在 Objective C 中丢失 viewDidLoad 的属性 [英] Losing properties on viewDidLoad in Objective C

问题描述

我正在尝试构建一个简单的应用程序，用户可以在其中从 tableViewController 中选择一个视频，然后加载一个播放视频的视图.我的问题是将视频的 URL 从 tableViewController 传输到 viewController.

我遵循了本教程，现在我正在尝试调整代码以进行播放视频而不是仅仅显示图像

我正在从 tableViewController 创建一个 viewController，如下所示:

- (void) prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {wwfpViewController * DVC = [[wwfpViewController alloc] init];NSIndexPath * path = [self.tableView indexPathForSelectedRow];NSString * theVideoName = [videoNames objectAtIndex:path.row];NSString * theVideoURL = [videoList objectAtIndex:path.row];DVC.videoNum = path.row;DVC.videoName = theVideoName;DVC.videoURL = theVideoURL;【DVC播放】；}

然后触发此 viewController 的播放消息，当我 NSLog 从此消息中时，videoURL 存在且正确.

然后在这个 viewController 上触发 viewDidLoad，此时当我 NSLog 的 videoURL 是返回为 (null).

我像这样声明 videoURL:

@property (strong, nonatomic) NSString * videoURL;

所以我有几个问题:

• 当 viewDidLoad 被触发时，viewController 是否会丢失其属性?
• 是否有更好的方法将属性发送到 viewController?
• 我这样做完全错了吗?
• 我还需要提供更多代码吗?

解决方案

问题是，尽管您使用的是 segue，但您手动分配/初始化控制器并设置其属性(这当然是完全不同的实例来自将要呈现的那个).

您应该这样做:

- (void) prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {//获取目标控制器//(它将自动从故事板实例化)wwfpViewController * DVC = (wwfpViewController *)segue.destinationViewController;NSIndexPath * path = [self.tableView indexPathForSelectedRow];NSString * theVideoName = [videoNames objectAtIndex:path.row];NSString * theVideoURL = [videoList objectAtIndex:path.row];//设置属性DVC.videoNum = path.row;DVC.videoName = theVideoName;DVC.videoURL = theVideoURL;【DVC播放】；//为什么不在目标控制器的 viewDidLoad 上调用它?}

PS: 在 Objective-C 中，您通常会命名 ivars(按照惯例)，以便它们以小写字母开头，而类以大写字母开头.所以 WWFPViewController 和 dvc 将是更合适的名称.

I'm trying to build a simple App where users can choose a video from a tableViewController, which then loads a view that plays a video. My problem is transferring the URL of the video from the tableViewController to the viewController.

I followed this tutorial, and I am now trying to adapt the code to play videos instead of just show images

I am creating a viewController from a tableViewController like this:

- (void) prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    wwfpViewController * DVC = [[wwfpViewController alloc] init];
    NSIndexPath * path = [self.tableView indexPathForSelectedRow];
    NSString * theVideoName = [videoNames objectAtIndex:path.row];
    NSString * theVideoURL = [videoList objectAtIndex:path.row];
    DVC.videoNum = path.row;
    DVC.videoName = theVideoName;
    DVC.videoURL = theVideoURL;
    [DVC play];
}

The play message for this viewController is then fired, and when I NSLog from this message the videoURL is present and correct.

Then viewDidLoad is fired on this viewController, and at this point when I NSLog the videoURL it is returned as (null).

I'm declaring the videoURL like this:

@property (strong, nonatomic) NSString * videoURL;

So I have a few questions:

• Does a viewController lose its properties when viewDidLoad is fired?
• Is there a better approach to sending properties to a viewController?
• Am I doing this completely wrong?
• And do I need to provide any more code?

解决方案

The problem is that despite the fact that you're using segues, you manually alloc/init a controller and set its properties (which of course is a totally different instance from the one that will be presented).

Here is what you should do instead:

- (void) prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    // Grab the destination controller
    // (it will be instantiated from the Storyboard automatically)
    wwfpViewController * DVC = (wwfpViewController *)segue.destinationViewController;
    NSIndexPath * path = [self.tableView indexPathForSelectedRow];
    NSString * theVideoName = [videoNames objectAtIndex:path.row];
    NSString * theVideoURL = [videoList objectAtIndex:path.row];
    // Set the properties
    DVC.videoNum = path.row;
    DVC.videoName = theVideoName;
    DVC.videoURL = theVideoURL;
    [DVC play]; // Why don't you call this on viewDidLoad of the destination controller?
}

PS: In Objective-C you usually name the ivars (by convention) so that they start with a small letter and the Classes with a capital one. So WWFPViewController and dvc would be more appropriate names in your case.

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