为了为不同的服务提供不同的项目，你将 UIActivityItemProvider 子类化.它符合UIActivityItemSource协议，所以你实现方法:

To provide different items for different services, you subclass UIActivityItemProvider. It conforms to UIActivityItemSource protocol, so you implement method:

- (id)activityViewController:(UIActivityViewController *)activityViewController
         itemForActivityType:(NSString *)activityType {
    // compare `activityType` and return what you want
}