stretchableImageWithLeftCapWidth通常用來(lái)做圖片的局部拉伸，常用的例子是聊天的背景圖的部分拉伸

stretchableImageWithLeftCapWidth這個(gè)方法 有2個(gè)參數(shù)
leftCapWidth: 左邊不拉伸的像素
topCapHeight:上邊不拉伸的像素

例如如下的點(diǎn)：

則取橫坐標(biāo)軸26的像素點(diǎn)橫向拉伸到一定寬度
取縱坐標(biāo)軸11的像素點(diǎn)縱向拉伸到一定長(zhǎng)度 （具體長(zhǎng)度又imageview的frame決定）
image = [image stretchableImageWithLeftCapWidth:25 topCapHeight:10];

    UIImage *image = [UIImage imageNamed:@"ic_nav_back_dark"];
    UIImageView *view = [[UIImageView alloc] init];
    view.backgroundColor = [UIColor whiteColor];
    view.image = image;
    
    view.frame = CGRectMake(60, 60, 100, 100);
    
    [self.view addSubview:view];
    
    
    UIImageView *view1 = [[UIImageView alloc] init];
    view1.backgroundColor = [UIColor whiteColor];
    image = [image stretchableImageWithLeftCapWidth:25 topCapHeight:10];
    view1.image = image;
    
    view1.frame = CGRectMake(60, 300, 100, 100);
    
    [self.view addSubview:view1];
    
    UIImageView *view2 = [[UIImageView alloc] init];
    view2.backgroundColor = [UIColor whiteColor];
    UIImage *image1 = [UIImage imageNamed:@"ic_nav_back_dark"];
    image1 = [image1 stretchableImageWithLeftCapWidth:1 topCapHeight:1];
    view2.image = image1;
    view2.frame = CGRectMake(200, 300, 100, 100);
    [self.view addSubview:view2];

效果如下：

855CAC19-ED4F-494C-BD32-21D00213D10A.png