ios - 为什么我们需要对 Objective-C 对象字段进行保留？

Question

这是Delphi媒体播放器的代码:

type
  TAVMedia = class(TMedia)
  private
    FPlayer: AVPlayer;
    FPlayerItem: AVPlayerItem;
  public
    constructor Create(const AFileName: string); override;
    destructor Destroy; override;
  end;

constructor TAVMedia.Create(const AFileName: string);
var aURL: NSUrl;
begin
  inherited Create(AFileName);
  FPlayerItem := TAVPlayerItem.Wrap(TAVPlayerItem.OCClass.playerItemWithURL(URL));
  FPlayerItem.retain;
  FPlayer := TAVPlayer.Wrap(TAVPlayer.OCClass.playerWithPlayerItem(FPlayerItem));
  FPlayer.retain;
end;

destructor TAVMedia.Destroy;
begin
  FPlayer.release;
  FPlayer := nil;
  FPlayerItem.release;
  FPlayerItem := nil;
  inherited Destroy;
end;

我不太明白为什么他们需要执行FPlayerItem.retain和FPlayer.retain？ FPlayerItem 和 FPlayer 是对象字段而不是局部变量，因此始终对它们有强引用。那么这里retain的目的是什么？

似乎执行FPlayer.release;也会释放FPlayerItem，因此当稍后调用FPlayerItem.release;时有时会触发访问冲突(奇怪的是并不总是)。

注意:我仍然不明白为什么我有 eaccessviolation，所以我决定将我所做的完整代码放在这里:

type
  TMyMedia = class(TObject)
  private
    FPlayer: AVPlayer;
    FPlayerItem: AVPlayerItem;
  public
    constructor Create;
    destructor Destroy; override;
  end;

constructor TMyMedia.Create;
begin
  inherited Create;

  P := TNSUrl.OCClass.URLWithString(StrToNSStr(aDataSource)); // Creates and returns an NSURL object initialized with a provided URL string
  if P = nil then raise EFileNotFoundException.Create(SFileNotFound); // If the URL string was malformed or nil, returns nil.
  aURL := TNSUrl.Wrap(P);
  try

    FPlayerItem := TAVPlayerItem.Wrap(TAVPlayerItem.OCClass.playerItemWithURL(URL));
    FPlayerItem.retain;

  finally
    aURL.release; // << if i don't do this then i will not have any exception at the end ??? 
    aURL := nil;  // <<
  end;


  FPlayer := TAVPlayer.Wrap(TAVPlayer.OCClass.playerWithPlayerItem(FPlayerItem));
  FPlayer.retain;
end;

destructor TAVMedia.Destroy;
begin

  ALLog('FPlayer.retainCount', inttostr(FPlayer.retainCount)); // => show 1
  ALLog('FPlayerItem.retainCount', inttostr(FPlayerItem.retainCount)); // => show 6

  FPlayer.release;
  FPlayer := nil;

  ALLog('FPlayerItem.retainCount', inttostr(FPlayerItem.retainCount)); // => show 1
  FPlayerItem.release; => here i receive Access violation at address 2156565 accessing address 68684458

  FPlayerItem := nil;
  inherited Destroy;
end;

Answer 1

TNSUrl.OCClass.URLWithString 和 TAVPlayerItem.OCClass.playerItemWithURL 将项目添加到自动释放池中。因此，保留计数为 1。当自动释放池释放其包含的项目时，它们将被释放，这通常发生在当前事件执行完成之后。

因此需要FPlayerItem.retain，因为函数退出后FPlayerItem不应该被释放。它已分配给 FPlayerItem，因此您希望使其保持事件状态。

一般来说，如果你用Create、alloc、copy、mutableCopy、new... 然后为您调用retain。然后你需要调用release或autorelease。

如果您使用其他函数(例如 fileUrlWithPath)创建这样的类，那么它将添加到自动释放池中。您的保留计数仍然为 1，但它将为您释放。如果你也释放它，那么你就会崩溃。

如果您在类上调用 retain，则该调用必须与 release 保持平衡。