最近專案中用到了“圖隨表動”，如下圖所示：

        這是一個訂單，需要add shipping insurance以上的部分隨著表格的滾動而滾動，這裡表格是group型別的，所以背景加在cell上是達不到效果的，在code4App上找到了一個例子（code4App中搜索“圖隨表動”），實現如下：

#define bgHeight table.frame.size.height
#define beginBgY -table.frame.size.height
- (void)addBgLayer{
    UIImage *img = [UIImage imageNamed:@"bgTable.png"];
    
    CGFloat imgHeight = img.size.height;
    CGFloat contentHeight = table.contentSize.height;
   
    NSInteger needNum = (contentHeight+bgHeight - beginBgY)/imgHeight + 1;
    if (needNum > alreadyAddBg) {
        for(int i = alreadyAddBg; i < needNum; i++) {
            CGRect rect = CGRectMake(0, beginBgY+i*imgHeight, table.frame.size.width,imgHeight);
            [self addLayerWithFrame:rect];
        }
        alreadyAddBg = needNum;
    }
}

- (void)addLayerWithFrame:(CGRect)frame{
    CALayer *background = [CALayer layer];
    background.zPosition = -1;
    background.frame = frame;
    background.contents = (id)([[UIImage imageNamed:@"bgTable.png"] CGImage]);
    [table.layer addSublayer:background];
}
 

       但是弄成收縮的之後，背景的大小又要隨變化，我開始直接想把layer 給remove掉，然後重新新增，當然，結果是可以預見的，程式崩潰掉了，因為layer的引用計數已經只有1了，再remove當然就崩掉了。

我想了一個辦法：因為從要求來看，只有列表部分的高度在變化，其他地方不變，所以背景的大小隻有兩種，這樣我就定義了兩個layer的成員變數並初始化分配記憶體，將背景圖分別加在兩個不一樣大小的layer上，然後需要用哪個的時候就將哪個載入在table的layer上，將不用的remove掉，具體程式碼如下：

- (void)addBgLayer
{
    
    UIImage *img0 = [UIImage imageNamed:@"tbg.png"];
    UIImage *img1 = [UIImage imageNamed:@"pbg.png"];
    UIImage *img2 = [UIImage imageNamed:@"bbg.png"];

    CGFloat imgHeight = img1.size.height;
    CGFloat contentHeightOpen = 0;;//減兩個16是頂部和底部鋸齒
    CGFloat contentHeightNotOpen = 0;;
    if (_order.isNewOrder)
    {
        contentHeightOpen = 90+[_order.productList count]*127+95+65+65+50*3-16-16;//減兩個16是頂部和底部鋸齒
        contentHeightNotOpen = 90+95+65+65+50*3-16-16;
    }else
    {
        contentHeightOpen = 90+[_order.productList count]*127+95+50*3-16-16;//減兩個16是頂部和底部鋸齒
        contentHeightNotOpen = 90+95+50*3-16-16;
    }
    self.layerOpen.frame = EZRECT(0, 0, 320,  contentHeightOpen);
    self.layerNotOpen.frame =EZRECT(0, 0, 320,  contentHeightOpen);


    NSInteger needNum;
    
    {//開啟的layer
        [self addLayerWithFrame:EZRECT(0, 0, 320, 16) withImage:img0 toLayer:self.layerOpen];
        needNum = contentHeightOpen/imgHeight + 1+1;//為了美觀，多加1個
        
        for(int i = 0; i < needNum; i++) {
            //CGRect rect = CGRectMake(0, beginBgY+i*imgHeight, 320,imgHeight);
            CGRect rect = CGRectMake(0, i*imgHeight+16, 320,imgHeight);
            [self addLayerWithFrame:rect withImage:img1 toLayer:self.layerOpen];
        }
        [self addLayerWithFrame:EZRECT(0, 16+needNum*imgHeight, 320, 16) withImage:img2 toLayer:self.layerOpen];
    }
    
    {//收起的layer
        [self addLayerWithFrame:EZRECT(0, 0, 320, 16) withImage:img0 toLayer:self.layerNotOpen];
        
        needNum = contentHeightNotOpen/imgHeight + 1+1;//為了美觀，多加1個
        for(int i = 0; i < needNum; i++)
        {
            //CGRect rect = CGRectMake(0, beginBgY+i*imgHeight, 320,imgHeight);
            CGRect rect = CGRectMake(0, i*imgHeight+16, 320,imgHeight);
            [self addLayerWithFrame:rect withImage:img1 toLayer:self.layerNotOpen];
        }
        
        [self addLayerWithFrame:EZRECT(0, 16+needNum*imgHeight, 320, 16) withImage:img2 toLayer:self.layerNotOpen];
    }
    
    BOOL isOpen = [[isOpenDic objectForKey:@"sectionProductList"] boolValue];
    if (isOpen)
    {
        self.layerOpen.zPosition = -2;
        [table.layer addSublayer:self.layerOpen];
    }
    else
    {
        self.layerNotOpen.zPosition = -2;
        [table.layer addSublayer:self.layerNotOpen];
    }
}

- (void)addLayerWithFrame:(CGRect)frame withImage:(UIImage *)image toLayer:(CALayer *)layer
{
    CALayer *background = [CALayer layer];
    background.zPosition = -1;
    background.frame = frame;
    background.contents = (id)([image CGImage]);
    [layer addSublayer:background];
}
 

       這裡我又碰到了一個問題，就是

       self.layerOpen.zPosition = -2;

       圖層的層次問題，注意到新增到table上的layer與新增到layerOpen（新增到table上的layer）層次是不同的。