Question

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());

Answer 1

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/