在滚动视图中延迟加载图像似乎会增加内存

Question

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 在instruments 中执行Generation Analysis 后，UIImage 似乎在增加。罪魁祸首类是 loadPage 和 loadVisiblePages。

编辑 2 @Wain 指出 imageNamed: 做了一些缓存。

Answer 1

您当前正在使用imageNamed:，它会缓存加载的图像以提高性能。这并不总是理想的，您可能需要避免使用这种方便的方法。

为避免使用imageNamed:，您需要获取图像的路径（可能来自bundle），然后使用initWithContentsOfFile:。

请注意，删除所有缓存可能过于极端，您可能希望创建自己的缓存（如使用字典），您可以控制在任何时候缓存多少图像。此外，根据您显示图像的方式，您可能希望将一些不同尺寸的图像添加到您的应用中，以便您可以为每种情况加载最合适的尺寸。

另外请注意，如果需要，您可以在后台加载图像，然后将图像推送到主线程以更新 UI。