Flutter載入圖片流程之ImageProvider原始碼範例解析

/// Resolves this image provider using the given `configuration`, returning
/// an [ImageStream].
///
/// This is the public entry-point of the [ImageProvider] class hierarchy.
///
/// Subclasses should implement [obtainKey] and [load], which are used by this
/// method. If they need to change the implementation of [ImageStream] used,
/// they should override [createStream]. If they need to manage the actual
/// resolution of the image, they should override [resolveStreamForKey].
///
/// See the Lifecycle documentation on [ImageProvider] for more information.
@nonVirtual
ImageStream resolve(ImageConfiguration configuration) {
  assert(configuration != null);
  final ImageStream stream = createStream(configuration);
  // Load the key (potentially asynchronously), set up an error handling zone,
  // and call resolveStreamForKey.
  _createErrorHandlerAndKey(
    configuration,
    (T key, ImageErrorListener errorHandler) {
      resolveStreamForKey(configuration, stream, key, errorHandler);
    },
    (T? key, Object exception, StackTrace? stack) async {
      await null; // wait an event turn in case a listener has been added to the image stream.
      InformationCollector? collector;
      assert(() {
        collector = () => <DiagnosticsNode>[          DiagnosticsProperty<ImageProvider>('Image provider', this),          DiagnosticsProperty<ImageConfiguration>('Image configuration', configuration),          DiagnosticsProperty<T>('Image key', key, defaultValue: null),        ];
        return true;
      }());
      if (stream.completer == null) {
        stream.setCompleter(_ErrorImageCompleter());
      }
      stream.completer!.reportError(
        exception: exception,
        stack: stack,
        context: ErrorDescription('while resolving an image'),
        silent: true, // could be a network error or whatnot
        informationCollector: collector,
      );
    },
  );
  return stream;
}

/// Converts an ImageProvider's settings plus an ImageConfiguration to a key
/// that describes the precise image to load.
///
/// The type of the key is determined by the subclass. It is a value that
/// unambiguously identifies the image (_including its scale_) that the [load]
/// method will fetch. Different [ImageProvider]s given the same constructor
/// arguments and [ImageConfiguration] objects should return keys that are
/// '==' to each other (possibly by using a class for the key that itself
/// implements [==]).
Future<T> obtainKey(ImageConfiguration configuration);

@protected
void resolveStreamForKey(ImageConfiguration configuration, ImageStream stream, T key, ImageErrorListener handleError) {
  // This is an unusual edge case where someone has told us that they found
  // the image we want before getting to this method. We should avoid calling
  // load again, but still update the image cache with LRU information.
  if (stream.completer != null) {
    final ImageStreamCompleter? completer = PaintingBinding.instance.imageCache.putIfAbsent(
      key,
      () => stream.completer!,
      onError: handleError,
    );
    assert(identical(completer, stream.completer));
    return;
  }
  final ImageStreamCompleter? completer = PaintingBinding.instance.imageCache.putIfAbsent(
    key,
    /// 載入
    () => loadBuffer(key, PaintingBinding.instance.instantiateImageCodecFromBuffer),
    onError: handleError,
  );
  if (completer != null) {
    /// 關鍵是解析並設定ImageStreamCompleter物件
    stream.setCompleter(completer);
  }
}

   PaintingBinding.instance.imageCache.putIfAbsent(
     key,
     () => loadBuffer(key, PaintingBinding.instance.instantiateImageCodecFromBuffer),
     onError: handleError,
   )

/// Converts a key into an [ImageStreamCompleter], and begins fetching the
/// image.
///
/// For backwards-compatibility the default implementation of this method calls
/// through to [ImageProvider.load]. However, implementors of this interface should
/// only override this method and not [ImageProvider.load], which is deprecated.
///
/// The [decode] callback provides the logic to obtain the codec for the
/// image.
///
/// See also:
///
///  * [ResizeImage], for modifying the key to account for cache dimensions.
@protected
ImageStreamCompleter loadBuffer(T key, DecoderBufferCallback decode) {
  return load(key, PaintingBinding.instance.instantiateImageCodec);
}

/// Converts a key into an [ImageStreamCompleter], and begins fetching the
/// image.
///
/// This method is deprecated. Implement [loadBuffer] for faster image
/// loading. Only one of [load] and [loadBuffer] must be implemented, and
/// [loadBuffer] is preferred.
///
/// The [decode] callback provides the logic to obtain the codec for the
/// image.
///
/// See also:
///
///  * [ResizeImage], for modifying the key to account for cache dimensions.
@protected
@Deprecated(
  'Implement loadBuffer for faster image loading. '
  'This feature was deprecated after v2.13.0-1.0.pre.',
)
ImageStreamCompleter load(T key, DecoderCallback decode) {
  throw UnsupportedError('Implement loadBuffer for faster image loading');
}

/// Evicts an entry from the image cache.
///
/// Returns a [Future] which indicates whether the value was successfully
/// removed.
///
/// The [ImageProvider] used does not need to be the same instance that was
/// passed to an [Image] widget, but it does need to create a key which is
/// equal to one.
///
/// The [cache] is optional and defaults to the global image cache.
///
/// The [configuration] is optional and defaults to
/// [ImageConfiguration.empty].
///
/// {@tool snippet}
///
/// The following sample code shows how an image loaded using the [Image]
/// widget can be evicted using a [NetworkImage] with a matching URL.
///
/// ```dart
/// class MyWidget extends StatelessWidget {
///   const MyWidget({
///     super.key,
///     this.url = ' ... ',
///   });
///
///   final String url;
///
///   @override
///   Widget build(BuildContext context) {
///     return Image.network(url);
///   }
///
///   void evictImage() {
///     final NetworkImage provider = NetworkImage(url);
///     provider.evict().then<void>((bool success) {
///       if (success) {
///         debugPrint('removed image!');
///       }
///     });
///   }
/// }
/// ```
/// {@end-tool}
Future<bool> evict({ ImageCache? cache, ImageConfiguration configuration = ImageConfiguration.empty }) async {
  cache ??= imageCache;
  final T key = await obtainKey(configuration);
  return cache.evict(key);
}