超出范围后销毁UIImageView对象

Question

In my application I have some images falling from above inside a UIView. The UIImageView objects are created dynamically and made to move downwards. My question is, do the objects destroy themselves after they move off below the screen area? Or should I do it manually to improve performance?

Answer 1

Once there are no more strong references to an object, it will be deallocated. The two references you probably need clear will be the variable pointer, and the superview's reference to it. So you'd need to do something like:

[imageView removeFromSuperView];
imageView = nil;

It's possible there are more as you did not provide any code (if for instance you have a pointer to the object in an array, you'd need to remove that too).

Answer 2

Removing from superview is all that's needed if you don't have any other pointers to the image view (in ARC). The most performant pattern is to keep a pool of pointers to offscreen image views. In pseudo code:

// this assumes the number onscreen is relatively small, in the tens or low hundreds
// this works better when the number on screen is relatively constant (low variance)

// say the animation looks something like this:

- (void)makeAnImageFall:(UIImage *)image {

    CGRect startFrame = // probably some rect above the superview's bounds
    UIImageView *imageView = [self addImageViewWithImage:image frame:frame];  // see below
    [UIView animateWithDuration:1.0 animations:^{
        imageView.frame = // probably some rect below the superview's bounds
    } completion:^(BOOL finished) {
        [self removeImageView:imageView];  // see below
    }];
}

Then these methods do everything to handle the pool:

- (UIImageView *)addImageViewWithImage:(UIImage *)image frame:(CGRect)frame {

    UIImageView *imageView;
    // assume you've declared and initialized imageViewPool as an NSMutableArray
    // or do that here:
    if (!self.imageViewPool) self.imageViewPool = [NSMutableArray array];

    if (self.imageViewPool.count) {
        imageView = [self.imageViewPool lastObject];
        [self.imageViewPool removeLastObject];
    } else {
        imageView = [[UIImageView alloc] init];
    }
    imageView.image = image;
    imageView.frame = frame;
    [self.view addSubview:imageView];
    return imageView;
}

- (void)removeImageView:(UIImageView *)imageView {
    imageView.image = nil;
    [self.imageViewPool addObject:imageView];
    [imageView removeFromSuperview];
}

The nice idea is that we avoid the relatively expensive churn of create-destroy of many image views. Instead, we create them (lazily) when they're first needed. Once we hit the high water mark of image views, we don't ever allocate another.