With reference to the code below, once a block has been put into an array, how could you take that block object and run the actual code in the block.
Another Question: If I call a method in a block like below, does that block encapsulate the code in that method or capture the signature of the method and call it that way?
-(void)blockCalledMethod
{
NSLog(#"Hello World");
}
-(void)programStart
{
NSArray * array = [[NSArray alloc] initWithObjects:[[^ { [self blockCalledMethod];} copy] autorelease],nil];
id pointerToBlock = [array lastObject];
}
Call it like this:
void (^pointerToBlock)(void) = [array lastObject];
pointerToBlock(); // because ^ {} is void(^)(void)
You cannot declare pointerToBlock as an id if you want to call it directly, because the compiler has to recognize it as a block type and not just an object.
If I call a method in a block like above, does that block encapsulate the code in that method or capture the signature of the method and call it that way?
I should think self refers to the calling class.
Related
I'm trying to write a mock of HKHealthStore. In the stubbed executeQuery: I need to call the result handler block of a HKSampleQuery instance passed to it. The block is private so I need to get it at runtime. This is what I have so far:
- (void)executeQuery:(HKQuery *)query {
NSAssert([query isKindOfClass:HKSampleQuery.class], #"Mock executeQuery: not implemented yet for other query types than HKSampleQuery");
HKSampleQuery *sampleQuery = (HKSampleQuery *)query;
NSMutableArray<HKObject *> *queryResults = [NSMutableArray new];
for (HKObject *o in self.storedObjects) {
if ([sampleQuery.predicate evaluateWithObject:o]) {
[queryResults addObject:o];
}
}
SEL selector = NSSelectorFromString(#"resultHandler");
Method m = class_getInstanceMethod(HKSampleQuery.class, selector);
IMP imp = method_getImplementation(m);
typedef void(*resultHandler_t)(id, SEL, void(^)(HKQuery*, NSArray*, NSError*));
resultHandler_t f = (resultHandler_t)imp;
// here, I need to invoke the result handler block with sampleQuery, queryResults and nil as arguments
}
Note the selector name is "resultHandler" even though the parameter of the initializer of HKSampleQuery is called "resultsHandler".
Is there any way to invoke the block with appropriate arguments?
You're not doing what you think you are doing. You are getting the implementation of the method resultHandler (the getter method of the property resultHandler). The block you want is the value of the property resultHandler, which is the return value of running the getter method. In other words, you need to run the getter and get the result, not get the getter itself.
Simplest way to call the method and get the return value (since in this case the return value is a regular object pointer type) would be
typedef void (^resultHandler_t)(HKSampleQuery *query, NSArray *results, NSError *error);
resultHandler_t f = [sampleQuery performSelector:#selector(resultHandler)];
f(sampleQuery, queryResults, nil);
Alternately, if you declare (but not implement) the instance method or property resultHandler in a dummy category of HKSampleQuery, you can then access the property directly like resultHandler_t f = sampleQuery.resultHandler;
I am passing an ivar (NSMutableArray) into some method. I was expecting that if I modify the object inside the function, it would be reflected outside the function, but in this case I need to set the object; something like the following:
- (void) someMethod:(SMResponseObject *)response onData:(NSMutableArray *)imAnIvar {
imAnIvar = [response objects];
//Some other stuff
}
But I noticed that the memory reference of imAnIvar inside the function changes when I set it, and given that, the actual ivar doesn't change. I understand that the problem is that I'm changing the reference of the object inside the method, so it stops pointing to the ivar and then it points to some other random memory direction.
I thought about one solution to this problem, and it can be to ensure that the ivar is not nil before calling the function and do something like this:
- (void) someMethod:(SMResponseObject *)response onData:(NSMutableArray *)imAnIvar {
NSMutableArray *data = [response objects];
[arrayForTableView removeAllObjects];
for(id element in data){
[imAnIvar addObject:element];
}
//Some other stuff
}
So I use the original object instead of setting it directly. The problem is that in order for this to work I need to ensure that the ivar is not nil, which I think is not clean, because I'll need to do something like this on every call to the method:
if(!_ivar){
//alloc it
}
So my question is: Is there a way to force the local scope variable to point to the original variable even if I'm setting it? if not, is there any cleaner way to make this work?
Do you mean this?
- (void)setFoo:(SomeClass **)objPtr
{
*objPtr = someOtherObject;
}
// call it as:
SomeClass *foo = someObject;
NSLog(#"Before: %#", foo);
[self setFoo:&foo];
NSLog(#"After: %#", foo);
Why not use a getter for the array so that you need not check for the array being nil while using it?
-(NSMutableArray *)iAmAnIvar {
if(_iAmAnIvar == nil) {
_iAmAnIvar = [NSMutableArray array];
}
return _iAmAnIvar;
}
And when you have to set a value to the array, as you mentioned in your question, you could use
[self.iAmAnIvar removeAllObjects];
[self.iAmAnIvar addObject:someObj];
I believe you can use the - (id)copy; function of NSObject
so your code might look like this:
- (void)someFunction:(NSString *)someArg
{
NSString *str = [someArg copy];
}
How can I can I call a function which name I stored as a value an an NSDictionary? Basically I need to typecast a String as a method name/call. Is this possible?
[thisCellContent objectForKey:#"callFunction"] //the object contains the function name to be called
//call this value as function, something similar to this ..
[self [thisCellContent objectForKey:#"callFunction"]]
Is something like this possible?
Not like this, but you can use the runtime/reflection:
NSString *s = [thisCellContent objectForKey:#"callFunction"];
SEL sel = NSSelectorFromString(s);
[self performSelector:sel withObject:nil];
I would suggest putting an Objective-C block into the dictionary. You can then call the block (and it can call other things if you like). Something like this:
//In code that sets up the dictionary
void (^thingToCall)() = [^{ /* any code you want to run here */ } copy];
[dictionary setObject:thingToCall forKey:#"callFunction"];
//In code that uses the dictionary
void (^thingToCall)() = [dictionary objectForKey:#"callFunction"];
thingToCall(); //call the block
Obj-C blocks are something I'm just using for the first time recently. I'm trying to understand the following block syntax:
In the header file:
#property (nonatomic, copy) void (^completionBlock)(id obj, NSError *err);
In the main file:
-(void)something{
id rootObject = nil;
// do something so rootObject is hopefully not nil
if([self completionBlock])
[self completionBlock](rootObject, nil); // What is this syntax referred to as?
}
I appreciate the assistance!
Blocks are Objects.
In your case inside the method you are checking if the block is not nil and then you are calling it passing the two required arguments ...
Keep in mind that blocks are called in the same way a c function is ...
Below i have split the statement in two to let you understand better :
[self completionBlock] //The property getter is called to retrieve the block object
(rootObject, nil); //The two required arguments are passed to the block object calling it
Its a block property, you can set a block at runtime.
Here is the syntax to set
As it is void type, so within the class you can set a method by following code
self.completionBlock = ^(id aID, NSError *err){
//do something here using id aID and NSError err
};
With following code you can call the method/block set previously.
if([self completionBlock])//only a check to see if you have set it or not
{
[self completionBlock](aID, nil);//calling
}
I have:
[self schedule:#selector(tickhealth)];
And tickHealth method only has one line of code:
-(void)tickHealth
{
[hm decreaseBars:0.5];
}
is it possible to use block objects in place of a selector. for example something like:
[self schedule:^{
[hm decreaseBars:0.5];
}];
As Caleb & bbum correctly pointed out you cannot simply pass a block to your existing (and unchanged) - (void)schedule:(SEL)selector; method.
You can however do this:
Define block type:
typedef void(^ScheduleBlock)();
Change schedule: method to be defined similar to this:
- (void)schedule:(ScheduleBlock)block {
//blocks get created on the stack, thus we need to declare ownership explicitly:
ScheduleBlock myBlock = [[block copy] autorelease];
//...
myBlock();
}
Then call it like this:
[self schedule:^{
[hm decreaseBars:0.5];
}];
Further Objective-C block goodness compiled by Mike Ash that will get you kickstarted with blocks:
http://mikeash.com/pyblog/friday-qa-2008-12-26.html
http://mikeash.com/pyblog//friday-qa-2009-08-14-practical-blocks.html
http://mikeash.com/pyblog/friday-qa-2011-06-03-objective-c-blocks-vs-c0x-lambdas-fight.html
You can't just pass a block in place of a selector because those two things have different types. However, if you have control over the -schedule: method, you can easily modify it to accept and use a block in place of a selector.