从.plist检索的Objective-C样式在Swift中不起作用? [英] Objective-C style retrieving from .plist not working in Swift?

问题描述

每当通过以下方法加载视图时，我就是在Objective-C的.plist中检索数据-

I was was retrieving data in a .plist in Objective-C whenever my view loaded by the following method -

@interface ListTableViewController ()

@end

@implementation ListTableViewController
NSArray *dict;

-(void)viewDidAppear:(BOOL)animated
{
    NSString *arr= [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];

    arr=[arr stringByAppendingPathComponent:@"datalist.plist"];
    if ([[NSFileManager defaultManager]fileExistsAtPath:arr])
    {
       dict=[NSArray arrayWithContentsOfFile:arr];
        NSLog(@"%@",dict);

    }
    [self.tableView reloadData];

}

现在，对于Swift我正在做的事情与-

Now for Swift I am doing the same thing as -

class ListTableViewController: UITableViewController{
  var arr = NSArray()
  var plistfinalpath = String()

  override func viewDidAppear(animated: Bool){
      let path = NSSearchPathForDirectoriesInDomains(NSSearchPathDirectory.DocumentDirectory, .UserDomainMask, true)[0]
      plistfinalpath = path.stringByAppendingString("/login.plist")
      print(plistfinalpath)
      print("time of appearing\(arr)")

      arr = NSArray(contentsOfFile: plistfinalpath)!
      self.tableView.reloadData()
  }
}

但是，由于我必须在第一次启动应用程序时在plist为空时解开arr = NSArray(contentsOfFile: plistfinalpath)!行中的值，因此数组为空，并且由于我将其解包，因此显示错误.那么如何实现目标C中的工作呢?

But since I have to unwrap the value in the line arr = NSArray(contentsOfFile: plistfinalpath)! when the plist is empty when the app is launched for the first time the array is empty and since I am unwrapping it, it shows an error. So how can I achieve what I was doing in objective C?

推荐答案

只需将arr变量定义为可选变量，如下所示:
var arr:NSArray? = NSArray()

Just define your arr variable as optional, like this :
var arr:NSArray? = NSArray()

将变量定义为非可选变量时，不能将其设置为nil.您使用的NSArray初始化程序定义为public convenience init?(contentsOfFile path: String)-?表示它实际上可以返回nil.当您使用!时，您将强制解开该值，当该值为nil时，将导致运行时崩溃.

When a variable is defined as non optional, you cannot make it nil. The NSArray initializer you used is defined as public convenience init?(contentsOfFile path: String) - the ? means, that it can in fact return a nil. When you used !, you force unwrapped the value, which results in runtime crash when the value is nil.

与往常一样，您可以在文档中阅读更多内容

As always, you can read more in the docs

或者，您可以执行以下操作:

Alternatively, you can do something like this :

if let tempArr = NSArray(contentsOfFile: plistfinalpath) {
    arr = tempArr
}

这样，您无需将arr定义为可选，而仅在文件内容不为空时分配给它.

This way, you don't need to have your arr defined as optional and will only assign to it, when the contents of file where not empty.

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