sdwebimage源码解析（一）

This library provides an async image downloader with cache support. For convenience, we added categories for UI elements like UIImageView, UIButton, MKAnnotationView.

1. Categories for UIImageView, UIButton, MKAnnotationView adding web image and cache management

• An asynchronous image downloader
• An asynchronous memory + disk image caching with automatic cache expiration handling
• A background image decompression
• A guarantee that the same old won’t be downloaded several times
• A guarantee that bogus URLs won’t be retried again and again
• A guarantee that main thread will never be blocked
• Performances!
• Use GCD and architecture

Objective-C:

[imageView sd_setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"] placeholderImage:[UIImage imageNamed:@"placeholder.png"]];

Swift:

imageView.sd_setImage(with: URL(string: "http://www.domain.com/path/to/image.jpg"), placeholderImage: UIImage(named: "placeholder.png"))