NSCondition ->Objective C

Question

I am a newbie to Objective-C. I'm currently working on threads.

I have to make a synchronous execution of threads. I'm using NSInvocationOperaion to spawn a thread.

I have two threads. I need to wait for the 1st thread to signal a event or the timeout. Signalling a event can be done by NSConditionLock. How to signal a timeout. I could not use waitUntilDate method here as the timeout is not a fixed value. Is there any way to do this?

EDITED

main.m
------
#import "PseudoSerialQueue.h"
#import "PseudoTask.h"

int main()
{
    PseudoSerialQueue* q = [[[PseudoSerialQueue alloc] init] autorelease];
    [q addTask:self selector:@selector(test0)];
    [q addTask:self selector:@selector(test1)];
    [q addTask:self selector:@selector(test2)];
    [q quit];
    return 0;
}

PseudoTask.h
-----------------

#import <Foundation/Foundation.h>


@interface PseudoTask : NSObject {

    id target_;
    SEL selector_;
    id queue_;

}

@property(nonatomic,readonly)id target;

-(id)initWithTarget:(id)target selector:(SEL)selector queue:(id)queue;
-(void)exec;

@end

PseudoTask.m
-----------------

#import "PseudoTask.h"


@implementation PseudoTask

@synthesize target = target_;

-(id)initWithTarget:(id)target selector:(SEL)selector queue:(id)queue
{
    self = [super init];
    if (self) {
        target_ = [target retain];
        selector_ = selector;
        queue_ = [queue retain];
    }
    return self;
}

-(void)exec
{
    [target_ performSelector:selector_];
}

-(void)dealloc
{
    [super dealloc];
    [target_ release];
    [queue_ release];
}

@end


PseudoSerialQueue.h
----------------------------

#import <Foundation/Foundation.h>
#import "PseudoTask.h"

@interface PseudoSerialQueue : NSObject {

    NSCondition* condition_;
    NSMutableArray* array_;
    NSThread* thread_;

}

-(void)addTask:(id)target selector:(SEL)selector;

@end

PseudoSerialQueue.m
----------------------------

#import "PseudoSerialQueue.h"

@implementation PseudoSerialQueue

-(id)init
{
    self = [super init];
    if (self) {
        array_ = [[NSMutableArray alloc]init];
        condition_ = [[NSCondition alloc]init];
        thread_ = [[NSThread alloc] initWithTarget:self selector:@selector(execQueue) object:nil];
        [thread_ start];
    }
    return self;
}

-(void)addTask:(id)target selector:(SEL)selector
{
    [condition_ lock];
    PseudoTask* task = [[PseudoTask alloc] initWithTarget:target selector:selector queue:self];
    [array_ addObject:task];
    [condition_ signal];
    [condition_ unlock];
}

-(void)quit
{
    [self addTask:nil selector:nil];
}

-(void)execQueue
{
    for(;;)
    {
        NSAutoreleasePool* pool = [[NSAutoreleasePool alloc]init];

        [condition_ lock];

        if (array_.count == 0) {
            [condition_ wait];
        }

        PseudoTask* task = [array_ objectAtIndex:0];
        [array_ removeObjectAtIndex:0];

        [condition_ unlock];

        if (!task.target) {
            [pool drain];
            break;
        }

        [task exec];
        [task release];

        [pool drain];
    }
}

-(void)dealloc
{
    [array_ release];
    [condition_ release];
    [super dealloc];
}

@end

I could not pass self from main.Hope i'm mistakenly calling it. Error:'self' undeclared is coming.

I could not understand -(void)exec { [target_ performSelector:selector_]; } in PseudoTask.m

target_ is not a method and its an ivar. I am not getting any error or warning.But i could not understand that code.

I am writing what i have understood from your program.Please correct me if i my way of understanding the program is wrong.

The Thread execQueue is spawned when the PseudoSerialQueue is initialised and it waits for the signal from the addTask method. The addTask method is called in the quit method and the parameters passed are nil.I could not understand why to pass a nil parameter.

It would be helpful if you explain about it.Thanks.

Answer 1

You mean NSCondition? You can use waitUntilDate: as relative time.

[condition lock];
// wait 5 seconds. 
[condition waitUntilDate:[NSDate dateWithTimeIntervalSinceNow:5]];
[condition unlock];

EDITED:

My PseudoSerialQueue class requires to be called from a class that is derived from NSObject like the following.

@interface Test : NSObject
@end

@implementation Test
- (void)test0
{
}

- (void)test1
{
}

- (id)init
{
    self = [super init];
    return self;
}

- (void)exec
{
    PseudoSerialQueue *q = [[PseudoSerialQueue alloc] init];
    [q addTask:self selector:@selector(test0)];
    [q addTask:self selector:@selector(test1)];
    [q addTask:self selector:@selector(test0)];
    [q quit];
}
@end

You can call it from main function.

Test *test = [[Test alloc] init];
[test exec];

I could not understand why to pass a nil parameter.

I just only chose it for the message of quitting the loop in the PseudoSerialQueue.

Answer 2

Let the 1st thread signal the 2nd one in both cases; then in the second thread you can tell in which case you are based on some read-only flag in the 1st controller or in your model (say, isDataAvailable).