- (UIImage *)stretchableImageWithLeftCapWidth:(NSInteger)leftCapWidth topCapHeight:(NSInteger)topCapWidth;

  Creates and returns a new image object with the specified cap values.     During scaling or resizing of the image, areas covered by a cap are not scaled or resized. Instead, the 1-pixel wide area not covered by the cap in each direction is what is scaled or resized. This technique is often used to create variable-width buttons, which retain the same rounded corners but whose center region grows or shrinks as needed.   上面是官方文檔上得解釋。意思就是用來創建一個內容可拉伸，而邊角不拉伸的圖片，需要兩個參數，第一個是左邊不拉伸區域的寬度，第二個參數是上面不拉伸的高度。那麼接下來的一個像素會被拉伸。例如，leftCapHeight為6，topCapHeight為8。那麼，圖片左邊的6個像素，上邊的8個像素。不會被拉伸，而左邊的第7個像素，上邊的第9個像素這一塊區域將會被拉伸。剩余的部分也不會被拉伸。