Question

I have been looking around online, doing research into how to use blocks. I have also decided to set up a basic example to try and understand the way in which they work.

Essentially what I want to do is have a ‘block variable’ (no sure if thats the correct term) in which I can store a block of code. I then want to be able to set the code in this block at pointX (methodA or methodB) in my code, then run the block of code at pointY (methodX).

So to be specific, my question is 3-fold

1. Using the example below is the setup / usage of blocks correct and valid?
2. In methodX how do I execute the code inside the block (self.completionBlock)?
3. When creating the block in methodA and methodB will the code be called there and then? If so how can I stop this from happening (all I want to do is set up the code in the block to be called later)?

I may have completely misunderstood how blocks are used, apologies if this is the case, however I’m relatively new to Objective-C and I’m trying to learn.

Here is my code so far:

.h

typedef void (^ CompletionBlock)();

@interface TestClass : NSObject
{   
    CompletionBlock completionBlock;

    NSString *stringOfText;
    NSString *otherStringOfText;
}

@property(nonatomic, copy)CompletionBlock completionBlock;

@property(nonatomic, retain)NSString *stringOfText;
@property(nonatomic, retain)NSString *otherStringOfText;

- (void)methodA:(NSString *)myText;
- (void)methodB:(NSString *)myText and:(NSString *)myOtherText;
- (void)methodX;

@end

.m

- (void)methodA:(NSString *)myText;
{
    if ([self.stringOfText isEqualToString:@""])
    {
        // Set the variable to be used by the completion block
        self.stringOfText = @"I visited methodA"; // normally make use of myText

        // Create the completion block
        __block TestClass *blocksafeSelf = self;
        self.completionBlock = ^()
        {
            [blocksafeSelf methodA:blocksafeSelf.stringOfText];
            blocksafeSelf.stringOfText = nil;
        };
    }
    else
    {
        // Do some other stuff with self.stringOfText
    }
}

- (void)methodB:(NSString *)myText and:(NSString *)myOtherText;
{
    if ([self.stringOfText isEqualToString:@""] || [self.otherStringOfText isEqualToString:@""])
    {
        // Set the variable to be used by the completion block
        self.stringOfText = @"I visited methodB"; // normally make use of myText
        self.otherStringOfText = @"I also visited methodB"; // normally make use of myOtherText

        // Create the completion block
        __block TestClass *blocksafeSelf = self;
        self.completionBlock = ^()
        {
            [blocksafeSelf methodB:blocksafeSelf.stringOfText and:blocksafeSelf.otherStringOfText];
            blocksafeSelf.stringOfText = nil;
            blocksafeSelf.otherStringOfText = nil;
        };
    }
    else
    {
        // Do some other stuff with self.stringOfText and self.otherStringOfText
    }
}

- (void)methodX
{
    // At this point run the block of code in self.completionBlock...how?!
}

In my example either methodA or methodB will be called first. Then some time later (perhaps from a different class) methodX will be called (only ever after methodA or methodB have been called).

It’s worth noting that the methods methodA, methodB and methodX are all in a singleton class.

NOTE: This is just a dummy example to try and understand the workings of blocks, I’m fully aware there are other ways to achieve the same result.

Answer 1

Here’s the code, just to be clear:

- (void)methodX
{
    if(self.completionBlock)
        self.completionBlock();
}