我在滚动视图中延迟加载图像。然而，当我滚动内容内存分配增加时，它相当缓慢。这是我的代码；
- (void)scrollViewSetUp
{
self.scrollview.delegate = self;
self.automaticallyAdjustsScrollViewInsets = NO;
NSInteger pageCount = self.saleImages.count;
self.pageControl = [[UIPageControl alloc] initWithFrame:CGRectMake(0.0,0.0,self.view.frame.size.width,40.0)];
[self.pageControl setNumberOfPages:pageCount];
[self.pageControl setCurrentPage:0];
self.pageControl.pageIndicatorTintColor = [UIColor lightGrayColor];
self.pageControl.currentPageIndicatorTintColor = [UIColor whiteColor];
[self.pageControl setBackgroundColor:[UIColor clearColor]];
//Next, you set up the array that holds the UIImageView instances. At first, no pages have been lazily loaded and so you just fill it with the right amount of NSNull objects that are needed – one for each page.
//You’re using [NSNull null] because it’s a lightweight singleton object that can be added to an array to signify a placeholder.
//Later on, you’ll use the fact that there is an NSNull in there to know if that page is loaded or not.
self.pageViews = [[NSMutableArray alloc] init];
for (NSInteger i = 0; i < pageCount; i++) {
[self.pageViews addObject:[NSNull null]];
}
}
这是我的loadPage:方法；
-(void)loadPage:(NSInteger)page{
if (page < 0 || (page >= self.saleImages.count)) {
//if its outside the range of what you have to display, then do nothing
return;
}
//First, you check if you've already loaded the view. If you haven't, then the object in the pageViews array will be an NSNull ( remember, [NSNull null] is a special singleton which is why == works).
UIView * pageView = [self.pageViews objectAtIndex:page];
if ((NSNull*) pageView == [NSNull null]) {
CGRect frame = self.view.bounds;//set to views bounds instead of scrollviews bounds because it doesnt work very well with autolayout
frame.origin.x = (frame.size.width+kScrollViewPadding) * page;
frame.origin.y = 0.0f;
UIImageView * newPageView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:[self.saleImages objectAtIndex:page]]];
newPageView.contentMode = UIViewContentModeScaleToFill;
newPageView.frame = CGRectMake(frame.origin.x,frame.origin.y,frame.size.width, frame.size.height);
[self.scrollview addSubview:newPageView];
//Finally, you replace the NSNull in pageViews array with the view you've kust created, so that if this page was asked to load again you would now not go into the if statement and instead do nothing since the view for the page has already been created.
[self.pageViews replaceObjectAtIndex:page withObject:newPageView];
}
}
这是我的purgePage和loadVisiblePages；
-(void)purgePage:(NSInteger)page{
if (page <0 || page >= self.saleImages.count) {
//if it's outside the range of what you have to display, then do nothing
return;
}
//Remove a page from the scroll view and reset the container array
UIView * pageView = [self.pageViews objectAtIndex:page];
if ((NSNull *)pageView != [NSNull null]) {
[pageView removeFromSuperview];
pageView = nil;
[self.pageViews replaceObjectAtIndex:page withObject:[NSNull null]];
}
}
-(void)loadVisiblePages{
//first determine which page is currently visible
CGFloat pageWidth = self.scrollview.frame.size.width;
NSInteger page = (NSInteger)floor((self.scrollview.contentOffset.x * 2.0f + pageWidth)/(pageWidth * 2.0f));
//update the page control
self.pageControl.currentPage = page;
//Work out which pages you want to load
NSInteger firstPage = page -1;
NSInteger lastPage = page+1;
//purge anything before the first page
for (NSInteger i = 0; i<firstPage; i++) {
[self purgePage:i];
}
//load pages in our range
for (NSInteger i = firstPage; i<=lastPage; i++) {
[self loadPage:i];
}
//purge anything after the last page
for (NSInteger i = lastPage +1; i<self.saleImages.count; i++) {
[self purgePage:i];
}
}
我个人觉得我的purgePage方法有问题。在这里，我从 superview 中删除 pageView 并将其设置为 nil。我希望有人能帮忙，因为我把头发扯掉了。
编辑 1后Generation Analysis在仪器中似乎UIImage正在增加。罪魁祸首类是 loadPage 和 loadVisiblePages。
编辑 2 @Wain 指出imageNamed:会进行一些缓存。