Support WeChat mini app environment (tensorflow#1510)

To compensate the differences between browser and WeChat mini app:

- WeChat mini app runs on JS core (ios) which does not have document, window, and setImmediate function or objects.
- When creating a GPGUContext with a existing context, it needs to store the context for the GL version, otherwise it would be picked later.

This PR also fix the inconsistency issue with GPGPUContext constructor, it should always cache the rendering context.

Author: pyu10055

src/browser_util.ts

@@ -15,9 +15,14 @@
  * =============================================================================
  */
 
-const delayCallback: Function = typeof requestAnimationFrame !== 'undefined' ?
-    requestAnimationFrame :  // Browsers
-    setImmediate;            // Node.js
+const delayCallback: Function = (() => {
+  if (typeof requestAnimationFrame !== 'undefined') {
+    return requestAnimationFrame;
+  } else if (typeof setImmediate !== 'undefined') {
+    return setImmediate;
+  }
+  return (f: Function) => f();  // no delays
+})();
 
 /**
  * Returns a promise that resolve when a requestAnimationFrame has completed.

src/canvas_util.ts

@@ -27,6 +27,11 @@ const WEBGL_ATTRIBUTES: WebGLContextAttributes = {
   failIfMajorPerformanceCaveat: true
 };
 
+export function setWebGLContext(
+    webGLVersion: number, gl: WebGLRenderingContext) {
+  contexts[webGLVersion] = gl;
+}
+
 export function getWebGLContext(webGLVersion: number): WebGLRenderingContext {
   if (!(webGLVersion in contexts)) {
     contexts[webGLVersion] = getWebGLRenderingContext(webGLVersion);

src/environment.ts

@@ -457,19 +457,29 @@ export class Environment {
           new Engine(backend, false /* safeMode */, () => this.get('DEBUG'));
     }
   }
+
+  get global(): {ENV: Environment} {
+    return getGlobalNamespace();
+  }
 }
 
+let _global: {ENV: Environment};
 function getGlobalNamespace(): {ENV: Environment} {
-  // tslint:disable-next-line:no-any
-  let ns: any;
-  if (typeof (window) !== 'undefined') {
-    ns = window;
-  } else if (typeof (process) !== 'undefined') {
-    ns = process;
-  } else {
-    throw new Error('Could not find a global object');
+  if (_global == null) {
+    // tslint:disable-next-line:no-any
+    let ns: any;
+    if (typeof (window) !== 'undefined') {
+      ns = window;
+    } else if (typeof (global) !== 'undefined') {
+      ns = global;
+    } else if (typeof (process) !== 'undefined') {
+      ns = process;
+    } else {
+      throw new Error('Could not find a global object');
+    }
+    _global = ns;
   }
-  return ns;
+  return _global;
 }
 
 function getOrMakeEnvironment(): Environment {

src/index.ts

@@ -23,8 +23,7 @@ import './kernels/backend_cpu';
 
 import {nextFrame} from './browser_util';
 import * as environment from './environment';
-import {Environment, enableProdMode} from './environment';
-
+import {enableProdMode, Environment} from './environment';
 // Serialization.
 import * as io from './io/io';
 import * as math from './math';
@@ -36,7 +35,6 @@ import {version} from './version';
 import * as webgl from './webgl';
 
 export {InferenceModel, ModelPredictConfig} from './model_types';
-
 // Optimizers.
 export {AdadeltaOptimizer} from './optimizers/adadelta_optimizer';
 export {AdagradOptimizer} from './optimizers/adagrad_optimizer';
@@ -70,7 +68,16 @@ export {version as version_core};
 export {nextFrame};
 
 // Second level exports.
-export {environment, io, math, serialization, test_util, util, webgl, enableProdMode};
+export {
+  environment,
+  io,
+  math,
+  serialization,
+  test_util,
+  util,
+  webgl,
+  enableProdMode
+};
 
 // Backend specific.
 export {KernelBackend, BackendTimingInfo, DataMover, DataStorage} from './kernels/backend';

src/io/browser_http.ts

@@ -22,6 +22,7 @@
  */
 
 import {assert} from '../util';
+
 import {concatenateArrayBuffers, getModelArtifactsInfoForJSON} from './io_utils';
 import {IORouter, IORouterRegistry} from './router_registry';
 import {IOHandler, ModelArtifacts, SaveResult, WeightsManifestConfig, WeightsManifestEntry} from './types';
@@ -375,7 +376,6 @@ IORouterRegistry.registerLoadRouter(httpRequestRouter);
  * import tensorflowjs as tfjs
  * import werkzeug.formparser
  *
- *
  * class ModelReceiver(object):
  *
  *   def __init__(self):

src/kernels/webgl/gpgpu_context.ts

@@ -15,7 +15,7 @@
  * =============================================================================
  */
 
-import {getWebGLContext} from '../../canvas_util';
+import {getWebGLContext, setWebGLContext} from '../../canvas_util';
 import {ENV} from '../../environment';
 import * as util from '../../util';
 import * as gpgpu_util from './gpgpu_util';
@@ -48,10 +48,12 @@ export class GPGPUContext {
   private textureConfig: TextureConfig;
 
   constructor(gl?: WebGLRenderingContext) {
+    const glVersion = ENV.get('WEBGL_VERSION');
     if (gl != null) {
       this.gl = gl;
+      setWebGLContext(glVersion, gl);
     } else {
-      this.gl = getWebGLContext(ENV.get('WEBGL_VERSION'));
+      this.gl = getWebGLContext(glVersion);
     }
     // WebGL 2.0 enables texture floats without an extension.
     if (ENV.get('WEBGL_VERSION') === 1) {