Introduce DEBUG module which traces public API calls

This patch improves on DEBUG module to trace all puppeteer's
public API calls.

References #89.

CONTRIBUTING.md

@@ -65,3 +65,8 @@ browser.newPage().then(async page => {
   browser.close();
 });
 ```
+
+Tips-n-tricks:
+- `DEBUG=*,-*:protocol node script.js` - dump everything BUT protocol messages
+- `DEBUG=*:page node script.js` - dump only Page's API calls
+- `DEBUG=*:mouse,*:keyboard node script.js` - dump only Mouse and Keyboard API calls

lib/Browser.js

@@ -16,6 +16,7 @@
 
 let {Duplex} = require('stream');
 let path = require('path');
+let helper = require('./helper');
 let removeRecursive = require('rimraf').sync;
 let Page = require('./Page');
 let childProcess = require('child_process');
@@ -139,6 +140,7 @@ class Browser {
 }
 
 module.exports = Browser;
+helper.tracePublicAPI(Browser);
 
 function waitForRemoteDebuggingPort(chromeProcess) {
   const rl = readline.createInterface({ input: chromeProcess.stderr });

lib/Dialog.js

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+const helper = require('./helper');
+
 class Dialog {
   /**
    * @param {!Connection} client
@@ -67,3 +69,4 @@ Dialog.Type = {
 };
 
 module.exports = Dialog;
+helper.tracePublicAPI(Dialog);

lib/FrameManager.js

@@ -358,5 +358,6 @@ class Frame {
     this._parentFrame = null;
   }
 }
+helper.tracePublicAPI(Frame);
 
 module.exports = FrameManager;

lib/Keyboard.js

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+const helper = require('./helper');
+
 class Keyboard {
   /**
    * @param {!Connection} client
@@ -236,3 +238,4 @@ function codeForKey(key) {
 }
 
 module.exports = Keyboard;
+helper.tracePublicAPI(Keyboard);

lib/NetworkManager.js

@@ -13,7 +13,8 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-let EventEmitter = require('events');
+const EventEmitter = require('events');
+const helper = require('./helper');
 
 class NetworkManager extends EventEmitter {
   /**
@@ -280,6 +281,7 @@ class Body {
     return content.buffer;
   }
 }
+helper.tracePublicAPI(Body);
 
 class Request extends Body {
   /**
@@ -300,6 +302,7 @@ class Request extends Body {
     return this._response;
   }
 }
+helper.tracePublicAPI(Request);
 
 class Response extends Body {
   /**
@@ -324,6 +327,7 @@ class Response extends Body {
     return this._request;
   }
 }
+helper.tracePublicAPI(Response);
 
 class InterceptedRequest {
   /**

lib/Page.js

@@ -721,3 +721,4 @@ Page.Events = {
 Page.Viewport;
 
 module.exports = Page;
+helper.tracePublicAPI(Page);

lib/helper.js

@@ -91,6 +91,41 @@ class Helper {
       });
     }
   }
+
+  /**
+   * @param {!Object} classType
+   */
+  static tracePublicAPI(classType) {
+    let className = classType.prototype.constructor.name;
+    className = className.substring(0, 1).toLowerCase() + className.substring(1);
+    const debug = require('debug')(`puppeteer:${className}`);
+    if (!debug.enabled)
+      return;
+    for (let methodName of Reflect.ownKeys(classType.prototype)) {
+      const method = Reflect.get(classType.prototype, methodName);
+      if (methodName === 'constructor' || methodName.startsWith('_') || typeof method !== 'function')
+        continue;
+      Reflect.set(classType.prototype, methodName, function(...args) {
+        let argsText = args.map(stringifyArgument).join(', ');
+        let callsite = `${className}.${methodName}(${argsText})`;
+        debug(callsite);
+        return method.call(this, ...args);
+      });
+    }
+
+    /**
+     * @param {!Object} arg
+     * @return {string}
+     */
+    function stringifyArgument(arg) {
+      if (typeof arg !== 'function')
+        return JSON.stringify(arg);
+      let text = arg.toString().split('\n').map(line => line.trim()).join('');
+      if (text.length > 20)
+        text = text.substring(0, 20) + '…';
+      return `"${text}"`;
+    }
+  }
 }
 
 module.exports = Helper;