1，如何一次dismiss2个ViewController
[self.presentingViewController.presentingViewController dismissViewControllerAnimated:true completion:^{
}];
2，查看各种机型调用@2x还是@3x
@1x（）
@2x（iPhone XR，iPhone SE，iPhone 6、iPhone 6s、iPhone 7、iPhone 8）
@3x（iPhone XS Max、iPhone XS，iPhone X，iPhone 8 Plus）
3,
- (CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath NS_AVAILABLE_IOS(7_0);
返回的数值要大于1.0 ，不然报错
Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'table view row height
must not be negative - provided height for index path
(<NSIndexPath: 0xc000000000000016> {length = 2, path = 0 - 0}) is -1.000000'**
**libc++abi.dylib: terminating with uncaught exception of type NSException**
