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问题:

I did the following code where three views can be reused during Pagination in UIScrollView in order to save live memory-->

    #pragma mark - UIScrollView Delegates
- (void)scrollViewDidScroll:(UIScrollView *)scrollView{

    CGFloat pageWidth = self.view.frame.size.width;
    CGPoint aContentOffSet = [[self scrollView] contentOffset] ;
    float currPos = aContentOffSet.x;
    int selectedPage = roundf(currPos/pageWidth);
    [[self pageControl] setCurrentPage:selectedPage];
    [self update:selectedPage];
}


#pragma mark - Custom methods
-(void)update:(int) selectedPage{

    BOOL view1FrameShallBeUnchanged = false;
    BOOL view2FrameShallBeUnchanged = false;
    BOOL view3FrameShallBeUnchanged = false;

    BOOL aFrame1Matched = false;
    BOOL aFrame2Matched = false;
    BOOL aFrame3Matched = false;

    CGRect aFrame1 = CGRectMake(selectedPage*self.view.frame.size.width, 0.0f, self.view.frame.size.width, self.scrollView.frame.size.height);
    CGRect aFrame2 = CGRectMake((selectedPage-1)*self.view.frame.size.width, 0.0f, self.view.frame.size.width, self.scrollView.frame.size.height);
    CGRect aFrame3 = CGRectMake((selectedPage+1)*self.view.frame.size.width, 0.0f, self.view.frame.size.width, self.scrollView.frame.size.height);

    ViewOnScrollView *aView1 = (ViewOnScrollView*)[[self scrollView] viewWithTag:1234];
    ViewOnScrollView *aView2 = (ViewOnScrollView*)[[self scrollView] viewWithTag:12345];
    ViewOnScrollView *aView3 = (ViewOnScrollView*)[[self scrollView] viewWithTag:123456];

    if(aView1 && aView2 && aView3){
    //Check for Frame 1
    if(aFrame1.origin.x == aView1.frame.origin.x){
        view1FrameShallBeUnchanged = true;
        aFrame1Matched = true;
    }
    else if(aFrame1.origin.x == aView2.frame.origin.x){
        view2FrameShallBeUnchanged = true;
        aFrame1Matched = true;
    }
    else if(aFrame1.origin.x ==aView3.frame.origin.x){
        view3FrameShallBeUnchanged = true;
        aFrame1Matched = true;
    }

    //Check for Frame 2
    if(aFrame2.origin.x == aView1.frame.origin.x){
        view1FrameShallBeUnchanged = true; 
        aFrame2Matched = true;
    }
    else if(aFrame2.origin.x == aView2.frame.origin.x){
        view2FrameShallBeUnchanged = true;
        aFrame2Matched = true;
    }
    else if(aFrame2.origin.x == aView3.frame.origin.x){
        view3FrameShallBeUnchanged = true;
        aFrame2Matched = true;
    }

    //Check for Frame 3
    if(aFrame3.origin.x == aView1.frame.origin.x){
        view1FrameShallBeUnchanged = true;
        aFrame3Matched = true;
    }
    else if(aFrame3.origin.x == aView2.frame.origin.x){
        view2FrameShallBeUnchanged = true;
        aFrame3Matched = true;
    }
    else if(aFrame3.origin.x == aView3.frame.origin.x){
        view3FrameShallBeUnchanged = true;
        aFrame3Matched = true;
    }


    if(!view1FrameShallBeUnchanged){
        if(!aFrame1Matched){
            [aView1 setFrame:aFrame1];
        }
        else if(!aFrame2Matched){
            [aView1 setFrame:aFrame2];
        }
        else{
            [aView1 setFrame:aFrame3];
        }
        [self hideOrShowTheTabs:aView1];
        [self hideShowView:aView1];
    }

    if(!view2FrameShallBeUnchanged){
        if(!aFrame1Matched){
            [aView2 setFrame:aFrame1];
        }
        else if(!aFrame2Matched){
            [aView2 setFrame:aFrame2];
        }
        else{
            [aView2 setFrame:aFrame3];
        }

        [self hideShowView:aView2];
    }

    if(!view3FrameShallBeUnchanged){
        if(!aFrame1Matched){
            [aView3 setFrame:aFrame1];
        }
        else if(!aFrame2Matched){
            [aView3 setFrame:aFrame2];
        }
        else{
            [aView3 setFrame:aFrame3];           
        }

        [self hideShowView:aView3];
    }
    }
}

-(void)hideShowView:(ViewOnScrollView*)theView{
    if(theView.frame.origin.x<0 || theView.frame.origin.x>[self.scrollView contentSize].width )
        theView.hidden = YES; 
    else{
        theView.hidden = NO;
    }
}

Comments/Suggestions/Better ways to do the same are welcome..

回答1:

Its ok, but you have too much code (~40 lines) and too much unnecessary processing. You only need to know when one frame matches (let's say the center frame), and also you should do this only when the page is about to change, not on every scroll event.

This way, whenever the left or right page becomes the current page, you move the opposite page to the other side.

Another bug you had is that you should hide the last+1 page when its frame.origin.x is equal (== , or >=) to the content size, not only bigger (>).

#pragma mark - UIScrollView Delegates
- (void)scrollViewDidScroll:(UIScrollView *)scrollView{

    int selectedPage = roundf(newsPagesView.contentOffset.x/_pageWidth);

    if (selectedPage != _currentPage) {

        _currentPage = selectedPage;        
        [self update:selectedPage];
    }


}


#pragma mark - Custom methods
-(void)update:(int) selectedPage{

    BOOL page1FrameMatched = false;
    BOOL page2FrameMatched = false;
    BOOL page3FrameMatched = false;

    BOOL frameCurrentMatched = false;


    CGRect frameCurrent = CGRectMake(selectedPage*_pageWidth, 0.0f, _pageWidth, _pageHeight);
    CGRect frameLeft = CGRectMake((selectedPage-1)*_pageWidth, 0.0f, _pageWidth, _pageHeight);
    CGRect frameRight = CGRectMake((selectedPage+1)*_pageWidth, 0.0f, _pageWidth, _pageHeight);

    NewsPage *page1 = (NewsPage*)[newsPagesView viewWithTag:100];
    NewsPage *page2 = (NewsPage*)[newsPagesView viewWithTag:101];
    NewsPage *page3 = (NewsPage*)[newsPagesView viewWithTag:102];

    if(page1 && page2 && page3){

        //Check for Current
        if(frameCurrent.origin.x == page1.frame.origin.x){
            page1FrameMatched = true;
            frameCurrentMatched = true;
        }
        else if(frameCurrent.origin.x == page2.frame.origin.x){
            page2FrameMatched = true;
            frameCurrentMatched = true;
        }
        else if(frameCurrent.origin.x ==page3.frame.origin.x){
            page3FrameMatched = true;
            frameCurrentMatched = true;
        }

        if(frameCurrentMatched){
            if(page1FrameMatched){
                [page1 setFrame:frameCurrent];
                [page2 setFrame:frameLeft];
                [page3 setFrame:frameRight];

            }
            else if(page2FrameMatched){
                [page1 setFrame:frameRight];
                [page2 setFrame:frameCurrent];
                [page3 setFrame:frameLeft];
            }
            else{
                [page1 setFrame:frameLeft];
                [page2 setFrame:frameRight];
                [page3 setFrame:frameCurrent];

            }

            [self hideShowView:page1];
            [self hideShowView:page2];
            [self hideShowView:page3];
        }

    }
}

/**
 * This method hides the view if it is outside the scrollview content bounds, i.e. the
 * view before page 0, or the view after last page.
 */
-(void)hideShowView:(NewsPage*)aPage{

    if(aPage.frame.origin.x<0 || aPage.frame.origin.x>=[newsPagesView contentSize].width )
        aPage.hidden = YES; 
    else{
        aPage.hidden = NO;
    }
}