UIScrollView and lazy loading

转自：

http://favo.asia/2010/06/uiscrollview-and-lazy-loading/

这个网址不稳定，时好时坏，就转到我的博客

If you have encountered memory problems loading too many images into your UIScrollView, lazy loading them is your answer.
Lazy loading describes an easy technique to load only what should be shown, no more.

Lazy loading of images in a UIScrollView is critical because of the iPhone/iPads/iPods low physical memory.

Doing it on the other hand is very easy if you follow these four steps:

1. Listen for scrollViewDidScroll on your delegate, like this:

-(void)scrollViewDidScroll:(UIScrollView *)myScrollView {
}

2. Calculate the current page using the known size of your images:

	/**
	 *	calculate the current page that is shown
	 *	you can also use myScrollview.frame.size.height if your image is the exact size of your scrollview
	 */
	int currentPage = (myScrollView.contentOffset.y / currentImageSize.height);

3. Look if the image already exists, if not, add it:

	// display the image and maybe +/-1 for a smoother scrolling
	// but be sure to check if the image already exists, you can do this very easily using tags
	if ( [myScrollView viewWithTag:(currentPage +1)] ) {
		return;
	}
	else {
		// view is missing, create it and set its tag to currentPage+1
	}

4. And don’t forget to clean your memory:

	/**
	 *	using your paging numbers as tag, you can also clean the UIScrollView
	 *	from no longer needed views to get your memory back
	 *	remove all image views except -1 and +1 of the currently drawn page
	 */
	for ( int i = 0; i < currentPages; i++ ) {
		if ( (i < (currentPage-1) || i > (currentPage+1)) && [myScrollView viewWithTag:(i+1)] ) {
			[[myScrollView viewWithTag:(i+1)] removeFromSuperview];
		}
	}

= And the final combination of all steps:

-(void)scrollViewDidScroll:(UIScrollView *)myScrollView {
	/**
	 *	calculate the current page that is shown
	 *	you can also use myScrollview.frame.size.height if your image is the exact size of your scrollview
	 */
	int currentPage = (myScrollView.contentOffset.y / currentImageSize.height);

	// display the image and maybe +/-1 for a smoother scrolling
	// but be sure to check if the image already exists, you can do this very easily using tags
	if ( [myScrollView viewWithTag:(currentPage +1)] ) {
		return;
	}
	else {
		// view is missing, create it and set its tag to currentPage+1
	}

	/**
	 *	using your paging numbers as tag, you can also clean the UIScrollView
	 *	from no longer needed views to get your memory back
	 *	remove all image views except -1 and +1 of the currently drawn page
	 */
	for ( int i = 0; i < currentPages; i++ ) {
		if ( (i < (currentPage-1) || i > (currentPage+1)) && [myScrollView viewWithTag:(i+1)] ) {
			[[myScrollView viewWithTag:(i+1)] removeFromSuperview];
		}
	}
}