loader中的this是什么

loader 怎么执行的全靠 loader-runner，查看 GitHub ​​地址​​

import { runLoaders } from "loader-runner";

runLoaders({
  resource: "/abs/path/to/file.txt?query", // 资源路径
  // String: Absolute path to the resource (optionally including query string)

  loaders: ["/abs/path/to/loader.js?query"], // loader路径
  // String[]: Absolute paths to the loaders (optionally including query string)
  // {loader, options}[]: Absolute paths to the loaders with options object

  context: { minimize: true }, // 执行loader函数this上下文
  // Additional loader context which is used as base context

  processResource: (loaderContext, resourcePath, callback) => { ... }, // 读取文件的内容
  // Optional: A function to process the resource
  // Must have signature function(context, path, function(err, buffer))
  // By default readResource is used and the resource is added a fileDependency

  readResource: fs.readFile.bind(fs) // 怎么读取，也就是读取内容的方法
  // Optional: A function to read the resource
  // Only used when 'processResource' is not provided
  // Must have signature function(path, function(err, buffer))
  // By default fs.readFile is used
}, function(err, result) {
  // err: Error?

  // result.result: Buffer | String
  // The result
  // only available when no error occured

  // result.resourceBuffer: Buffer
  // The raw resource as Buffer (useful for SourceMaps)
  // only available when no error occured

  // result.cacheable: Bool
  // Is the result cacheable or do it require reexecution?

  // result.fileDependencies: String[]  // An array of paths (existing files) on which the result depends on

  // result.missingDependencies: String[]  // An array of paths (not existing files) on which the result depends on

  // result.contextDependencies: String[]  // An array of paths (directories) on which the result depends on
})

context 就是 loader 中的 this，默认是个空对象，其实内部还有一些其他属性。

exports.runLoaders = function runLoaders(options, callback) {
  // read options
  var resource = options.resource || "";
  var loaders = options.loaders || [];
  var loaderContext = options.context || {};
  var processResource = options.processResource || ((readResource, context, resource, callback) => {
    context.addDependency(resource);
    readResource(resource, callback);
  }).bind(null, options.readResource || readFile);

  //
  var splittedResource = resource && parsePathQueryFragment(resource);
  var resourcePath = splittedResource ? splittedResource.path : undefined;
  var resourceQuery = splittedResource ? splittedResource.query : undefined;
  var resourceFragment = splittedResource ? splittedResource.fragment : undefined;
  var contextDirectory = resourcePath ? dirname(resourcePath) : null;

  // execution state
  var requestCacheable = true;
  var fileDependencies = [];
  var contextDependencies = [];
  var missingDependencies = [];

  // prepare loader objects
  loaders = loaders.map(createLoaderObject);

  loaderContext.context = contextDirectory;
  loaderContext.loaderIndex = 0;
  loaderContext.loaders = loaders;
  loaderContext.resourcePath = resourcePath;
  loaderContext.resourceQuery = resourceQuery;
  loaderContext.resourceFragment = resourceFragment;
  loaderContext.async = null;
  loaderContext.callback = null;
  loaderContext.cacheable = function cacheable(flag) {
    if(flag === false) {
      requestCacheable = false;
    }
  };
  loaderContext.dependency = loaderContext.addDependency = function addDependency(file) {
    fileDependencies.push(file);
  };
  loaderContext.addContextDependency = function addContextDependency(context) {
    contextDependencies.push(context);
  };
  loaderContext.addMissingDependency = function addMissingDependency(context) {
    missingDependencies.push(context);
  };
  loaderContext.getDependencies = function getDependencies() {
    return fileDependencies.slice();
  };
  loaderContext.getContextDependencies = function getContextDependencies() {
    return contextDependencies.slice();
  };
  loaderContext.getMissingDependencies = function getMissingDependencies() {
    return missingDependencies.slice();
  };
  loaderContext.clearDependencies = function clearDependencies() {
    fileDependencies.length = 0;
    contextDependencies.length = 0;
    missingDependencies.length = 0;
    requestCacheable = true;
  };
  Object.defineProperty(loaderContext, "resource", {
    enumerable: true,
    get: function() {
      if(loaderContext.resourcePath === undefined)
        return undefined;
      return loaderContext.resourcePath.replace(/#/g, "\0#") + loaderContext.resourceQuery.replace(/#/g, "\0#") + loaderContext.resourceFragment;
    },
    set: function(value) {
      var splittedResource = value && parsePathQueryFragment(value);
      loaderContext.resourcePath = splittedResource ? splittedResource.path : undefined;
      loaderContext.resourceQuery = splittedResource ? splittedResource.query : undefined;
      loaderContext.resourceFragment = splittedResource ? splittedResource.fragment : undefined;
    }
  });
  Object.defineProperty(loaderContext, "request", {
    enumerable: true,
    get: function() {
      return loaderContext.loaders.map(function(o) {
        return o.request;
      }).concat(loaderContext.resource || "").join("!");
    }
  });
  Object.defineProperty(loaderContext, "remainingRequest", {
    enumerable: true,
    get: function() {
      if(loaderContext.loaderIndex >= loaderContext.loaders.length - 1 && !loaderContext.resource)
        return "";
      return loaderContext.loaders.slice(loaderContext.loaderIndex + 1).map(function(o) {
        return o.request;
      }).concat(loaderContext.resource || "").join("!");
    }
  });
  Object.defineProperty(loaderContext, "currentRequest", {
    enumerable: true,
    get: function() {
      return loaderContext.loaders.slice(loaderContext.loaderIndex).map(function(o) {
        return o.request;
      }).concat(loaderContext.resource || "").join("!");
    }
  });
  Object.defineProperty(loaderContext, "previousRequest", {
    enumerable: true,
    get: function() {
      return loaderContext.loaders.slice(0, loaderContext.loaderIndex).map(function(o) {
        return o.request;
      }).join("!");
    }
  });
  Object.defineProperty(loaderContext, "query", {
    enumerable: true,
    get: function() {
      var entry = loaderContext.loaders[loaderContext.loaderIndex];
      return entry.options && typeof entry.options === "object" ? entry.options : entry.query;
    }
  });
  Object.defineProperty(loaderContext, "data", {
    enumerable: true,
    get: function() {
      return loaderContext.loaders[loaderContext.loaderIndex].data;
    }
  });

  // finish loader context
  if(Object.preventExtensions) {
    Object.preventExtensions(loaderContext);
  }

  var processOptions = {
    resourceBuffer: null,
    processResource: processResource
  };
  iteratePitchingLoaders(processOptions, loaderContext, function(err, result) {
    if(err) {
      return callback(err, {
        cacheable: requestCacheable,
        fileDependencies: fileDependencies,
        contextDependencies: contextDependencies,
        missingDependencies: missingDependencies
      });
    }
    callback(null, {
      result: result,
      resourceBuffer: processOptions.resourceBuffer,
      cacheable: requestCacheable,
      fileDependencies: fileDependencies,
      contextDependencies: contextDependencies,
      missingDependencies: missingDependencies
    });
  });
};