编译器警告：CGContextSaveGState: invalid context 0x0

一、问题描述

下载图片，然后用Quartz2D绘制缩放的图片，运行无法显示图片并且编译器警告:

Aug 18 21:41:50 02_计算UITableViewCell的行高[16777] <Error>: CGContextSaveGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Aug 18 21:41:50 02_计算UITableViewCell的行高[16777] <Error>: CGContextSetBlendMode: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Aug 18 21:41:50 02_计算UITableViewCell的行高[16777] <Error>: CGContextSetAlpha: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Aug 18 21:41:50 02_计算UITableViewCell的行高[16777] <Error>: CGContextTranslateCTM: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Aug 18 21:41:50 02_计算UITableViewCell的行高[16777] <Error>: CGContextScaleCTM: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Aug 18 21:41:50 02_计算UITableViewCell的行高[16777] <Error>: CGContextDrawImage: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Aug 18 21:41:50 02_计算UITableViewCell的行高[16777] <Error>: CGContextRestoreGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.

二、问题分析

有人说因为设置app的状态栏样式的使用了旧的方式，一般式iOS6的时候使用这种方式，iOS7、8也兼容，但是到了iOS9就报了警告。在info.plist里面设置了View controller-based status bar appearance为NO，要设置为YES。

但View controller-based status bar appearance设置YES后，问题还是没解决。

后来发现只要执行以下代码都会警告：

1 - (void)setImageView
2 {
3     NSString *url = @"http://ww3.sinaimg.cn/large/005P1ePojw1f6xzt7o6saj30go4w8wna.jpg";
4     // 设置图片
5      __weak typeof(self) weakSelf = self;
6     [self.imageView sd_setImageWithURL:[NSURL URLWithString:url] placeholderImage:nil options:0 progress:nil completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
7         // 开启图形上下文
8         UIGraphicsBeginImageContextWithOptions(weakSelf.size, YES, 0.0);
9         // 将下载完的image对象绘制到图形上下文
10         CGFloat width = weakSelf.size.width;
11         CGFloat height = width * 520 / 300;
12         [image drawInRect:CGRectMake(0, 0, width, height)];
13         // 获得图片
14         weakSelf.imageView.image = UIGraphicsGetImageFromCurrentImageContext();
15         // 结束图形上下文
16         UIGraphicsEndImageContext();
17     }];
18 }

后来发现问题出现在这处： UIGraphicsBeginImageContextWithOptions(weakSelf.size, YES, 0.0);

因为weakSelf.size的值CGSizeMake(0, 0)，导致错误。

三、问题解决

重新设置self.size的值。