前言
其實Flutter本身已具備載入圖片的能力，Image元件就滿足網路圖片、本地圖片、檔案圖片的載入。那為什麼我們還需要實現其他圖片載入方案呢？其實是因為Flutter圖片元件功能上存在一些缺陷：
圖片快取沒有持久化能力，無網環境下不支援顯示圖片。 檔案圖片與原生環境不共用，導致圖片資原始檔重複。
因此為了滿足日常開發需要和優化點，可以做點什麼讓圖片元件功能達到滿意的效果。接下來從Flutter原生元件再到外接紋理慢慢了解圖片元件功能演進的過程。
Flutter原生圖片元件
Flutter原生圖片支援多種載入形式：
Image.network（網路圖片） Image.file （本地圖片） Image.asset （檔案圖片） Image.memory （byte圖片）
圖片載入流程簡要（網路圖片為例）
第一步：網路圖片載入形式以NetworkImage，內部由network_image.NetworkImage構成。
Image.network(
String src, {
......省略不必要程式碼
}) : image = ResizeImage.resizeIfNeeded(cacheWidth, cacheHeight, NetworkImage(src, scale: scale, headers: headers)),
...... 省略不必要程式碼
super(key: key);
......
const factory NetworkImage(String url, { double scale, Map<String, String> headers }) = network_image.NetworkImage;
複製程式碼第二步：NetWorkImage實質上繼承於ImageProvider，其他載入形式也是如此。ImageProvider是處理圖片基本抽象類，繼承它的載入類主要實現load方法執行不同形式載入過程。
abstract class ImageProvider<T> {
const ImageProvider();
.......
@protected
ImageStreamCompleter load(T key, DecoderCallback decode);
.......
}
複製程式碼第三步：例如網路形式獲取圖片資料過程通過網路，通過Dart層網路請求HttpClient請求圖片獲取最終資料Uint8List。
class NetworkImage extends image_provider.ImageProvider<image_provider.NetworkImage> implements image_provider.NetworkImage {
.......
@override
ImageStreamCompleter load(image_provider.NetworkImage key, image_provider.DecoderCallback decode) {
final StreamController<ImageChunkEvent> chunkEvents = StreamController<ImageChunkEvent>();
return MultiFrameImageStreamCompleter(
codec: _loadAsync(key as NetworkImage, chunkEvents, decode),
chunkEvents: chunkEvents.stream,
scale: key.scale,
informationCollector: () {
return <DiagnosticsNode>[
DiagnosticsProperty<image_provider.ImageProvider>('Image provider', this),
DiagnosticsProperty<image_provider.NetworkImage>('Image key', key),
];
},
);
}
Future<ui.Codec> _loadAsync(
NetworkImage key,
StreamController<ImageChunkEvent> chunkEvents,
image_provider.DecoderCallback decode,
) async {
try {
final Uri resolved = Uri.base.resolve(key.url);
final HttpClientRequest request = await _httpClient.getUrl(resolved);
headers?.forEach((String name, String value) {
request.headers.add(name, value);
});
final HttpClientResponse response = await request.close();
if (response.statusCode != HttpStatus.ok) {
PaintingBinding.instance.imageCache.evict(key);
throw image_provider.NetworkImageLoadException(statusCode: response.statusCode, uri: resolved);
}
final Uint8List bytes = await consolidateHttpClientResponseBytes(
response,
onBytesReceived: (int cumulative, int total) {
chunkEvents.add(ImageChunkEvent(
cumulativeBytesLoaded: cumulative,
expectedTotalBytes: total,
));
},
);
if (bytes.lengthInBytes == 0)
throw Exception('NetworkImage is an empty file: $resolved');
return decode(bytes);
} finally {
chunkEvents.close();
}
}
}
複製程式碼第四步：獲取到圖片Uint8List資料之後就是解碼過程。通過DecoderCallback回撥方法得到圖片原始資料之後交付給全域性單例解碼器PaintingBinding.instance.instantiateImageCodec，然後由引擎層C++的instantiateImageCodec處理資料返回可被Flutter層渲染展示Image資料。
final ImageStreamCompleter completer = PaintingBinding.instance.imageCache.putIfAbsent(
key,
() => load(key, PaintingBinding.instance.instantiateImageCodec),
onError: handleError,
);
Future<ui.Codec> instantiateImageCodec(Uint8List bytes, {
int cacheWidth,
int cacheHeight,
}) {
assert(cacheWidth == null || cacheWidth > 0);
assert(cacheHeight == null || cacheHeight > 0);
return ui.instantiateImageCodec(
bytes,
targetWidth: cacheWidth,
targetHeight: cacheHeight,
);
}
String _instantiateImageCodec(Uint8List list, _Callback<Codec> callback, _ImageInfo imageInfo, int targetWidth, int targetHeight)
native 'instantiateImageCodec';
複製程式碼第五步：引擎層c++解碼器具體在codec.cc中，呼叫了Skia的SkCodec對圖片資料做處理。經過解碼器內部處理後執行ToDart將ui_codec返回到Dart層。
/// Dart層程式碼
_String _instantiateImageCodec(Uint8List list, _Callback<Codec> callback, _ImageInfo imageInfo, int targetWidth, int targetHeight)
native 'instantiateImageCodec';
/// c++層程式碼
static void InstantiateImageCodec(Dart_NativeArguments args) {
UIDartState::ThrowIfUIOperationsProhibited();
Dart_Handle callback_handle = Dart_GetNativeArgument(args, 1);
.......省略部分程式碼
Dart_Handle image_info_handle = Dart_GetNativeArgument(args, 2);
std::optional<ImageDecoder::ImageInfo> image_info;
/// 圖片資訊是否為空，不為空做一些處理
if (!Dart_IsNull(image_info_handle)) {
auto image_info_results = ConvertImageInfo(image_info_handle, args);
if (auto value =
std::get_if<ImageDecoder::ImageInfo>(&image_info_results)) {
image_info = *value;
} else if (auto error = std::get_if<std::string>(&image_info_results)) {
Dart_SetReturnValue(args, tonic::ToDart(*error));
return;
}
}
sk_sp<SkData> buffer;
{
/// 處理圖片資料
Dart_Handle exception = nullptr;
tonic::Uint8List list =
tonic::DartConverter<tonic::Uint8List>::FromArguments(args, 0,
exception);
if (exception) {
Dart_SetReturnValue(args, exception);
return;
}
/// 圖片資料做拷貝
buffer = MakeSkDataWithCopy(list.data(), list.num_elements());
}
if (image_info) {
const auto expected_size =
image_info->row_bytes * image_info->sk_info.height();
if (buffer->size() < expected_size) {
Dart_SetReturnValue(
args, ToDart("Pixel buffer size does not match image size"));
return;
}
}
/// 獲取圖片目標寬高
const int targetWidth =
tonic::DartConverter<int>::FromDart(Dart_GetNativeArgument(args, 3));
const int targetHeight =
tonic::DartConverter<int>::FromDart(Dart_GetNativeArgument(args, 4));
std::unique_ptr<SkCodec> codec;
bool single_frame;
if (image_info) {
single_frame = true;
} else {
/// 底層解碼器使用的是SkCodec解碼器，Android底層同樣使用的是它。
codec = SkCodec::MakeFromData(buffer);
if (!codec) {
Dart_SetReturnValue(args, ToDart("Could not instantiate image codec."));
return;
}
single_frame = codec->getFrameCount() == 1;
}
/// 解碼器同時解碼後得出幀數資訊，判斷圖片是否為動圖
fml::RefPtr<Codec> ui_codec;
if (single_frame) {
ImageDecoder::ImageDescriptor descriptor;
descriptor.decompressed_image_info = image_info;
if (targetWidth > 0) {
descriptor.target_width = targetWidth;
}
if (targetHeight > 0) {
descriptor.target_height = targetHeight;
}
descriptor.data = std::move(buffer);
ui_codec = fml::MakeRefCounted<SingleFrameCodec>(std::move(descriptor));
} else {
ui_codec = fml::MakeRefCounted<MultiFrameCodec>(std::move(codec));
}
/// 最後將解碼器結果返回到Dart層
tonic::DartInvoke(callback_handle, {ToDart(ui_codec)});
}
複製程式碼外接紋理渲染圖片
Flutter中有一個叫做Texture元件,該元件只有唯一入參textureId，寥寥無幾的幾行程式碼就實現外接紋理著實讓人摸不清頭腦。在分析外接紋理原理之前先簡單瞭解外接紋理渲染圖片功能實現。
Texture元件使用
Java層通過Channel外掛PluginRegistry.Registrar建立Surface、textureId。
/// 外掛介面獲取texture註冊器
TextureRegistry textureRegistry = registrar.textures();
/// 建立Texture例項
TextureRegistry.SurfaceTextureEntry surfaceTextureEntry = textureRegistry.createSurfaceTexture();
long textureId = surfaceTextureEntry.id();
SurfaceTexture surfaceTexture = surfaceTextureEntry.surfaceTexture();
/// 獲取圖片地址
String url = call.argument("url");
...... 省略圖片請求載入過程
/// 建立Surface例項載入surfaceTexture
Surface surface = new Surface(surfaceTexture);
/// 畫布繪製bitmap 紋理對映
Canvas canvas = surface.lockCanvas(rect);
canvas.drawBitmap(bitmap, null, rect, null);
bitmap.recycle();
surface.unlockCanvasAndPost(canvas);
/// Dart返回textureId
Map<String, Object> maps = new HashMap<>();
maps.put("textureId", textureId);
result.success(maps);
複製程式碼Dart層建立MethodChannel，向Native層傳遞載入圖片路徑。
static const MethodChannel _channel = const MethodChannel('texture_channel');
/// 原始載入圖片介面
static Future<Map> loadTexture({String url}) async {
var args = <String, dynamic>{
"url": url,
};
return await _channel.invokeMethod("loadTexture", args);
}
/// 執行載入
Map _textureResult = await TexturePlugin.loadTexture(
url: _uri.toString(),
width: url.width,
height: url.height,
);
/// 返回Native生成的textureId
int id = _textureResult['textureId'];
/// 例項化texture元件 顯示圖片
Texture( textureId: id);
複製程式碼程式碼解析
Dart層
從原始碼上可以看到Texture會建立渲染物件TextureBox。TextureBox會去繪製TextureLayer，TextureLayer則通過ui.SceneBuilder向Scene新增紋理，最終是呼叫引擎層SceneBuilder_addTexture方法實現紋理渲染。
Texture
class Texture extends LeafRenderObjectWidget {
const Texture({
Key key,
@required this.textureId,
}) : assert(textureId != null),
super(key: key);
final int textureId;
@override
TextureBox createRenderObject(BuildContext context) => TextureBox(textureId: textureId);
@override
void updateRenderObject(BuildContext context, TextureBox renderObject) {
renderObject.textureId = textureId;
}
}
複製程式碼TextureBox
class TextureBox extends RenderBox {
TextureBox({ @required int textureId })
: assert(textureId != null),
_textureId = textureId;
...... 省略程式碼
@override
void paint(PaintingContext context, Offset offset) {
if (_textureId == null)
return;
context.addLayer(TextureLayer(
rect: Rect.fromLTWH(offset.dx, offset.dy, size.width, size.height),
textureId: _textureId,
));
}
}
複製程式碼TextureLayer
class TextureLayer extends Layer {
TextureLayer({
@required this.rect,
@required this.textureId,
this.freeze = false,
}) : assert(rect != null),
assert(textureId != null);
......
@override
void addToScene(ui.SceneBuilder builder, [ Offset layerOffset = Offset.zero ]) {
final Rect shiftedRect = layerOffset == Offset.zero ? rect : rect.shift(layerOffset);
builder.addTexture(
textureId,
offset: shiftedRect.topLeft,
width: shiftedRect.width,
height: shiftedRect.height,
freeze: freeze,
);
}
}
複製程式碼SceneBuilder
class SceneBuilder extends NativeFieldWrapperClass2 {
void addTexture(
int textureId, {
Offset offset = Offset.zero,
double width = 0.0,
double height = 0.0,
bool freeze = false,
}) {
_addTexture(offset.dx, offset.dy, width, height, textureId, freeze);
}
/// SceneBuilder_addTexture對應scene_builder.cc下的SceneBuilder::addTexture方法
void _addTexture(double dx, double dy, double width, double height, int textureId, bool freeze)
native 'SceneBuilder_addTexture';
複製程式碼scene_builder.cc
void SceneBuilder::addTexture(double dx,
double dy,
double width,
double height,
int64_t textureId,
bool freeze) {
auto layer = std::make_unique<flutter::TextureLayer>(
SkPoint::Make(dx, dy), SkSize::Make(width, height), textureId, freeze);
AddLayer(std::move(layer));
}
複製程式碼texture_layer.cc
/// 建立紋理層物件
TextureLayer::TextureLayer(const SkPoint& offset,
const SkSize& size,
int64_t texture_id,
bool freeze)
: offset_(offset), size_(size), texture_id_(texture_id), freeze_(freeze) {}
/// 紋理物件繪製方法
void TextureLayer::Paint(PaintContext& context) const {
TRACE_EVENT0("flutter", "TextureLayer::Paint");
/// texture物件繪製時會從GetTexture方法的map中找到紋理物件進行繪製。
std::shared_ptr<Texture> texture =
context.texture_registry.GetTexture(texture_id_);
if (!texture) {
TRACE_EVENT_INSTANT0("flutter", "null texture");
return;
}
texture->Paint(*context.leaf_nodes_canvas, paint_bounds(), freeze_,
context.gr_context);
}
複製程式碼Java層
FlutterRenderer
public class FlutterRenderer implements TextureRegistry {
public FlutterRenderer(@NonNull FlutterJNI flutterJNI) {
this.flutterJNI = flutterJNI;
this.flutterJNI.addIsDisplayingFlutterUiListener(flutterUiDisplayListener);
}
@Override
public SurfaceTextureEntry createSurfaceTexture() {
/// 建立SurfaceTexture
final SurfaceTexture surfaceTexture = new SurfaceTexture(0);
surfaceTexture.detachFromGLContext();
final SurfaceTextureRegistryEntry entry =
new SurfaceTextureRegistryEntry(nextTextureId.getAndIncrement(), surfaceTexture);
registerTexture(entry.id(), surfaceTexture);
return entry;
}
///向Native JNI層註冊
private void registerTexture(long textureId, @NonNull SurfaceTexture surfaceTexture) {
flutterJNI.registerTexture(textureId, surfaceTexture);
}
}
複製程式碼FlutterJNI
public void registerTexture(long textureId, @NonNull SurfaceTexture surfaceTexture) {
/// 這裡需要注意必須在主執行緒執行否則會報錯
ensureRunningOnMainThread();
ensureAttachedToNative();
nativeRegisterTexture(nativePlatformViewId, textureId, surfaceTexture);
}
複製程式碼C++層
platform_view_android_jni.cc
{
.name = "nativeRegisterTexture",
.signature = "(JJLandroid/graphics/SurfaceTexture;)V",
.fnPtr = reinterpret_cast<void*>(&RegisterTexture),
}
static void RegisterTexture(JNIEnv* env,
jobject jcaller,
jlong shell_holder,
jlong texture_id,
jobject surface_texture) {
ANDROID_SHELL_HOLDER->GetPlatformView()->RegisterExternalTexture(
static_cast<int64_t>(texture_id), //
fml::jni::JavaObjectWeakGlobalRef(env, surface_texture) //
);
}
複製程式碼platform_view_android.cc
void PlatformViewAndroid::RegisterExternalTexture(
int64_t texture_id,
const fml::jni::JavaObjectWeakGlobalRef& surface_texture) {
RegisterTexture(
std::make_shared<AndroidExternalTextureGL>(texture_id, surface_texture));
}
複製程式碼texture.cc
// 註冊紋理方法
void TextureRegistry::RegisterTexture(std::shared_ptr<Texture> texture) {
if (!texture) {
return;
}
// 內部map儲存texture例項
mapping_[texture->Id()] = texture;
}
// 獲取到紋理物件，從map中提取
std::shared_ptr<Texture> TextureRegistry::GetTexture(int64_t id) {
auto it = mapping_.find(id);
return it != mapping_.end() ? it->second : nullptr;
}
複製程式碼android_external_texture_gl.cc
/// 外接紋理物件例項
AndroidExternalTextureGL::AndroidExternalTextureGL(
int64_t id,
const fml::jni::JavaObjectWeakGlobalRef& surfaceTexture)
: Texture(id), surface_texture_(surfaceTexture), transform(SkMatrix::I()) {}
AndroidExternalTextureGL::~AndroidExternalTextureGL() {
if (state_ == AttachmentState::attached) {
glDeleteTextures(1, &texture_name_);
}
}
/// 繪製方法
void AndroidExternalTextureGL::Paint(SkCanvas& canvas,
const SkRect& bounds,
bool freeze,
GrContext* context) {
if (state_ == AttachmentState::detached) {
return;
}
if (state_ == AttachmentState::uninitialized) {
glGenTextures(1, &texture_name_);
Attach(static_cast<jint>(texture_name_));
state_ = AttachmentState::attached;
}
if (!freeze && new_frame_ready_) {
Update();
new_frame_ready_ = false;
}
GrGLTextureInfo textureInfo = {GL_TEXTURE_EXTERNAL_OES, texture_name_,
GL_RGBA8_OES};
GrBackendTexture backendTexture(1, 1, GrMipMapped::kNo, textureInfo);
sk_sp<SkImage> image = SkImage::MakeFromTexture(
canvas.getGrContext(), backendTexture, kTopLeft_GrSurfaceOrigin,
kRGBA_8888_SkColorType, kPremul_SkAlphaType, nullptr);
if (image) {
SkAutoCanvasRestore autoRestore(&canvas, true);
canvas.translate(bounds.x(), bounds.y());
canvas.scale(bounds.width(), bounds.height());
if (!transform.isIdentity()) {
SkMatrix transformAroundCenter(transform);
transformAroundCenter.preTranslate(-0.5, -0.5);
transformAroundCenter.postScale(1, -1);
transformAroundCenter.postTranslate(0.5, 0.5);
canvas.concat(transformAroundCenter);
}
canvas.drawImage(image, 0, 0);
}
}
複製程式碼①Java層FlutterRenderer建立SurfaceTexture和textureId。 ②將urfaceTexture和textureId通過JNI向引擎層註冊 ③向引擎註冊過程中通過層層方法最後在texture.cc的TextureRegistry由map以鍵值對形式快取例項物件。 ④將需要顯示圖片在SurfaceTexture上離屏渲染。 ⑤Java層建立的textureId通過Channel傳遞到Dart層作為Texture元件入參。 ⑥Dart的Texture元件接收textureId入參後向下層元件例項化。 ⑦在SceneBuilder呼叫addTexture時執行引擎層建立TextureLayer。 ⑧最終在texture.cc中TextureRegistry的map根據TextureId獲取SurfaceTexture例項。
Image VS Texture
Image元件和Texture元件實質上都是對RenderBox的實現，已知Flutter渲染樹實際上就是RenderObject合併為Layer最終通過Flutter引擎進行繪製上屏顯示頁面內容。兩者區別在於實現RenderObject的paint有所不同，Image的實現將ui.Image內容繪製在ui.Canvas上，Texture是將TextureLayer新增到ui.Scene中。不同之處就在於Image做了繪製操作，Texture是做新增操作，紋理方案的圖片載入和繪製完全由原生平臺完成。