessing you are trying to use some third party (Google?) code. That code defines a callback protocol - a C function type. You cannot just redefine that C function type adding an extra argument and expect the third party code to magically cope!
So unless you intend to change the C you can use the second approach - store the reference to Objective-C object in a global. In your case this will be something like:
static MT2AppDelegate *sharedWithCAppDelegateReference;
int callback(...)
{
...
[sharedWithCAppDelegateReference->L1 setStringValue:#"Hellofff"];
...
}
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
sharedWithCAppDelegateReference = self; // store so C can pick it up
...
MTRegisterContactFrameCallback(dev, callback);
...
}
But remember this is not thread or re-entrant safe - you are effectively passing a function parameter via a global variable. If you need it to be thread/re-entrant safe you need to get a bit more involved.
Sometimes I encounter code that has *, sometimes **. Can anyone explain what they mean in Objective C? (I used to be a Java programmer, with experience in C/C++.)
The * denotes that you are using a pointer to a variable, and is most commonly used to store a reference to an Objective-C object, objects which can only live on the heap and not the stack.
Pointers are not a part of Objective-C exclusively, but rather a feature of C (and therefore its derived languages, of which Objective-C is one of them).
If you are questioning the difference between * and **, the first denotes a pointer, whereas the second denotes a pointer to a pointer; the advantage of the latter to the former is that when passing in an object using ** in a method parameter, the method can then change this parameter and the new value is accessible in the calling method.
Perhaps the most common use of ** in Cocoa is when using NSError objects. When a method is called that can return an NSError object on failure, the method signature would look something like this:
- (id)someMethodThatUsesObject:(id)object error:(NSError**)error;
What this means is that the calling function can pass in a pointer to an NSError object, but someMethodThatUsesObject: can change the value of error to another NSError object if it needs to, which can then be accessed by the calling method.
This is often used as a workaround for the fact that functions can only return one value.
A * in Objective-C means exactly the same as in C; and you'll usually see it (or not) in these situations:
// Method signatures:
// Here the asterisk (*) shows that you have a pointer to an NSString instance.
+ (NSString *)stringWithString:(NSString *)aString;
// Method signatures, part two:
// Here the double asterisk (**) signifies that you should pass in a pointer
// to an area of memory (NSError *) where outError can be written.
- (BOOL)writeToURL:(NSURL *) atomically:(BOOL) error:(NSError **)outError;
// Method signatures make for good examples :)
// Here the asterisk is hidden; id is a typedef for void *; and it signifies that
// a pointer to some object of an indeterminate class will be returned
- (id)init;
// And a forth example to round it all out: C strings!
// Here the asterisk signifies, you guessed it, a pointer! This time, it's a
// pointer to the first in a series of const char; terminated by a \0, also known
// as a C string. You probably won't need to work with this a lot.
- (const char *)UTF8String;
// For a bit of clarity, inside example two, the outError is used as follows:
// Here the asterisk is used to dereference outError so you can get at and write
// to the memory it points to. You'd pass it in with:
// NSError *anError;
// [aString writeToURL:myURL atomically:YES error:&anError];
- (BOOL)writeToURL:(NSURL *)url atomically:(BOOL)atom error:(NSError **)outError {
// do some writing, and if it went awry:
if (outError != NULL)
*outError = [NSError errorWithName:#"NSExampleErrorName"];
return NO;
}
I'm trying to build a NSArray of methods in Objective-C.
(What I'm trying to accomplish here is something like the following in C)
typedef (void)(*handler)(int command);
void handleCommandA(void) { ... }
void handleCommandB(void) { ... }
static const handler handler_table[10] = {
handleCommandA, handleCommandB, handleCommandC
};
I have to port this to Objective-C and I don't know how to
build an array of function pointers (in Objective-c world,
class methods) at compile-time.
In Objective-C I have the following.
- (void)handleCommandA { ... }
- (void)handleCommandB { ... }
/* Now how to add above 2 functions into NSArray? */
NSArray *handler_table = [NSArray arrayWithObjects:... ]; /* This doesn't seem to work. */
The problem here is that to bind those functions you must use the selector keyword which returns a SEL type. This is a pointer type whereas NSArray stores objects.
You thus have three options;
Use a regular C-type array
Fold the functions into an NSObject derived class that will call them.
Use a protocol.
The second is likely the nicer and for this you can use the NSValue class to hold the selector results. E.g;
NSValue* selCommandA = [NSValue valueWithPointer:#selector(handleCommandA:)];
NSValue* selCommandB = [NSValue valueWithPointer:#selector(handleCommandB:)];
NSArray *handler_table = [NSArray arrayWithObjects:selCommandA, selCommandB, nil ];
When you have retrieved the correct entry from the array, to convert back you would do;
SEL mySelector = [selCommand pointerValue];
[someObject performSelector:mySelector];
(Note I'm assuming that from your objective-c syntax that these are intended to be used as methods on an object and not global functions. If you wish to use them globally then you should write them as you would in plain C.)
Another option is to formalize the command methods into a protocol. This allows you to write functionality that will work on any object which implements that protocol and the compiler will provide more checking than if you were just calling selectors.
E.g.
// some header
#protocol CommandHandler
#required
-(void) handleCommandA;
-(void) handleCommandB;
#end
// some other header
#interface someClass : NSObject<CommandHandler>
{
// you will receive compiler warnings if you do not implement the protocol functions
}
Your handling and dispatch code is then written to work with objects of type "CommandHandler". E.g
-(void) registerForCommands:(CommandHandler*)handler
Use NSValue.
For example:
NSArray* handlers = [NSArray arrayWithObjects:[NSValue valueWithPointer:handleA] ... ];
then to access :
handleptr* handle = (handlerptr*)[[handlers objectAtIndex:0] pointerValue];
handle(foo_bar);
In Objective-C, you don't pass around methods; you pass around selectors, which are basically the canonical names of methods. Then, to make an object respond to a selector message, you send it performSelector:. For example:
NSString *exampleString = [NSString stringWithString:#"Hello"];
SEL methodName = #selector(stringByAppendingString:);
// ^This is the selector. Note that it just represents the name of a
// message, and doesn't specify any class or implementation
NSString *combinedString = [exampleString performSelector:methodName withObject:#" world!"];
What you'll want is to make an array of NSStrings containing the names of the selectors you're interested in. You can use the function NSStringFromSelector() to do this. Then, when you want to use them, call NSSelectorFromString() on the strings to get the original selector back and pass it to the appropriate object's performSelector:. (As shown in the example above, the receiver isn't encoded in a selector — just the method name — so you might need to store the receiver as well.)
I have a method in an objective-C class. It has 2 callback functions written in C. The class pointer i.e. self is passed to these functions as void *. In the C functions I create a pointer of type class and assign the void * parameter.
The first callback function executes successfully. But the void * pointer becomes nil in the 2nd callback function. Note that I haven't tweaked pointer in the first callback but still I get nil in 2nd callback.
Any ideas what might be going wrong?
For example:
kr = IOServiceAddMatchingNotification(gNotifyPort, kIOFirstMatchNotification,
matchingDict, RawDeviceAdded, NULL,
&gRawAddedIter);
RawDeviceAdded(NULL, gRawAddedIter, self);
This works fine. But below function receives self as nil.
kr = IOServiceAddMatchingNotification(gNotifyPort, kIOFirstMatchNotification,
matchingDict, BulkTestDeviceAdded, NULL,
&gBulkTestAddedIter);
BulkTestDeviceAdded(NULL, gBulkTestAddedIter, self);
Are your problems specifically with the IOKit callback routines? The problem with the specific example you gave is that the IOServiceMatchingCallback takes only 2 parameters, not 3. You need your RawDeviceAdded() and BulkTestDeviceAdded() callback functions to match the IOServiceMatchingCallback prototype and to accept self as the first parameter (refCon), not the 3rd. Also, you need to pass in self as the second-to-last parameter of IOServiceAddMatchingNotification() to get it passed back to you by the callback.
A common method for handling C callbacks in Objective-C code is just to have a static function that forwards the callback to your instance. So, your example callback code would look like this:
static RawDeviceAdded(void* refcon, io_iterator_t iterator)
{
[(MyClass*)refcon rawDeviceAdded:iterator];
}
#implementation MyClass
- (void)setupCallbacks
{
// ... all preceding setup snipped
kr = IOServiceAddMatchingNotification(gNotifyPort,kIOFirstMatchNotification, matchingDict,RawDeviceAdded,(void*)self,&gRawAddedIter );
// call the callback method once to 'arm' the iterator
[self rawDeviceAdded:gRawAddedIterator];
}
- (void)rawDeviceAdded:(io_iterator_t)iterator
{
// take care of the iterator here, making sure to complete iteration to re-arm it
}
#end
Generally, callbacks in Objective-C are handled by passing a delegate object and a selector to perform on that delegate. For example, this method will call a method on its delegate after logging a message, passing both itself and the message that was logged.
- (void)logMessage:(NSString *)message
delegate:(id)delegate
didLogSelector:(SEL)didLogSelector
{
NSLog(#"%#", message);
if (delegate && didLogSelector && [delegate respondsToSelector:didLogSelector]) {
(void) [delegate performSelector:didLogSelector
withObject:self
withObject:message];
}
}
You might call it in code like this:
- (void)sayHello
{
[logger logMessage:#"Hello, world"
delegate:self
didLogSelector:#selector(messageLogger:didLogMessage:)];
}
- (void)messageLogger:(id)logger
didLogMessage:(NSString *)message
{
NSLog(#"Message logger %# logged message '%#'", logger, message);
}
You can also use objc_msgSend() directly instead, though you need to understand the Objective-C runtime enough to choose which variant to use and how to construct the prototype and function pointer through which to call it. (It's the mechanism by which message sends are actually implemented in Objective-C — what the compiler normally generates calls to in order to represent [] expressions.)
This is what Objective-C's selector is for:
http://developer.apple.com/iphone/library/documentation/Cocoa/Reference/NSInvocationOperation_Class
The API isn't very intuitive, but its fine once you understand it
You might need to do some refactoring as well, now there might be a better way, but when I had this problem my solution was to refactor and use InvoationOperation.