用于mask遮罩效果的图片配合resizableImage使用

效果：

作为素材用的图片：

源码：

//
//  ViewController.m
//  Rect
//
//  Created by YouXianMing on 15/3/29.
//  Copyright (c) 2015年 YouXianMing. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    // 背景图
    UIImageView *bgView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"psb.jpeg"]];
    bgView.frame        = CGRectMake(0, 0,
                                     bgView.frame.size.width / 2.f,
                                     bgView.frame.size.height / 2.f);
    bgView.center       = self.view.center;
    [self.view addSubview:bgView];

    // 作为mask用的图片
    UIImage *maskImage   = [UIImage imageNamed:@"rect_new"];
    maskImage            = [maskImage resizableImageWithCapInsets:UIEdgeInsetsMake(13, 13, 13, 13)
                                                     resizingMode:UIImageResizingModeTile];

    // 作为mask用的view
    UIImageView *maskView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
    maskView.image        = maskImage;
    bgView.maskView       = maskView;

    // 执行动画
    [UIImageView animateWithDuration:3.f animations:^{
        maskView.frame = bgView.bounds;
    }];
}

@end

需要注意的细节：

这对于做alpha通道的渐变效果十分便利。