Introduce logLevel option

Part of #1560

src/common/services/LogService.ts

@@ -0,0 +1,78 @@
+/**
+ * Copyright (c) 2019 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+import { ILogService, IOptionsService } from 'common/services/Services';
+
+interface IConsole {
+  log(message?: any, ...optionalParams: any[]): void;
+  error(message?: any, ...optionalParams: any[]): void;
+  info(message?: any, ...optionalParams: any[]): void;
+  trace(message?: any, ...optionalParams: any[]): void;
+  warn(message?: any, ...optionalParams: any[]): void;
+}
+
+// console is available on both node.js and browser contexts but the common
+// module doesn't depend on them so we need to explicitly declare it.
+declare const console: IConsole;
+
+
+export enum LogLevel {
+  Debug = 0,
+  Info = 1,
+  Warn = 2,
+  Error = 3,
+  Off = 4
+}
+
+const optionsKeyToLogLevel: { [key: string]: LogLevel } = {
+  debug: LogLevel.Debug,
+  info: LogLevel.Info,
+  warn: LogLevel.Warn,
+  error: LogLevel.Error,
+  off: LogLevel.Off
+};
+
+export class LogService implements ILogService {
+  private _logLevel!: LogLevel;
+
+  constructor(
+    private readonly _optionsService: IOptionsService
+  ) {
+    this._updateLogLevel();
+    this._optionsService.onOptionChange(key => {
+      if (key === 'logLevel') {
+        this._updateLogLevel();
+      }
+    })
+  }
+
+  private _updateLogLevel(): void {
+    this._logLevel = optionsKeyToLogLevel[this._optionsService.options.logLevel];
+  }
+
+  debug(message: any, ...optionalParams: any[]): void {
+    if (this._logLevel <= LogLevel.Debug) {
+      console.log.call(console, message, optionalParams);
+    }
+  }
+
+  info(message: any, ...optionalParams: any[]): void {
+    if (this._logLevel <= LogLevel.Info) {
+      console.info.call(console, message, optionalParams);
+    }
+  }
+
+  warn(message: any, ...optionalParams: any[]): void {
+    if (this._logLevel <= LogLevel.Warn) {
+      console.warn.call(console, message, optionalParams);
+    }
+  }
+
+  error(message: any, ...optionalParams: any[]): void {
+    if (this._logLevel <= LogLevel.Error) {
+      console.error.call(console, message, optionalParams);
+    }
+  }
+}