• Ian Hickson's avatar
    Refactor everything to do with images (#4583) · 2dfdc840
    Ian Hickson authored
    Overview
    ========
    
    This patch refactors images to achieve the following goals:
    
    * it allows references to unresolved assets to be passed
      around (previously, almost every layer of the system had to know about
      whether an image came from an asset bundle or the network or
      elsewhere, and had to manually interact with the image cache).
    
    * it allows decorations to use the same API for declaring images as the
      widget tree.
    
    It requires some minor changes to call sites that use images, as
    discussed below.
    
    Widgets
    -------
    
    Change this:
    
    ```dart
          child: new AssetImage(
            name: 'my_asset.png',
            ...
          )
    ```
    
    ...to this:
    
    ```dart
          child: new Image(
            image: new AssetImage('my_asset.png'),
            ...
          )
    ```
    
    Decorations
    -----------
    
    Change this:
    
    ```dart
          child: new DecoratedBox(
            decoration: new BoxDecoration(
              backgroundImage: new BackgroundImage(
                image: DefaultAssetBundle.of(context).loadImage('my_asset.png'),
                ...
              ),
              ...
            ),
            child: ...
          )
    ```
    
    ...to this:
    
    ```dart
          child: new DecoratedBox(
            decoration: new BoxDecoration(
              backgroundImage: new BackgroundImage(
                image: new AssetImage('my_asset.png'),
                ...
              ),
              ...
            ),
            child: ...
          )
    ```
    
    DETAILED CHANGE LOG
    ===================
    
    The following APIs have been replaced in this patch:
    
    * The `AssetImage` and `NetworkImage` widgets have been split in two,
      with identically-named `ImageProvider` subclasses providing the
      image-loading logic, and a single `Image` widget providing all the
      widget tree logic.
    
    * `ImageResource` is now `ImageStream`. Rather than configuring it with
      a `Future<ImageInfo>`, you complete it with an `ImageStreamCompleter`.
    
    * `ImageCache.load` and `ImageCache.loadProvider` are replaced by
      `ImageCache.putIfAbsent`.
    
    The following APIs have changed in this patch:
    
    * `ImageCache` works in terms of arbitrary keys and caches
      `ImageStreamCompleter` objects using those keys. With the new model,
      you should never need to interact with the cache directly.
    
    * `Decoration` can now be `const`. The state has moved to the
      `BoxPainter` class. Instead of a list of listeners, there's now just a
      single callback and a `dispose()` method on the painter. The callback
      is passed in to the `createBoxPainter()` method. When invoked, you
      should repaint the painter.
    
    The following new APIs are introduced:
    
    * `AssetBundle.loadStructuredData`.
    
    * `SynchronousFuture`, a variant of `Future` that calls the `then`
      callback synchronously. This enables the asynchronous and
      synchronous (in-the-cache) code paths to look identical yet for the
      latter to avoid returning to the event loop mid-paint.
    
    * `ExactAssetImage`, a variant of `AssetImage` that doesn't do anything clever.
    
    * `ImageConfiguration`, a class that describes parameters that configure
      the `AssetImage` resolver.
    
    The following APIs are entirely removed by this patch:
    
    * `AssetBundle.loadImage` is gone. Use an `AssetImage` instead.
    
    * `AssetVendor` is gone. `AssetImage` handles everything `AssetVendor`
      used to handle.
    
    * `RawImageResource` and `AsyncImage` are gone.
    
    The following code-level changes are performed:
    
    * `Image`, which replaces `AsyncImage`, `NetworkImage`, `AssetImage`,
      and `RawResourceImage`, lives in `image.dart`.
    
    * `DecoratedBox` and `Container` live in their own file now,
      `container.dart` (they reference `image.dart`).
    
    DIRECTIONS FOR FUTURE RESEARCH
    ==============================
    
    * The `ImageConfiguration` fields are mostly aspirational. Right now
      only `devicePixelRatio` and `bundle` are implemented. `locale` isn't
      even plumbed through, it will require work on the localisation logic.
    
    * We should go through and make `BoxDecoration`, `AssetImage`, and
      `NetworkImage` objects `const` where possible.
    
    * This patch makes supporting animated GIFs much easier.
    
    * This patch makes it possible to create an abstract concept of an
      "Icon" that could be either an image or a font-based glyph (using
      `IconData` or similar). (see
      https://github.com/flutter/flutter/issues/4494)
    
    RELATED ISSUES
    ==============
    
    Fixes https://github.com/flutter/flutter/issues/4500
    Fixes https://github.com/flutter/flutter/issues/4495
    Obsoletes https://github.com/flutter/flutter/issues/4496
    2dfdc840
implicit_animations.dart 19.6 KB