Add MutableDisposable

Part of #4685

src/common/Lifecycle.ts

@@ -50,6 +50,42 @@ export abstract class Disposable implements IDisposable {
   }
 }
 
+export class MutableDisposable<T extends IDisposable> implements IDisposable {
+  private _value?: T;
+  private _isDisposed = false;
+
+  /**
+   * Gets the value if it exists.
+   */
+  public get value(): T | undefined {
+    return this._isDisposed ? undefined : this._value;
+  }
+
+  /**
+   * Sets the value, disposing of the old value if it exists.
+   */
+  public set value(value: T | undefined) {
+    if (this._isDisposed || value === this._value) {
+      return;
+    }
+    this._value?.dispose();
+    this._value = value;
+  }
+
+  /**
+   * Resets the stored value and disposes of the previously stored value.
+   */
+  public clear(): void {
+    this.value = undefined;
+  }
+
+  public dispose(): void {
+    this._isDisposed = true;
+    this._value?.dispose();
+    this._value = undefined;
+  }
+}
+
 /**
  * Wrap a function in a disposable.
  */