I was going over this example in which a selector is used. I have copied the code from there for convenience.
// MYTapGestureRecognizer.h
#interface MYTapGestureRecognizer : UITapGestureRecognizer
#property (nonatomic, strong) NSString *data;
#end
// MYTapGestureRecognizer.m
#implementation MYTapGestureRecognizer
#end
// =====================
....
MYTapGestureRecognizer *singleTap = [[MYTapGestureRecognizer alloc] initWithTarget:self action:#selector(tapDetected:)];
singleTap.data = #"Hello";
.....
// ====================
-(void)tapDetected:(UITapGestureRecognizer *)tapRecognizer {
MYTapGestureRecognizer *tap = (MYTapGestureRecognizer *)tapRecognizer;
NSLog(#"data : %#", tap.data);
}
My question is
1-When self calls the selector what parameter does it pass in the above case ?
2- Also if a selector (pointing to a method that requires parameters) is called (see example below) and no parameters are passed are there any defaults in that case ? If possible is there any documentation for that ?
Suppose the signature of MyTest is
- (void) MyTest : (NSString*) a;
Now constructing and calling a selector
SEL a = NSSelectorFromString(#"MyTest:");
[t performSelector:a]; //Works Fine and the call is made - However Notice no parameter is passed . In this case what would the value of the parameter be in the method ?
I checked the following but I could not find this information
Apple docs
Rys Tutorials
Answers to your questions:-
When self calls the selector what parameter does it pass in the above case ?
If a tap is detected and the selector is called, the parameter will be an object of UITapGestureRecognizer. This will be the same instance on which the tap gesture is detected.
Also if a selector (pointing to a method that requires parameters) is called (see example below) and no parameters are passed are there any defaults in that case ? If possible is there any documentation for that ?
Why do you want to call the method like that, is there any special purpose?. If not, you can call the method just like
[self tapDetected:nil];
or
[self performSelector:#selector(tapDetected:) withObject:nil];
If you call the method as provided in the question, most probably it will crash.
If you wish to call the method on self, pass nil parameter to it. But i do not understand what purpose is it serving you.
Also if you do not send parameters to your methods, it is going to fail at your builds. You have to pass either the parameter or nil.
Also if your method does not accept nil parameters it might cause an exception - 'NSInvalidArgumentException'
Normally if you want to access that selector via self, use it like :
[self tapDetected:nil];
You need to handle this case in your selector, like :
-(void)tapDetected:(UITapGestureRecognizer *)tapRecognizer {
if (tapRecognizer)
{
MYTapGestureRecognizer *tap = (MYTapGestureRecognizer *)tapRecognizer;
NSLog(#"data : %#", tap.data);
}
else
{
//Do your work
}
}
Also not only this, if you are not sure of parameter you are passing change your selector decalartion as id, like :
-(void)tapDetected:(id)sender {
NSString *className = NSStringFromClass([id class]);
NSLog(#"Object passed is of class : %#", className);
//And make check here
if ([id isKindOfClass:[MYTapGestureRecognizer class]])
{
//Do your work here
}
}
There are no default cases, you need to handle every case manually or else app will crash.
SEL a = NSSelectorFromString(#"MyTest:");
[t performSelector:a]; //Works Fine and the call is made - However Notice no parameter is passed . In this case what would the value of the parameter be in the method ?
It will be undefined junk. You have no guarantees about what it might contain. Most likely, it will be an invalid pointer. If you're unlucky, it might be a valid pointer to some arbitrary object and operating on it will corrupt your app's state. If you're lucky, it will crash so you can find the problem.
Related
This question already has answers here:
NSInvocation for Dummies?
(4 answers)
Closed 7 years ago.
I came across this code in a project, where it is using NSInvocation. I want to know what it is supposed to do, and why would we ever need that. Simple explanations would be appreciated. I am posting the code.
// Public interface
#interface CCDelegateSplitter : NSObject
- (void) addDelegate: (id) delegate;
- (void) addDelegates: (NSArray*) delegates;
#end
// Private interface
#interface CCDelegateSplitter ()
#property(strong) NSMutableSet *delegates;
#end
#implementation CCDelegateSplitter
- (id) init
{
self = [super init];
_delegates = [NSMutableSet set];
return self;
}
- (void) addDelegate: (id) delegate
{
[_delegates addObject:delegate];
}
- (void) addDelegates: (NSArray*) delegates
{
[_delegates addObjectsFromArray:delegates];
}
- (void) forwardInvocation: (NSInvocation*) invocation
{
for (id delegate in _delegates) {
[invocation invokeWithTarget:delegate];
}
}
- (NSMethodSignature*) methodSignatureForSelector: (SEL) selector
{
NSMethodSignature *our = [super methodSignatureForSelector:selector];
NSMethodSignature *delegated = [(NSObject *)[_delegates anyObject] methodSignatureForSelector:selector];
return our ? our : delegated;
}
- (BOOL) respondsToSelector: (SEL) selector
{
return [[_delegates anyObject] respondsToSelector:selector];
}
#end
I'll assume you know what an NSInvocation is (if not, it's a data structure that holds all the information needed to make a method call; think "blocks" from long before blocks were added to the language).
forwardInvocation: is one of the methods that the runtime will call if it cannot find an implementation for a method. So if you pass a -doSomething message to an object [object doSomething], it will first check whether it has a doSomething method. Then it will check its superclasses. It'll try dynamic method resolution (resolveInstanceMethod for instance). It'll look for a forwarding target (forwardingTargetForSelector:), and it'll finally, if everything else fails, it'll create an invocation (using methodSignatureForSelector: and punt to forwardInvocation:. By default, forwardInvocation: just calls doesNotRecognizeSelector: which crashes you on iOS (or terminates the current thread on OS X). But you can override it to do something else (like they have here).
methodSignatureForSelector: is necessary so that the runtime system can create an invocation out of a message. This one either returns a method signature from this object or its superclasses, or it asks one of its targets for the appropriate method signature. A selector by itself isn't enough to figure out exactly how to call a method. The system needs to ask an object how the method actually works (what types it takes and what type it returns).
The code you've posted is a multi-delegate trampoline. It will accept any selector that its targets respond to (technically it'll pick a random target and see if it responds), and it'll forward that message to all of its targets.
For a similar trampoline with some comments about usage, you may want to look at RNObserverManager.
Take a look at the link in bfitch's comment. That covers what NSInvocation is, and hints at why you would use it, but doesn't cover the why in much detail.
NSInvocation lets you do fairly advanced things like creating a proxy object that actually forwards messages to another object. With NSInvocation it's possible to take ANY message at runtime and forward it to another object.
Another example is the performSelector family of methods. There's performSelector:, performSelector:withObject:, and performSelector:withObject:withObject:. (plus variants like performSelector:withObject:afterDelay:, performSelector:onThread:, etc.) Those methods take 0, 1, or or 2 objects as parameters. If you need to invoke a method on another object that takes scalar parameters, or anything other than 0, 1 or 2 objects, you're out of luck. However you can send messages with ANY type of parameters using NSInvocation.
Note that when blocks were added to Objective-C the need for tricks like performSelector and NSInvocation become less. Blocks can reference variables from their enclosing scope, which makes them more flexible.
I love blocks and it makes me sad when I can't use them. In particular, this happens mostly every time I use delegates (e.g.: with UIKit classes, mostly pre-block functionality).
So I wonder... Is it possible -using the crazy power of ObjC-, to do something like this?
// id _delegate; // Most likely declared as class variable or it will be released
_delegate = [DelegateFactory delegateOfProtocol:#protocol(SomeProtocol)];
_delegate performBlock:^{
// Do something
} onSelector:#selector(someProtocolMethod)]; // would execute the given block when the given selector is called on the dynamic delegate object.
theObject.delegate = (id<SomeProtocol>)_delegate;
// Profit!
performBlock:onSelector:
If YES, how? And is there a reason why we shouldn't be doing this as much as possible?
Edit
Looks like it IS possible. Current answers focus on the first part of the question, which is how. But it'd be nice to have some discussion on the "should we do it" part.
Okay, I finally got around to putting WoolDelegate up on GitHub. Now it should only take me another month to write a proper README (although I guess this is a good start).
The delegate class itself is pretty straightforward. It simply maintains a dictionary mapping SELs to Block. When an instance recieves a message to which it doesn't respond, it ends up in forwardInvocation: and looks in the dictionary for the selector:
- (void)forwardInvocation:(NSInvocation *)anInvocation {
SEL sel = [anInvocation selector];
GenericBlock handler = [self handlerForSelector:sel];
If it's found, the Block's invocation function pointer is pulled out and passed along to the juicy bits:
IMP handlerIMP = BlockIMP(handler);
[anInvocation Wool_invokeUsingIMP:handlerIMP];
}
(The BlockIMP() function, along with other Block-probing code, is thanks to Mike Ash. Actually, a lot of this project is built on stuff I learned from his Friday Q&A's. If you haven't read those essays, you're missing out.)
I should note that this goes through the full method resolution machinery every time a particular message is sent; there's a speed hit there. The alternative is the path that Erik H. and EMKPantry each took, which is creating a new clas for each delegate object that you need, and using class_addMethod(). Since every instance of WoolDelegate has its own dictionary of handlers, we don't need to do that, but on the other hand there's no way to "cache" the lookup or the invocation. A method can only be added to a class, not to an instance.
I did it this way for two reasons: this was an excercise to see if I could work out the part that's coming next -- the hand-off from NSInvocation to Block invocation -- and the creation of a new class for every needed instance simply seemed inelegant to me. Whether it's less elegant than my solution, I will leave to each reader's judgement.
Moving on, the meat of this procedure is actually in the NSInvocation category that's found in the project. This utilizes libffi to call a function that's unknown until runtime -- the Block's invocation -- with arguments that are also unknown until runtime (which are accessible via the NSInvocation). Normally, this is not possible, for the same reason that a va_list cannot be passed on: the compiler has to know how many arguments there are and how big they are. libffi contains assembler for each platform that knows/is based on those platforms' calling conventions.
There's three steps here: libffi needs a list of the types of the arguments to the function that's being called; it needs the argument values themselves put into a particular format; then the function (the Block's invocation pointer) needs to be invoked via libffi and the return value put back into the NSInvocation.
The real work for the first part is handled largely by a function which is again written by Mike Ash, called from Wool_buildFFIArgTypeList. libffi has internal structs that it uses to describe the types of function arguments. When preparing a call to a function, the library needs a list of pointers to these structures. The NSMethodSignature for the NSInvocation allows access of each argument's encoding string; translating from there to the correct ffi_type is handled by a set of if/else lookups:
arg_types[i] = libffi_type_for_objc_encoding([sig getArgumentTypeAtIndex:actual_arg_idx]);
...
if(str[0] == #encode(type)[0]) \
{ \
if(sizeof(type) == 1) \
return &ffi_type_sint8; \
else if(sizeof(type) == 2) \
return &ffi_type_sint16; \
Next, libffi wants pointers to the argument values themselves. This is done in Wool_buildArgValList: get the size of each argument, again from the NSMethodSignature, and allocate a chunk of memory that size, then return the list:
NSUInteger arg_size;
NSGetSizeAndAlignment([sig getArgumentTypeAtIndex:actual_arg_idx],
&arg_size,
NULL);
/* Get a piece of memory that size and put its address in the list. */
arg_list[i] = [self Wool_allocate:arg_size];
/* Put the value into the allocated spot. */
[self getArgument:arg_list[i] atIndex:actual_arg_idx];
(An aside: there's several notes in the code about skipping over the SEL, which is the (hidden) second passed argument to any method invocation. The Block's invocation pointer doesn't have a slot to hold the SEL; it just has itself as the first argument, and the rest are the "normal" arguments. Since the Block, as written in client code, could never access that argument anyways (it doesn't exist at the time), I decided to ignore it.)
libffi now needs to do some "prep"; as long as that succeeds (and space for the return value can be allocated), the invocation function pointer can now be "called", and the return value can be set:
ffi_call(&inv_cif, (genericfunc)theIMP, ret_val, arg_vals);
if( ret_val ){
[self setReturnValue:ret_val];
free(ret_val);
}
There's some demonstrations of the functionality in main.m in the project.
Finally, as for your question of "should this be done?", I think the answer is "yes, as long as it makes you more productive". WoolDelegate is completely generic, and an instance can act like any fully written-out class. My intention for it, though, was to make simple, one-off delegates -- that only need one or two methods, and don't need to live past their delegators -- less work than writing a whole new class, and more legible/maintainable than sticking some delegate methods into a view controller because it's the easiest place to put them. Taking advantage of the runtime and the language's dynamism like this hopefully can increase your code's readability, in the same way, e.g., Block-based NSNotification handlers do.
I just put together a little project that lets you do just this...
#interface EJHDelegateObject : NSObject
+ (id)delegateObjectForProtocol:(Protocol*) protocol;
#property (nonatomic, strong) Protocol *protocol;
- (void)addImplementation:(id)blockImplementation forSelector:(SEL)selector;
#end
#implementation EJHDelegateObject
static NSInteger counter;
+ (id)delegateObjectForProtocol:(Protocol *)protocol
{
NSString *className = [NSString stringWithFormat:#"%s%#%i",protocol_getName(protocol),#"_EJH_implementation_", counter++];
Class protocolClass = objc_allocateClassPair([EJHDelegateObject class], [className cStringUsingEncoding:NSUTF8StringEncoding], 0);
class_addProtocol(protocolClass, protocol);
objc_registerClassPair(protocolClass);
EJHDelegateObject *object = [[protocolClass alloc] init];
object.protocol = protocol;
return object;
}
- (void)addImplementation:(id)blockImplementation forSelector:(SEL)selector
{
unsigned int outCount;
struct objc_method_description *methodDescriptions = protocol_copyMethodDescriptionList(self.protocol, NO, YES, &outCount);
struct objc_method_description description;
BOOL descriptionFound = NO;
for (int i = 0; i < outCount; i++){
description = methodDescriptions[i];
if (description.name == selector){
descriptionFound = YES;
break;
}
}
if (descriptionFound){
class_addMethod([self class], selector, imp_implementationWithBlock(blockImplementation), description.types);
}
}
#end
And using an EJHDelegateObject:
self.alertViewDelegate = [EJHDelegateObject delegateObjectForProtocol:#protocol(UIAlertViewDelegate)];
[self.alertViewDelegate addImplementation:^(id _self, UIAlertView* alertView, NSInteger buttonIndex){
NSLog(#"%# dismissed with index %i", alertView, buttonIndex);
} forSelector:#selector(alertView:didDismissWithButtonIndex:)];
UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:#"Example" message:#"My delegate is an EJHDelegateObject" delegate:self.alertViewDelegate cancelButtonTitle:#"Cancel" otherButtonTitles:#"OK", nil];
[alertView show];
Edit: This is what I've come up after having understood your requirement. This is just a quick hack, an idea to get you started, it's not properly implemented, nor is it tested. It is supposed to work for delegate methods that take the sender as their only argument. It works It is supposed to work with normal and struct-returning delegate methods.
typedef void *(^UBDCallback)(id);
typedef void(^UBDCallbackStret)(void *, id);
void *UBDDelegateMethod(UniversalBlockDelegate *self, SEL _cmd, id sender)
{
UBDCallback cb = [self blockForSelector:_cmd];
return cb(sender);
}
void UBDelegateMethodStret(void *retadrr, UniversalBlockDelegate *self, SEL _cmd, id sender)
{
UBDCallbackStret cb = [self blockForSelector:_cmd];
cb(retaddr, sender);
}
#interface UniversalBlockDelegate: NSObject
- (BOOL)addDelegateSelector:(SEL)sel isStret:(BOOL)stret methodSignature:(const char *)mSig block:(id)block;
#end
#implementation UniversalBlockDelegate {
SEL selectors[128];
id blocks[128];
int count;
}
- (id)blockForSelector:(SEL)sel
{
int idx = -1;
for (int i = 0; i < count; i++) {
if (selectors[i] == sel) {
return blocks[i];
}
}
return nil;
}
- (void)dealloc
{
for (int i = 0; i < count; i++) {
[blocks[i] release];
}
[super dealloc];
}
- (BOOL)addDelegateSelector:(SEL)sel isStret:(BOOL)stret methodSignature:(const char *)mSig block:(id)block
{
if (count >= 128) return NO;
selectors[count] = sel;
blocks[count++] = [block copy];
class_addMethod(self.class, sel, (IMP)(stret ? UBDDelegateMethodStret : UBDDelegateMethod), mSig);
return YES;
}
#end
Usage:
UIWebView *webView = [[UIWebView alloc] initWithFrame:CGRectZero];
UniversalBlockDelegate *d = [[UniversalBlockDelegate alloc] init];
webView.delegate = d;
[d addDelegateSelector:#selector(webViewDidFinishLoading:) isStret:NO methodSignature:"v#:#" block:^(id webView) {
NSLog(#"Web View '%#' finished loading!", webView);
}];
[webView loadRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:#"http://google.com"]]];
I'm trying to have another object call a selector. I'm attempting to define this selector from another class by defining the selector property. It doesn't seem to be working like I expect.
ComboBox.h
#property (nonatomic) SEL onComboSelect;
ComboBox.m
-(void)doneClicked:(id) sender
{
[textField resignFirstResponder]; //hides the pickerView
NSLog(#"DONE CLICKED CALLED");
[self performSelector:#selector(onComboSelect)];
}
OtherClass.h
#interface OtherClass : BaseViewController
{
ComboBox *combo;
}
-(void)comboSelector;
OtherClass.m
// in viewDidLoad
combo = [[ComboBox alloc] init];
combo.onComboSelect = #selector(comboSelector);
-(void)comboSelector
{
NSLog(#"COMBO SELECTOR");
}
I see "DONE CLICK CALLED" in the logs, but not "COMBO SELECTOR". So I know doneClicked is being called, but the selector doesn't seem to be working. What am I doing wrong? Is there a better way to do this?
A #selector is just a method name - it does not include any context about the class on which it is defined. So this [self performSelector:#selector(onComboSelect)] is just invoking the method on self. In addition to the selector, you also need a reference to the object on which you want to call it.
Notice how some built-in classes (like UIControl) take both a target object and action selector.
There are a 2 major issues in your code.
1.
onComboSelect is a SEL so no need to use the #selector again.
Instead of:
[self performSelector:#selector(onComboSelect)];
Use :
[self performSelector:onComboSelect];
2.
You are calling the selector on self from ComboBox class, so it'll call the selector on ComboBox object (if defined) not on OtherClass object
Your answers were helpful. Here is what I did:
added to ComboBox.h
#property (nonatomic, weak) UIViewController *parentViewController;
added to ComboBox.m
-(void)doneClicked:(id) sender
{
[textField resignFirstResponder]; //hides the pickerView
if ([parentViewController respondsToSelector:#selector(comboSelector)])
[parentViewController performSelector:#selector(comboSelector)];
}
added to OtherClass.m
combo.parentViewController = self;
#property (nonatomic) SEL onComboSelect <-- This property is not needed in ComboBox.h.
SEL is a point of objc_seletor,and in object_seletor runtime can find the
objc_method ,that define
objc_method {
SEL method_name OBJC2_UNAVAILABLE;
char *method_types OBJC2_UNAVAILABLE;
IMP method_imp OBJC2_UNAVAILABLE;
}
and IMP is the point of method,and you can find this in "runtime.h" file.
Any noe kow how to find objc_method by object_seletor ? and I cannot find the define of objc_seletor struct.
I need to add additional behavior to methods I need to extend, i.e. implement method that looks like
- (void)extendMethod:(SEL)selector forClass:(Class)class withCompletionBlock:(void (^)(void))completionBlock;
So every time Class instance call a method with SEL selector in addition should be invoked my completion block.
I've tried method swizzling, but ran into some problems: I want original method implementation to be called.
What I need is very similar with subclassing, but this should be implemented without subclassing.
UPDATE:
For example I have subclass of UIViewController named MyViewController. MyViewController have - (void)viewDidLoad method. Somewhere in application I call method
[methodExtender extendMethod:#selector(viewDidLoad)
forClass:[MyViewController class]
withCompletionBlock:^{
NSLog(#"view did load called");
}];
So after viewDidLoad method of every instance of MyViewController my completion block invoked.
I'm not sure how you want to use selector, but you can try extend any class(even ones that you don't have implementation file) by using mechanism in Objective-C know as "Categories".
From Xcode click on File->New->File (command+n)
From Cocoa Touch choose Objective-C category
Type name of your category and choose class on which you want to make category (I choosed UIButton)
Then next and Create.
Xcode will create two files for example: UIButton+extendMethod.h
Declare your method in header file and implement it in *.m file.
Using
If you want to use in let's say your View Controller in *.h file import
#import "UIButton+extendMethod.h"
and then you can call your method like this:
UIButton *button = [[UIButton alloc] init];
[button extendMethod:#selector(yourMethod:)];
Swizzling does allow you to call the original implementation, though it is just a bit confusing. Because the implementations are swapped after swizzling, you call the original implementation using the selector of the swizzled method:
- (void)mySwizzledImplementation {
// do stuff
// now call original implementation (using swizzled selector!)
[self mySwizzledImplementation];
// do more stuff
}
See also http://cocoadev.com/wiki/MethodSwizzling
There is no way (anymore)to simulate inheritance without subclassing. There use to be Posing, method swizzling is all that is left (not as elegant as posing though). Here is one way to do method swizzling correctly while being able to call the original implementation.
int swizzle_instance_methods(Class class, SEL selector, IMP replacement, IMP *store) {
#synchronized(class) {
Method method = class_getInstanceMethod(class, selector);
IMP original_imp = NULL;
if (method != NULL) {
const char *type = method_getTypeEncoding(method);
IMP original_imp = class_replaceMethod(class, selector, replacement, type);
if (original_imp == NULL)
original_imp = method_getImplementation(method);
if (original_imp != NULL && store != NULL) {
*store = original_imp;
}
}
return (original_imp != NULL);
}
}
+ (void) load
{
static IMP originalMethodImpl = NULL;
IMP customMethodImpl = imp_implementationWithBlock(^(id self_) {
NSString *descr = ((NSString(*)(id,SEL))originalMethodImpl)(self_, #selector(description);
return [NSString stringWithFormat:#"<--- %# --->",descr];
});
swizzle_instance_methods([self class], #selector(description), customMethodImpl, &originalMethodImpl);
}
I might add that this is nice for debugging and I think that it can be greate for building excellent frameworks. Alas, Apple seems to think differently and using method swizzling can result in your app being excluded from the App store. If you are not aiming for the app store then all the power to you.
It is be possible with ObjC categories. For ex, you can extend hasPrefix method of NSString as follows,
-(BOOL)hasPrefixx:(NSString *)aString
{
NSLog(#"Checking has prefix");
return [self hasPrefix:aString];
}
If you import the category, you should be able to call this method. But his means you got change the method in your call.
By the way, Method swizzling should work. Bit of explanation here.
I love blocks and it makes me sad when I can't use them. In particular, this happens mostly every time I use delegates (e.g.: with UIKit classes, mostly pre-block functionality).
So I wonder... Is it possible -using the crazy power of ObjC-, to do something like this?
// id _delegate; // Most likely declared as class variable or it will be released
_delegate = [DelegateFactory delegateOfProtocol:#protocol(SomeProtocol)];
_delegate performBlock:^{
// Do something
} onSelector:#selector(someProtocolMethod)]; // would execute the given block when the given selector is called on the dynamic delegate object.
theObject.delegate = (id<SomeProtocol>)_delegate;
// Profit!
performBlock:onSelector:
If YES, how? And is there a reason why we shouldn't be doing this as much as possible?
Edit
Looks like it IS possible. Current answers focus on the first part of the question, which is how. But it'd be nice to have some discussion on the "should we do it" part.
Okay, I finally got around to putting WoolDelegate up on GitHub. Now it should only take me another month to write a proper README (although I guess this is a good start).
The delegate class itself is pretty straightforward. It simply maintains a dictionary mapping SELs to Block. When an instance recieves a message to which it doesn't respond, it ends up in forwardInvocation: and looks in the dictionary for the selector:
- (void)forwardInvocation:(NSInvocation *)anInvocation {
SEL sel = [anInvocation selector];
GenericBlock handler = [self handlerForSelector:sel];
If it's found, the Block's invocation function pointer is pulled out and passed along to the juicy bits:
IMP handlerIMP = BlockIMP(handler);
[anInvocation Wool_invokeUsingIMP:handlerIMP];
}
(The BlockIMP() function, along with other Block-probing code, is thanks to Mike Ash. Actually, a lot of this project is built on stuff I learned from his Friday Q&A's. If you haven't read those essays, you're missing out.)
I should note that this goes through the full method resolution machinery every time a particular message is sent; there's a speed hit there. The alternative is the path that Erik H. and EMKPantry each took, which is creating a new clas for each delegate object that you need, and using class_addMethod(). Since every instance of WoolDelegate has its own dictionary of handlers, we don't need to do that, but on the other hand there's no way to "cache" the lookup or the invocation. A method can only be added to a class, not to an instance.
I did it this way for two reasons: this was an excercise to see if I could work out the part that's coming next -- the hand-off from NSInvocation to Block invocation -- and the creation of a new class for every needed instance simply seemed inelegant to me. Whether it's less elegant than my solution, I will leave to each reader's judgement.
Moving on, the meat of this procedure is actually in the NSInvocation category that's found in the project. This utilizes libffi to call a function that's unknown until runtime -- the Block's invocation -- with arguments that are also unknown until runtime (which are accessible via the NSInvocation). Normally, this is not possible, for the same reason that a va_list cannot be passed on: the compiler has to know how many arguments there are and how big they are. libffi contains assembler for each platform that knows/is based on those platforms' calling conventions.
There's three steps here: libffi needs a list of the types of the arguments to the function that's being called; it needs the argument values themselves put into a particular format; then the function (the Block's invocation pointer) needs to be invoked via libffi and the return value put back into the NSInvocation.
The real work for the first part is handled largely by a function which is again written by Mike Ash, called from Wool_buildFFIArgTypeList. libffi has internal structs that it uses to describe the types of function arguments. When preparing a call to a function, the library needs a list of pointers to these structures. The NSMethodSignature for the NSInvocation allows access of each argument's encoding string; translating from there to the correct ffi_type is handled by a set of if/else lookups:
arg_types[i] = libffi_type_for_objc_encoding([sig getArgumentTypeAtIndex:actual_arg_idx]);
...
if(str[0] == #encode(type)[0]) \
{ \
if(sizeof(type) == 1) \
return &ffi_type_sint8; \
else if(sizeof(type) == 2) \
return &ffi_type_sint16; \
Next, libffi wants pointers to the argument values themselves. This is done in Wool_buildArgValList: get the size of each argument, again from the NSMethodSignature, and allocate a chunk of memory that size, then return the list:
NSUInteger arg_size;
NSGetSizeAndAlignment([sig getArgumentTypeAtIndex:actual_arg_idx],
&arg_size,
NULL);
/* Get a piece of memory that size and put its address in the list. */
arg_list[i] = [self Wool_allocate:arg_size];
/* Put the value into the allocated spot. */
[self getArgument:arg_list[i] atIndex:actual_arg_idx];
(An aside: there's several notes in the code about skipping over the SEL, which is the (hidden) second passed argument to any method invocation. The Block's invocation pointer doesn't have a slot to hold the SEL; it just has itself as the first argument, and the rest are the "normal" arguments. Since the Block, as written in client code, could never access that argument anyways (it doesn't exist at the time), I decided to ignore it.)
libffi now needs to do some "prep"; as long as that succeeds (and space for the return value can be allocated), the invocation function pointer can now be "called", and the return value can be set:
ffi_call(&inv_cif, (genericfunc)theIMP, ret_val, arg_vals);
if( ret_val ){
[self setReturnValue:ret_val];
free(ret_val);
}
There's some demonstrations of the functionality in main.m in the project.
Finally, as for your question of "should this be done?", I think the answer is "yes, as long as it makes you more productive". WoolDelegate is completely generic, and an instance can act like any fully written-out class. My intention for it, though, was to make simple, one-off delegates -- that only need one or two methods, and don't need to live past their delegators -- less work than writing a whole new class, and more legible/maintainable than sticking some delegate methods into a view controller because it's the easiest place to put them. Taking advantage of the runtime and the language's dynamism like this hopefully can increase your code's readability, in the same way, e.g., Block-based NSNotification handlers do.
I just put together a little project that lets you do just this...
#interface EJHDelegateObject : NSObject
+ (id)delegateObjectForProtocol:(Protocol*) protocol;
#property (nonatomic, strong) Protocol *protocol;
- (void)addImplementation:(id)blockImplementation forSelector:(SEL)selector;
#end
#implementation EJHDelegateObject
static NSInteger counter;
+ (id)delegateObjectForProtocol:(Protocol *)protocol
{
NSString *className = [NSString stringWithFormat:#"%s%#%i",protocol_getName(protocol),#"_EJH_implementation_", counter++];
Class protocolClass = objc_allocateClassPair([EJHDelegateObject class], [className cStringUsingEncoding:NSUTF8StringEncoding], 0);
class_addProtocol(protocolClass, protocol);
objc_registerClassPair(protocolClass);
EJHDelegateObject *object = [[protocolClass alloc] init];
object.protocol = protocol;
return object;
}
- (void)addImplementation:(id)blockImplementation forSelector:(SEL)selector
{
unsigned int outCount;
struct objc_method_description *methodDescriptions = protocol_copyMethodDescriptionList(self.protocol, NO, YES, &outCount);
struct objc_method_description description;
BOOL descriptionFound = NO;
for (int i = 0; i < outCount; i++){
description = methodDescriptions[i];
if (description.name == selector){
descriptionFound = YES;
break;
}
}
if (descriptionFound){
class_addMethod([self class], selector, imp_implementationWithBlock(blockImplementation), description.types);
}
}
#end
And using an EJHDelegateObject:
self.alertViewDelegate = [EJHDelegateObject delegateObjectForProtocol:#protocol(UIAlertViewDelegate)];
[self.alertViewDelegate addImplementation:^(id _self, UIAlertView* alertView, NSInteger buttonIndex){
NSLog(#"%# dismissed with index %i", alertView, buttonIndex);
} forSelector:#selector(alertView:didDismissWithButtonIndex:)];
UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:#"Example" message:#"My delegate is an EJHDelegateObject" delegate:self.alertViewDelegate cancelButtonTitle:#"Cancel" otherButtonTitles:#"OK", nil];
[alertView show];
Edit: This is what I've come up after having understood your requirement. This is just a quick hack, an idea to get you started, it's not properly implemented, nor is it tested. It is supposed to work for delegate methods that take the sender as their only argument. It works It is supposed to work with normal and struct-returning delegate methods.
typedef void *(^UBDCallback)(id);
typedef void(^UBDCallbackStret)(void *, id);
void *UBDDelegateMethod(UniversalBlockDelegate *self, SEL _cmd, id sender)
{
UBDCallback cb = [self blockForSelector:_cmd];
return cb(sender);
}
void UBDelegateMethodStret(void *retadrr, UniversalBlockDelegate *self, SEL _cmd, id sender)
{
UBDCallbackStret cb = [self blockForSelector:_cmd];
cb(retaddr, sender);
}
#interface UniversalBlockDelegate: NSObject
- (BOOL)addDelegateSelector:(SEL)sel isStret:(BOOL)stret methodSignature:(const char *)mSig block:(id)block;
#end
#implementation UniversalBlockDelegate {
SEL selectors[128];
id blocks[128];
int count;
}
- (id)blockForSelector:(SEL)sel
{
int idx = -1;
for (int i = 0; i < count; i++) {
if (selectors[i] == sel) {
return blocks[i];
}
}
return nil;
}
- (void)dealloc
{
for (int i = 0; i < count; i++) {
[blocks[i] release];
}
[super dealloc];
}
- (BOOL)addDelegateSelector:(SEL)sel isStret:(BOOL)stret methodSignature:(const char *)mSig block:(id)block
{
if (count >= 128) return NO;
selectors[count] = sel;
blocks[count++] = [block copy];
class_addMethod(self.class, sel, (IMP)(stret ? UBDDelegateMethodStret : UBDDelegateMethod), mSig);
return YES;
}
#end
Usage:
UIWebView *webView = [[UIWebView alloc] initWithFrame:CGRectZero];
UniversalBlockDelegate *d = [[UniversalBlockDelegate alloc] init];
webView.delegate = d;
[d addDelegateSelector:#selector(webViewDidFinishLoading:) isStret:NO methodSignature:"v#:#" block:^(id webView) {
NSLog(#"Web View '%#' finished loading!", webView);
}];
[webView loadRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:#"http://google.com"]]];