import bg from './bg.png';
// 1. 加載本地圖片資源
<Image source={bg}/>
<Image source={require('./bg.png)}/>
// 2. 加載網(wǎng)絡(luò)圖片資源
<Image source={{ uri: 'https://reactnative.dev/img/tiny_logo.png' }}/>

// src/index.js
import { Image } from 'react-native';
import bg from './bg.png';
const jsx = <Image source={bg}/>

.
└── src/
    ├── index.js
    ├── bg.png
    ├── bg@1.5x.png
    ├── bg@2x.png
    └── bg@3x.png

// ios
react-native bundle --entry-file src/index.ts --platform ios --bundle-output dist/ios/ios.bundle.js --assets-dest dist/ios 
// android
react-native bundle --entry-file src/index.ts --platform android --bundle-output dist/android/android.bundle.js --assets-dest dist/android

 modules = Object.create(null);
 function define(factory, moduleId, dependencyMap) {
    if (modules[moduleId] != null) {
      return;
    }
    var mod = {
      dependencyMap: dependencyMap,
      factory: factory,
      hasError: false,
      importedAll: EMPTY,
      importedDefault: EMPTY,
      isInitialized: false,
      publicModule: {
        exports: {},
      },
    };
    modules[moduleId] = mod;
  }

function metroRequire(moduleId) {
    var moduleIdReallyIsNumber = moduleId;
    var module = modules[moduleIdReallyIsNumber];
    return module && module.isInitialized
      // 如果已經(jīng)初始化過，直接返回緩存
      ? module.publicModule.exports // 這里其實(shí)就是 module.exports
      // 如果沒有初始化過，則內(nèi)部調(diào)用module的factory方法初始化
      : guardedLoadModule(moduleIdReallyIsNumber, module);
  }

__d(
  // factory
  function (
    global,
    _$$_REQUIRE, //__r
    _$$_IMPORT_DEFAULT,
    _$$_IMPORT_ALL,
    module,
    exports,
    _dependencyMap
  ) {
    module.exports = _$$_REQUIRE(
      _dependencyMap[0],
      'react-native/Libraries/Image/AssetRegistry'
    ).registerAsset({
      __packager_asset: true,
      httpServerLocation: '/assets/src',
      width: 295,
      height: 153,
      scales: [1, 1.5, 2, 3],
      hash: '615a107224f6f73b539078be1c162c6c',
      name: 'bg',
      type: 'png',
    });
  },
  479,
  [223], // 223 就是react-native/Libraries/Image/AssetRegistry 模塊
  'src/bg.png'
);

// react-native/Libraries/Image/AssetRegistry
__d(
  function (
    global,
    _$$_REQUIRE,
    _$$_IMPORT_DEFAULT,
    _$$_IMPORT_ALL,
    module,
    exports,
    _dependencyMap
  ) {
    'use strict';
    var assets = [];
    function registerAsset(asset) {
      return assets.push(asset);
    }
    function getAssetByID(assetId) {
      return assets[assetId - 1];
    }
    module.exports = {
      registerAsset: registerAsset,
      getAssetByID: getAssetByID,
    };
  },
  223,
  [],
  'node_modules/react-native/Libraries/Image/AssetRegistry.js'
);

// react-native/Libraries/Image/Image.ios.js  代碼有刪減
const BaseImage = (props) => {
  const source = getImageSourcesFromImageProps(props) || {
    uri: undefined,
    width: undefined,
    height: undefined,
  };
  let sources;
  let style: ImageStyleProp;
  if (Array.isArray(source)) {
    style = flattenStyle([styles.base, props.style]) || {};
    sources = source;
  } else {
    const {width = props.width, height = props.height, uri} = source;
    style = flattenStyle([{width, height}, styles.base, props.style]) || {};
    sources = [source];
    if (uri === '') {
      console.warn('source.uri should not be an empty string');
    }
  }
  const objectFit =
    style && style.objectFit
      ? convertObjectFitToResizeMode(style.objectFit)
      : null;
  const resizeMode =
    objectFit || props.resizeMode || (style && style.resizeMode) || 'cover';
  const {
    height,
    width,
    ...restProps
  } = props;
  return (
     return (
       <ImageViewNativeComponent
          {...restProps}
          style={style}
          resizeMode={resizeMode}
          source={sources}
        />
     );
  );
};

// 代碼有刪減
function getImageSourcesFromImageProps(imageProps) {
  return resolveAssetSource(imageProps.source);
}

/**
 * `source` is either a number (opaque type returned by require('./foo.png'))
 * or an `ImageSource` like { uri: '<http location || file path>' }
 */
function resolveAssetSource(source: any): ?ResolvedAssetSource {
  if (typeof source === 'object') {
    return source;
  }
  const asset = AssetRegistry.getAssetByID(source);
  if (!asset) {
    return null;
  }
  const resolver = new AssetSourceResolver(
    getDevServerURL(),
    getScriptURL(),
    asset,
  );
  // 如果存在自定義處理函數(shù)_customSourceTransformer，就返回它的執(zhí)行結(jié)果。
  // 可以通過setCustomSourceTransformer來設(shè)置。
  if (_customSourceTransformer) {
    return _customSourceTransformer(resolver);
  }
  return resolver.defaultAsset();
}

// 代碼有刪減
class AssetSourceResolver {
  constructor(serverUrl: ?string, jsbundleUrl: ?string, asset: PackagerAsset) {
    this.serverUrl = serverUrl;
    this.jsbundleUrl = jsbundleUrl;
    this.asset = asset;
  }
  isLoadedFromServer(): boolean {
    return !!this.serverUrl;
  }
  isLoadedFromFileSystem(): boolean {
    return !!(this.jsbundleUrl && this.jsbundleUrl.startsWith('file://'));
  }
  defaultAsset(): ResolvedAssetSource {
    // 如果是本地開發(fā)
    if (this.isLoadedFromServer()) {
      return this.assetServerURL();
    }
    // 非本地開發(fā)，Native內(nèi)嵌
    if (Platform.OS === 'android') {
      return this.isLoadedFromFileSystem()
        ? this.drawableFolderInBundle()
        : this.resourceIdentifierWithoutScale();
    } else {
      return this.scaledAssetURLNearBundle();
    }
  }
  assetServerURL(): ResolvedAssetSource {
    return this.fromSource(
      this.serverUrl +
        getScaledAssetPath(this.asset) +
        '?platform=' +
        Platform.OS +
        '&hash=' +
        this.asset.hash,
    );
  }
  /**
   * If the jsbundle is running from a sideload location, this resolves assets
   * relative to its location
   * E.g. 'file:///sdcard/xxx/drawable-xxhdpi/src_bg.png'
   */
  drawableFolderInBundle(): ResolvedAssetSource {
    const path = this.jsbundleUrl || 'file://';
    return this.fromSource(path + getAssetPathInDrawableFolder(this.asset));
  }
   /**
   * The default location of assets bundled with the app, located by
   * resource identifier
   * The Android resource system picks the correct scale.
   * E.g. 'src_bg'
   */
  resourceIdentifierWithoutScale(): ResolvedAssetSource {
    return this.fromSource(getAndroidResourceIdentifier(this.asset));
  }
   /**
   * Resolves to where the bundle is running from, with a scaled asset filename
   * E.g. 'file:///sdcard/bundle/assets/src/bg@3x.png'
   */
  scaledAssetURLNearBundle(): ResolvedAssetSource {
    const path = this.jsbundleUrl || 'file://';
    return this.fromSource(
      // Assets can have relative paths outside of the project root.
      // When bundling them we replace `../` with `_` to make sure they
      // don't end up outside of the expected assets directory.
      path + getScaledAssetPath(this.asset).replace(/\.\.\//g, '_')
    );
  }
  fromSource(source: string): ResolvedAssetSource {
    return {
      __packager_asset: true,
      width: this.asset.width,
      height: this.asset.height,
      uri: source,
      scale: pickScale(this.asset.scales, PixelRatio.get()),
    };
  }
}

/**
 * 返回圖片在服務(wù)器中的路徑，比如 'assets/src/bg@3x.png'
 */
function getScaledAssetPath(asset: PackagerAsset): string {
  const scale = pickScale(asset.scales, PixelRatio.get());
  const scaleSuffix = scale === 1 ? '' : '@' + scale + 'x';
  // 這里的assetDir其實(shí)就是 之前通過__d定義的圖片信息中的httpServerLocation，即assets_src
  const assetDir = getBasePath(asset); 
  return assetDir + '/' + asset.name + scaleSuffix + '.' + asset.type;
}

// 判斷選擇哪種尺寸的圖片
// RN根據(jù)當(dāng)前手機(jī)的ratio加載對(duì)應(yīng)的scale圖片。如果當(dāng)前手機(jī)的ratio沒有匹配到正確的scale圖片，則會(huì)獲取第一個(gè)大于當(dāng)前手機(jī)ratio的scale圖片。
// 例如當(dāng)前手機(jī)的scale為2，如果存在2x圖片，則返回2x圖片。如果沒有2x圖，則會(huì)向上獲取3x圖
export function pickScale(scales: Array<number>, deviceScale?: number): number {
  if (deviceScale == null) {
    deviceScale = PixelRatio.get();
  }
  // Packager guarantees that `scales` array is sorted
  for (let i = 0; i < scales.length; i++) {
    if (scales[i] >= deviceScale) {
      return scales[i];
    }
  }
  // If nothing matches, device scale is larger than any available
  // scales, so we return the biggest one. Unless the array is empty,
  // in which case we default to 1
  return scales[scales.length - 1] || 1;
}

this.serverUrl +
        getScaledAssetPath(this.asset) +
        '?platform=' +
        Platform.OS +
        '&hash=' +
        this.asset.hash,

//image.ios.js
const source = getImageSourcesFromImageProps(props) || {
    uri: undefined,
    width: undefined,
    height: undefined,
  };
const {width, height, uri} = source;
style = flattenStyle([{width, height}, styles.base, props.style]) || {};

registerAsset({
      __packager_asset: true,
      httpServerLocation: '/assets/src',
      width: 295,
      height: 153,
      scales: [1, 1.5, 2, 3],
      hash: '615a107224f6f73b539078be1c162c6c',
      name: 'bg',
      type: 'png',
    });