Objective C Block Not Equal to Its Own Copy - ios

In my app, I associate an NSTimer with a block passed to a method; the block is also added to a an array of blocks. When the timer fires, its associated block is called and should be removed from the array. So my setup looks like this:
#interface MyObject : NSObject
#property(strong, nonatomic) NSMutableArray *allBlocks;
- (void)myMethodWithBlock:(void(^)(void))block;
- (void)timerFired:(NSTimer *)timer;
#end
#implementation MyObject
- (id)init
{
self = [super init];
if (self)
{
self.allBlocks = [NSMutableArray array];
}
return self;
}
- (void)myMethodWithBlock:(void(^)(void))block
{
[NSTimer scheduledTimerWithTimeInterval:5.0f
target:self
selector:#selector(timerFired:)
userInfo:block
repeats:NO];
[self.allBlocks addObject:block];
}
- (void)timerFired:(NSTimer *)timer
{
void(^block)(void) = timer.userInfo;
[self.allBlocks removeObject:block];
block();
}
#end
My problem is that when timerFired: is called, the block is (sometimes) not removed. Why?

The problem here is that NSTimer copies the block assigned to userInfo, but the block passed to myMethodWithBlock: is probably an instance of NSStackBlock, which is not equal to its copies.
Let's consider three scenarios, where myObject is an instance of MyObject:
// A
void(^myBlock)(void) = ^{
NSLog(#"1");
};
[myObject myMethodWithBlock:myBlock];
// B
int one = 1;
void(^myBlock)(void) = ^{
NSLog(#"%d", one);
};
[myObject myMethodWithBlock:myBlock];
// C
int one = 1;
[myObject myMethodWithBlock:^{
NSLog(#"%d", one);
};];
In A, the block captures no variables from its context; the block will be an instance of NSGlobalBlock, which simply returns itself when copied.
In B, the block captures the variable one; it will be an instance of NSMallocBlock, which also returns itself when copied.
In C, the block again captures the variable one, but is also not assigned to a variable before being passed to myMethodWithBlock:. In this case, the block is an instance of NSStackBlock, which returns an instance of NSMallocBlock when copied.
The result of this is that in a situation like scenario C, the NSStackBlock will be added to allBlocks, while an NSMallocBlock will be assigned to the timer's userInfo. When the timer fires, removeObject: does nothing since the block assigned to the timer isn't equal to any of the blocks in the array.
The solution is to always copy the block before storing it in the array. This way, the same block instance will be stored in the array and assigned to the timer:
- (void)myMethodWithBlock:(void(^)(void))block
{
block = [block copy];
[NSTimer scheduledTimerWithTimeInterval:5.0f
target:self
selector:#selector(timerFired:)
userInfo:block
repeats:NO];
[self.allBlocks addObject:block];
}

A clearer approach is to mark the blocks with something whose isEqual: behavior is well known and more readable, like an NSNumber...
// keep state so these can be made unique
#property(nonatomic, assign) NSInteger blockIndex;
// change the blocks collection to record blocks' associations with numbers
#property(nonatomic, strong) NSMutableDictionary *allBlocks;
// in myMethod...
NSNumber *nextIndex = [NSNumber numberWithInt:++self.blockIndex];
self.allBlocks[nextIndex] = block;
// pass userInfo:nextIndex when you schedule the timer
Now the timer context never has a block, copied or otherwise. Then, when the timer fires...
- (void)timerFired:(NSTimer *)timer {
NSNumber *index = timer.userInfo;
void(^block)(void) = self.allBlocks[index];
[self.allBlocks removeObjectForKey:index];
block();
}

I think it will be safest to compare apples to apples.
- (void)myMethodWithBlock:(void(^)(void))block
{
NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:5.0f
target:self
selector:#selector(timerFired:)
userInfo:block
repeats:NO];
[self.allBlocks addObject:timer.userInfo];
}
- (void)timerFired:(NSTimer *)timer
{
[self.allBlocks removeObject:timer.userInfo];
void(^block)(void) = timer.userInfo;
block();
}

Related

attempt to insert nil object from objects

I have this below error
-[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[1539]
It happens sometimes I try to tap several times on screen, because code is little, so all the code is pasted below
#interface ViewController ()
#property (nonatomic,weak) NSTimer *timer;
#property (nonatomic,strong)NSMutableArray * testArray;
#property (nonatomic,strong) dispatch_queue_t queue1;
#property (nonatomic,strong) dispatch_queue_t queue2;
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.testArray = [NSMutableArray array];
_queue1 = dispatch_queue_create("test", DISPATCH_QUEUE_CONCURRENT);
_queue2 = dispatch_queue_create("test",DISPATCH_QUEUE_SERIAL);
NSTimer * timer = [NSTimer scheduledTimerWithTimeInterval:0.01 target:self selector:#selector(addObjectforArray) userInfo:nil repeats:YES];
[timer fire];
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
dispatch_async(_queue2, ^{
NSLog(#"touchesBeganThread:%#",[NSThread currentThread]);
NSArray * testTempArray = [NSArray arrayWithArray:self.testArray];
for (UIView *view in testTempArray) {
NSLog(#"%#",view);
}
});
}
- (void)addObjectforArray{
dispatch_async(_queue1, ^{
NSLog(#"addObjectThread:%#",[NSThread currentThread]);
[self.testArray addObject:[[UIView alloc]init]];
});
}
I can not understand why this happens, if I change _queue1 to DISPATCH_QUEUE_SERIAL, it becomes normal.
How can I understand this issue? If anyone could shed some light, that would be wonderful.
There are multiple problems in your code. They can cause all sorts of bugs randomly.
UIView should be created in the main thread using dispatch_get_main_queue().
https://developer.apple.com/documentation/uikit
For the most part, use UIKit classes only from your app’s main thread or main dispatch queue. This restriction applies to classes derived from
UIResponder
or that involve manipulating your app’s user interface in any way.
The property testArray is nonatomic but being accessed in two threads. The property should be atomic. It runs fine at this moment but it is fragile. If in the future testArray mutates, the app will crash randomly.
NSArray is not thread-safe. It should be locked while accessing in multiple threads or protected by other means.
As pointed out by #Nirmalsinh, the dispatch_async is redundant (actually harmful).
I am not sure if you have heavily simplified your code or only to test something. If you are not doing long running work, you might want to use dispatch_get_main_queue() in dispatch_async. It will save you from a lot of troubles.
It seems you are inserting nil value into your array. You cannot add nil to array or dictionary.
- (void)addObjectforArray{
NSLog(#"addObjectThread:%#",[NSThread currentThread]);
UIView *view = [[UIView alloc] init];
if(view != nil)
[self.testArray addObject:view];
}
There is no required to use a queue in the method. You are already using NSTimer for same.
Try to check above. It will help you.

pass info from Model to viewController without NSNotification

In my viewController, I have a button that calls a method to start a timer in a Timer class. Just like this
main view controller
[self.timer startTimer];
In the timer class, startTimer calls a countdownTime method, both of which you can see below. Very simple. At the end of the countdownTime, method, the time is put in the label in the view like this as I iterate through a loop of the clocks.
Timer *timerblah = self.gameClocks[i];
[self.gameClocks[i] setText: [NSString stringWithFormat:#"%#", self.time]];
In other words, the Timer class has an array property that holds all the clocks and that is connected to the view, hence the ability to set the text.
The problem with this code is that the model (i.e. Timer class) is setting the text in the view. I want the view Controller to get the time from the model and have the text set in the view controller i.e. have the viewController and only the viewController communicate with the view. It is no problem for me to get the array of clocks in the viewController, however, I'm sure how to pass the time back from the countdownTime method to the viewController. It would seem like overkill to set up an NSNotification class to send the time back every second, wouldn't it?
-(void)startTimer{
self.NStimer = [NSTimer scheduledTimerWithTimeInterval:1.0
target:self
selector:#selector(countdownTime:)
userInfo:nil
repeats:YES];
}
-(void)countdownTime:(NSTimer *)timer
{
#logic to countdown time
self.minutes = self.secondsRemaining / 60;
self.stringMinutes = [NSString stringWithFormat:#"%i", self.minutes];
self.seconds = self.secondsRemaining - (self.minutes * 60);
self.stringSeconds = [NSString stringWithFormat:#"%i", self.seconds];
if (self.seconds < 10) self.stringSeconds = [NSString stringWithFormat:#"0%i", self.seconds];
self.time = [NSString stringWithFormat:#"%#:%#", self.stringMinutes, self.stringSeconds];
self.secondsRemaining -= 1;
if (self.secondsRemaining == 0){
[self stopTimer];
}
#adding to the view (from the model i.e. in Timer.m class)
for(int i = 0; i < [self.gameClocks count]; i++){
Timer *timerblah = self.gameClocks[i];
if (timerblah.systemClock == YES){
[self.gameClocks[i] setText: [NSString stringWithFormat:#"%#", self.time]];
}
}
}
Update
There's a comment suggesting that I use a block to do this. In the timer class, I added a method like this to return a string with the time (the time is set to self.time above)
-(NSString *)passTheTime:(NSString (^)(void))returnBlock
{
return self.time;
}
I then, in view controller, call the method passTheTime on the timer class. I created a property in the view controller that will store the time, so I pass that as a parameter to the block,
[self.timer passTheFoo:^NSString * (self.timeToSet){
}];
a) I'm unsure of what to do in the block here.
b) I'm unsure of whether it was necessary to pass self.timeToSet into the block. How do I connect self.time from the Timer class to self.timeToSet in the view controller
c) there's an error incompatible block pointer type sending NSString *((^)(void)) to parameter of type NSString(^)(void)
d) alternatively, can I pass a block to startTimer, and have that block passed in the selector countdownTime and return the self.time once the calculations in countdownTime are finished?
You can use block to update the label in the view controller class as below
In Timer Class, create a block
TimerClass.h
#import <Foundation/Foundation.h>
typedef void(^TimerCallback)(NSString *time);
#interface TimerClass : NSObject{
TimerCallback timerCallback;
NSTimer *timer;
}
- (void)startTimerWithCallBack:(TimerCallback)callback;
#end
In TimerClass.m , update the code as below
#import "TimerClass.h"
#implementation TimerClass
- (void)startTimerWithCallBack:(TimerCallback)callback{
timerCallback = callback;
timer = [NSTimer scheduledTimerWithTimeInterval:1.0
target:self
selector:#selector(countdownTime:)
userInfo:nil
repeats:YES];
}
-(void)countdownTime:(NSTimer *)timer
{
// countdown logic here
// call the callback method with time as a parameter to the viewcontroller class
// here for demo purpose I am passing random number as a value in callback, you need to pass the time that you want to display here
NSString *str = [NSString stringWithFormat:#"%d",1+arc4random()%10];
timerCallback(str);
}
#end
In ViewController class, call the timer start method as follows
TimerClass *timer = [[TimerClass alloc] init];
[timer startTimerWithCallBack:^(NSString *time) {
lbl.text = time; // display time in label
}];

Keeping a strong pointer to NSTimer not working

I am having trouble with NSTimer. The below code is called from a method and it creates an NSTimer and sets it to the instance variable self.syncDelay. The if statement at the end of the method checking if the timer is valid gets called.
However when I call this method again before the timer runs out, the if statement at the top of the method doesn't get called. Which, I am guessing, is why [self.syncDelay invalidate]; doesn't invalidate the timer.
I have a property defined as: #property (nonatomic, strong) NSTimer *syncDelay;
The functionality I am looking for is the timer being invalidated when the method is called for the second time before the timer runs out and creating a new timer. Thus effectively resetting the timer.
EDIT: When the method gets called for the second the NSTimer is nil. Which is strange as I am keeping a strong pointer to it.
else {
if ([self.syncDelay isValid]) {
NSLog(#"Timer valid at start of method");
}
[self.syncDelay invalidate];
self.syncDelay = nil;
NSInvocation *timerInvocation = [NSInvocation invocationWithMethodSignature:
[self methodSignatureForSelector:#selector(testMethod:)]];
// configure invocation
[timerInvocation setSelector:#selector(testMethod:)];
[timerInvocation setTarget:self];
[timerInvocation setArgument:&className atIndex:2]; // argument indexing is offset by 2 hidden args
self.syncDelay = [NSTimer scheduledTimerWithTimeInterval:10
invocation:timerInvocation
repeats:NO];
if ([self.syncDelay isValid]) {
NSLog(#"Timer valid at end of method");
}
}

Objective-C: Accessing integer value of a ViewController in View?

I'm trying to start a NSTimer in my UIView class called "ClockView" with a method as selector that manipulates an initial float which was declared in the ViewController "ClockViewController".
My ClockViewController declares int timerIntWhite as an integer (for example 500). My ClockView needs this Value for the - (void)start method which runs a method called - (void)updateWhiteClock every second:
- (void)start {
timerIntWhite = PLEASE HELP ME AT THIS POINT!;
randomTimerWhite = [NSTimer scheduledTimerWithTimeInterval:(1.0/1.0)target:self selector:#selector(updateWhiteClock) userInfo:nil repeats:YES];
}
Is it possible to access the integer of ClockViewController in ClockView?
Try the following:
In your ClockView also declare the variable (property) int timerIntWhite and set this variable from your View Controller after the View gets created, for example, in viewDidLoad.
-(void)viewDidLoad {
[super viewDidLoad];
self.view.timerIntWhite = self.timerIntWhite;
}
After doing this, ClockView can access it's own timerIntWhite variable:
- (void)start {
timerIntWhite = self.timerIntWhite;
randomTimerWhite = [NSTimer scheduledTimerWithTimeInterval:(1.0/1.0)target:self selector:#selector(updateWhiteClock) userInfo:nil repeats:YES];
}
I'm assuming that your ClockViewController class knows that its view IS-A ClockView. This is very important! Otherwise you'll get a warning.
I also want to mention that according to the MVC rules it's a better idea if your ClockViewController class takes care of the NSTimer. Views should be used to display information to the user only.
Hope this helps!

NSTimer Category + Blocks implementation to replace selector

I am quite new to blocks and objective-c, and i am trying to write my first category using both. My idea is to create a category on NSTimer that will receive a block as a parameter and this block will be used in the selector call. Right now I have this.
// NSTimer+Additions.h
#import <Foundation/Foundation.h>
typedef void (^VoidBlock)();
#interface NSTimer (NSTimer_Additions)
+ (NSTimer *)scheduleTimerWithTimeInterval:(NSTimeInterval)theSeconds repeats:(BOOL)repeats actions:(VoidBlock)actions;
#end
#import "NSTimer+Additions.h"
static VoidBlock _voidBlock;
#interface NSTimer (AdditionsPrivate) // Private stuff
- (void)theBlock;
#end
#implementation NSTimer (NSTimer_Additions)
+ (NSTimer *)scheduleTimerWithTimeInterval:(NSTimeInterval)theSeconds repeats:(BOOL)repeats actions:(VoidBlock)actions {
[_voidBlock release];
_voidBlock = [actions copy];
NSTimer* timer = [[NSTimer alloc] initWithFireDate:[NSDate date]
interval:theSeconds
target:self
selector:#selector(theBlock)
userInfo:nil
repeats:repeats];
[timer fire];
return [timer autorelease];
}
- (void)theBlock {
_voidBlock();
}
#end
Gist for the code: https://gist.github.com/1065235
Everything compiles fine but i have the following error:
2011-07-05 14:35:47.068 TesteTimer[37716:903] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[NSTimer theBlock]: unrecognized selector sent to class 0x7fff70bb0a18'
How can I make this category work?
Your major flaw besides the wrong target is your use of a static variable. You won't be able to support beyond a single timer.
Using block as argument to the invoked method.
#interface NSTimer (AdditionsPrivate) // Private stuff
- (void)theBlock:(VoidBlock)voidBlock;
#end
#implementation NSTimer (NSTimer_Additions)
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)theSeconds repeats:(BOOL)repeats actions:(VoidBlock)actions {
NSInvocation * invocation = [NSInvocation invocationWithMethodSignature:[self instanceMethodSignatureForSelector:#selector(theBlock:)]];
NSTimer * timer = [NSTimer scheduledTimerWithTimeInterval:theSeconds
invocation:invocation
repeats:repeats];
[invocation setTarget:timer];
[invocation setSelector:#selector(theBlock:)];
Block_copy(actions);
[invocation setArgument:&actions atIndex:2];
Block_release(actions);
return timer;
}
- (void)theBlock:(VoidBlock)voidBlock {
voidBlock();
}
#end
The problem with using associative references was the leak as there was no good point to release the block.
Earlier Approach using associative references
You can use associative references to attach the block to that particular instance of NSTimer.
#implementation NSTimer (NSTimer_Additions)
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)theSeconds repeats:(BOOL)repeats actions:(VoidBlock)actions {
NSInvocation * invocation = [NSInvocation invocationWithMethodSignature:[self instanceMethodSignatureForSelector:#selector(theBlock)]];
NSTimer * timer = [NSTimer scheduledTimerWithTimeInterval:theSeconds
invocation:invocation
repeats:repeats];
[invocation setTarget:timer];
[invocation setSelector:#selector(theBlock)];
objc_setAssociatedObject(timer, #"Block", actions, OBJC_ASSOCIATION_COPY);
return timer;
}
- (void)theBlock {
VoidBlock _voidBlock = (VoidBlock)objc_getAssociatedObject(self, #"Block");
_voidBlock();
}
#end
What about leveraging userInfo to carry your block? (this is done with ARC)
void (^callback)(void) = ^{
NSLog(#"do stuff");
}
NSTimer *timer = [NSTimer timerWithTimeInterval:10.0 target:self selector:#selector(handleTimeout:) userInfo:[NSDictionary dictionaryWithObject:[callback copy] forKey:#"block"] repeats:NO];
[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];
And then add the static selector of:
+ (void)handleTimeout:(NSTimer *)timer
{
void (^callback)(void) = [timer.userInfo objectForKey:#"block"];
callback();
[timer invalidate];
timer = nil;
};
This should work:
NSTimer* timer = [[NSTimer alloc] initWithFireDate:[NSDate date]
interval:theSeconds
target:timer
selector:#selector(theBlock)
userInfo:nil
repeats:repeats];
The problem is that you're setting the target of the new NSTimer instance to be self. However, in the context of + scheduleTimerWithTimeInterval:repeats:actions: (notice the +), self is NSTimer, and not (as you probably thought) your newly-created NSTimer instance.
As you can see from the error message, your app is crashing because NSTimer doesn't respond to the class method + theBlock, which is of course correct since you only defined the instance method - theBlock.

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