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.
Related
Say I have two classes, BulbDevice and FanDevice, both are subclasses of Device and has a method signature like this:
+ (BOOL)isMyId:(NSInteger)someId;
If I wanted to create a class I could test it out:
if ([BulbDevice isMyId:someId]) {
Device *dev = [BulbDevice alloc] initWithId:someId];
}
But what I really want is to create a factory method inside a factory class, with minimum fuss when new device are added:
+ (Device)createDevice:(NSInteger)someId {
// say I have an array registered
NSArray *arr = #[[BulbDevice class], [FanDevice class]];
// Loop through it.
Device *device;
for (Class *c in arr) {
// The idea is kind of like this but I'm not sure how to make it work
if ([c isMyId]) {
device = [[c alloc] init];
}
}
}
The idea is that I only need to update arr in the factory method. So I think it is good to have something like this. But I am not sure how to make it work.
EDIT:
I took out the asterisk, but it won't work:
for (Class c in arr) {
// Now I want to access the isMyId which is a static method,
// but I how do I cast to that class? I mean not an object of the class, but to that class itself.
if ([(Device)c isMyId:]) {
}
}
But I still need a way to access that class method. Error says Used type 'Device' where arithmetic or pointer type is required, and even if it works, I want to access class method, not sending message to an object.
Or shall I store NSString in the array instead? But it is hard to find way to access the class method as well.
If I understand correctly what you are trying to achieve, then your approach seems to be correct.
There is only one thing that needs to be fixed:
for (Class c in arr)
c variable is not a pointer - the asterisk should be removed. Your code works.
The Class type is not an NSObject type, and although it is a bit special it is object-like or object-equivalent, so you are able to send it messages and store it in collections like you're doing.
You don't use the asterisk as #MaxPevsner says, because Class isn't used as a normal pointer-to-object. Think of Class as a special type like id which also doesn't get the * when you use it to reference an object.
According to this comment:
C functions inside #implementation blocks have the unique property of
being able to access private and protected ivars directly. Thus, from
my own experience, it's become a strong idiom to place C functions
that "belong" to a class inside the corresponding implementation.
My code, defining a private instance variable in the implementation only as suggested by this answer:
With the brackets:
#implementation ViewController{
MyTest *tt;
}
void testf(){
NSLog(#"hello back from c++ into obj c land");
[tt testcf: 5];
}
...
Will not build; the compiler indicates that tt in testf is undeclared. If I remove the brackets, then the C function works fine.
But... if I remove the brackets, do I understand that actually this is no longer an instance variable, but sneakily it is a global variable, disconnected from the class, despites its placement in the #implementation? It would appear that this is indeed true, since I can do this as the end:
#end
void testf2(){
NSLog(#"hello back from c++ into obj c land");
[tt testcf: 5];
}
And the compiler does not contain about tt being out of scope. So -- how to declare a private instance variable in an implementation and have it truly be an instance variable, but also have a C function be able to access it? For the purposes of this question, I am trying to get this to work based on the linked comments/answers without using the id pointer of the object itself.
You will need to pass a reference to self to the C-function:
void testf(ViewController *vc){
NSLog(#"hello back from c++ into obj c land");
[vc->tt testcf: 5];
}
and call it like this:
- (void)someMethodInViewController
{
testf(self);
}
However it's not clear why you are using C-functions at all.
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.
Are there any Objective-C runtime functions that will allow me to get a function (or block) pointer from a string identifying the function? I need some way to dynamically find and invoke a function or static method based on some form of string identifying it.
Ideally this function should be able to exist in any dynamically loaded library.
Looking at Objective-C Runtime Reference, the best bet looks like class_getClassMethod, but there don't appear to be any function-related functions in this reference. Are there other raw C ways of getting a pointer to a function by name?
if you want to invoke some static objc method, you can make it as a class method of a class
#interface MyClas : NSObject
+ (int)doWork;
#end
and call the method by
[[MyClass class] performSelector:NSSelectorFromString(#"doWork")];
if you real want to work with C-style function pointer, you can check dlsym()
dlsym() returns the address of the code or data location specified by
the null-terminated character
string symbol. Which libraries and bundles are searched depends on the handle
parameter If dlsym() is called with the special handle RTLD_DEFAULT,
then all mach-o images in the process
(except those loaded with dlopen(xxx, RTLD_LOCAL)) are searched in the order they were loaded. This
can be a costly search and should be avoided.
so you can use it to find the function pointer base on asymbol name
not sure why you want to do this, sometimes use function table can do
typedef struct {
char *name,
void *fptr // function pointer
} FuncEntry;
FuncEntry table[] = {
{"method", method},
{"method2", method2},
}
// search the table and compare the name to locate function, you get the idea
If you know method signature you can create selector to it with NSSelectorFromString function, e.g.:
SEL selector = NSSelectorFromString(#"doWork");
[worker performSelector:selector];
You may be able to do what you want with libffi. But unless you are doing something like create your own scripting language or something like that where you need to do this sort of thing a lot. It is probable overkill
I've wondered the SAME thing.. and I guess, after having researched it a bit.. there is NOT a "standard C" way to do such a thing.. (gasp).. but to the rescue? Objective C blocks!
An anonymous function.. that can be OUTSIDE any #implementation, etc...
void doCFunction() { printf("You called me by Name!"); }
Then, in your objective-C method… you can somehow "get" the name, and "call" the function...
NSDictionary *functionDict = #{ #"aName" : ^{ doCFunction(); } };
NSString *theName = #"aName";
((void (^)()) functionDict[theName] )();
Result: You called me by Name!
Loves it! 👓 ⌘ 🐻
I have problem that im trying to get solve for like week.
My goal is to get variable out of my IBAction, to use for example in -(void)viewDidLoad..
But as far as I am now I can use my variable only in my IBAction..
- (IBAction) changeLat:(NSNumber *)str {
longi = str;
double lop = longi.doubleValue;
NSLog(#"%f",lop);
}
- (void)viewDidLoad
{
[super viewDidLoad];
NSLog (#"%#",lop);
}
It NSLog shows everything fine in action, but in view did load it doesn't even recorganize it.
If you create a variable inside of -IBAction, the scope of that variable is only that method, so you cannot access to that variable outside it.
If you want your variable to be global to your class, you have to create it in the declaration of your class, like this:
#interface MainViewController () {
#private
double lop;
}
Put this at the beginning of your .m file, and then lop would be accesible in all your class.
You can read more about the scope of the variables here:
http://www.techotopia.com/index.php/Objective-C_Variable_Scope_and_Storage_Class
Actually, IBAction is converted to void by the preprocessor. It's used by Interface Builder as a label that identifies this method as an action able to be related from an IB Object.
There's no way (AFAIK) to use two return types in a function (for example `(IBAction double)´, equivalent to ´(void double)´), but a good practice could be something like this:
- (IBAction)changeLatAction:(id)sender {
NSNumber *str = <get the NSNumber from a valid place>;
[self changeLat:str];
}
- (double) changeLat:(NSNumber *)str {
longi = str;
double lop = longi.doubleValue;
NSLog(#"%f",lop);
return ????;
}
Your first declaration of changeLat seems to be wrong, because as a first parameter you'll always get the "sender" or "caller" object, related from IB (when called from an action, of course), so, you need to get the str value from a valid place.
Cheers.