I am wondering what is the scope of variables when using the same name of a variable within and outside a block. An example will talk from itself:
NSSet *test = [NSSet setWithObjects#"Test"];
void (^onComplete)(id) = ^(NSSet *test) {
// do we see the variable define as an argument of the block or the variable define outside of the block?
NSSet *test2 = test;
}
NSSet *test3 = test;
Is there any possible conficts here?
Local variables hide outer scopes. So in the block, test is the parameter, not the outside variable.
Related
Currently I am using the following syntax to explicitly access a private scoped variable, inside of a PowerShell function:
function MyPowershellFunction {
param($param1)
# ...
# lot of code here
# ...
# access variable x on private scope explicitly
if ($private:x) {
# do something
}
}
I am doing this so that I do not accidentally refer to a variable defined in parent scope. But the code looks ugly by having private all around. Is there a simpler way?
By making a variable private you restrict its visibility to the current scope. Since variables are looked up in the current scope first $x should give you the private variable after it was initially declared private as long as you don't leave the scope. Using the scope modifier all the time shouldn't be necessary.
Just use the Local scope modifier:
function MyPowershellFunction {
param($param1)
# access variable x on private scope explicitly
if ($local:x) {
# do something
}
}
The if statement does not see any $x defined in the parent scope.
My problem is that I'm using dispatch_async(dispatch_get_main_queue(), ^(void) { ... }); to call a method asynchronously, in this method depending on some conditions i set a boolean to YES. When this boolean is read in this method, it's always read by it's old value which is NO.
The weird thing is that when i made a breakpoint on the line where the bool is checked, everything went fine and as intended !
EDIT:
Here is the code where the threads are spawned
dispatch_async(dispatch_get_main_queue(), ^(void) {
[self drawFaceBoxesForFeatures:features forVideoBox:claporientation:curDeviceOrientation image:img];
});
The method itself
- (void)drawFaceBoxesForFeatures:(NSArray *)features forVideoBox:(CGRect)clap orientation: (UIDeviceOrientation)orientation image:(UIImage *)image;
{
if (![self getSendingRequestStatus]) {
NSLog(#"Sending req");
// send async request
dispatch_async(dispatch_get_main_queue(),^ {
sendingRequest = YES;
} );
}
}
It looks like you are modifying an ivar that was created outside of a block inside of the block. In order to do this and have the ivar hold the correct value, you are going to need to use the __block keyword like so:
#interface MyCoolClass : NSObject {
#private
__block int sendingRequest_;
}
As Jack Lawrence said in the commend above, "[the runtime] takes a snapshot of all of the relevant objects/variables at that point in time". The __block identifier will tell the runtime that it should not copy that ivar to the heap and will allow you to assign values to sendingRequest_ inside of a block, even if that block is simply being run on the main thread.
A lot of good information to start with (including the above) can be found in the Blocks Programming Guide.
When primitives are passed into a block they are copied. So if you put a primitive local or instance variable in a block and then later change it either in the same method that created the block (after the block creation) or another method it won't have any effect on the variable in the block. In the case of a local variable, just make sure you make any necessary changes before block creation. In the case of instance variables you could try accessing the instance variable by using some C: self->iVar or declare it as a property and access it through the property accessor: self.iVar.
Most of the documented examples of block usage demonstrate closure with simple variables, but I've been confounded by any attempts to access objects which are present in the surrounding code. For example, this crashes in an ugly, unhelpful way:
#interface VisualizerPreset : EyeCandyPreset {
float changeSourceRate;
float (^frontPanelSlider2DisplayValueBlock)(void);
}
....
VisualizerPreset *it;
it = [[VisualizerPreset alloc] init];
it.changeSourceRate = 0.4;
it.frontPanelSlider2DisplayValueBlock = ^(void) {
return it.changeSourceRate;
};
....
// this crashes
NSLog(#"%f",it.frontPanelSlider2DisplayValueBlock());
One possible reason is that you've lost the block. A block is created in stack, not in the heap. So if you want to keep the block, you have to copy it; this will make a copy of the block in the heap.
float (^aVar) = [^{return 0.0;} copy];
Of course, you will have to also release it later.
Be careful who owns the copy of the block. Inside a block, all referenced objects are automatically retained. So it is easy to create a reference cycle. You can use __block modifier for this problem. Consider reading this http://thirdcog.eu/pwcblocks/
Could some one tell me why my array is out of scope?
Here's my class:
// Paper.h
#interface Paper : NSObject {
NSMutableArray* items;
}
#property (retain) NSMutableArray* items;
// Paper.m
#import "Paper.h"
#implementation Paper {
#synthesize items;
}
// ParserUtil.m
#implementation ParserUtil {
+(Paper*) parsePaper:(NSString*)file {
...
Paper* paper = [[[Paper alloc] init] autorelease];
// does the following line is the best practice?
paper.items = [[[MutableArray alloc] init] autorelease];
Item* item = ...; // create item instance
[paper.items addObject:item];
return paper;
}
// call the parser method
...
Paper* paper = [[ParserUtil parsePaper:#"SomeFile"] retain];
// when run to this line, the paper.items is out of scope
// seems all the items in the array are dispear
NSMutableArray* items = paper.items;
...
Could someone point out what is wrong here?
Many thanks!
It isn't.
An object cannot be out of scope, because objects do not have scope. What they can be is unreachable, which is what happens when you don't have any variables holding the object's pointer.
Variables can be out of scope. You can only use a variable within the same scope in which you declared it; you can't begin a compound statement, declare a variable, finish the compound statement, and use the variable, and you can't declare a variable in one function or method and then use it in a different one.
You said in your other question that it's the debugger telling you the variable is out of scope. This means one of two three things:
The variable really is out of scope. Move the variable or move the code that uses it, or just interrupt the debugger earlier (with a breakpoint, if necessary).
The debugger is just being stupid. This happens a lot. Try the po command or sprinkle your code with NSLog statements instead.
You're trying to examine a property-access expression. A property-access expression, by definition, must send an accessor message, which may have side effects; for that reason, the debugger won't do that just for you hovering over the expression, because that's too easy to do by accident. You must use the po command in the Debugger Console to send the accessor message and print the description of the result.
i'm in a bit of a situation here...
i am passing a string to a function and in that function i need to create an array whose name is the value of the string.
Say, for example: -(void) function : (NSString *) arrayName; //let arrayName = #"foo";
In this function I need to create an array named "foo" i.e the value of the passed parameter.
Can anyone help please :|
Thanks in advance ;)
Arrays don't have names. Variables have names, but variables are local to their scope, so once you leave the scope of that method, having a variable named "foo" is pointless; you can name the variable whatever you want and it will work just fine. Ex:
- (void) function:(id)whatever {
NSArray * myVariable = [NSArray arrayWithStuff....];
//use myVariable
}
What are you really trying to do?
That is not possible in Objective-C, but you can use e.g. a dictionary that maps a string to an array.
E.g. assuming something like the following property:
#property (readonly, retain) NSMutableDictionary *arrays;
... you can store an array by name:
- (void)function:(NSString *)arrayName {
NSArray *array = [NSArray arrayWithObjects:#"foo", #"bar", nil];
[self.arrays setObject:array forKey:arrayName];
}
... and access it like so:
NSArray *array = [self.arrays objectForKey:arrayName];
C is a compiled language where any source code names (for variables, functions, etc.) are not available at runtime (except for perhaps optionally debugging, -g). The Objective C runtime adds to this the ability to look up Obj C methods and classes by name, but not objects, nor any C stuff. So you're out of luck unless you build your own mini-language-interpreter structure for reference-by-name. Lots of ways to do this, but simple languages usually build some sort of variable table, something like a dictionary, array, or linked-list of objects (structs, tuples, etc.) containing string name, object pointer (maybe also type, size, etc.).