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diff --git a/devtools/client/debugger/src/utils/context.js b/devtools/client/debugger/src/utils/context.js
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at <http://mozilla.org/MPL/2.0/>. */
+// @flow
+
+import type { ThreadId } from "../types";
+import type { State } from "../reducers/types";
+
+import { getThreadContext } from "../selectors";
+
+// Context encapsulates the main parameters of the current redux state, which
+// impact most other information tracked by the debugger.
+//
+// The main use of Context is to control when asynchronous operations are
+// allowed to make changes to the program state. Such operations might be
+// invalidated as the state changes from the time the operation was originally
+// initiated. For example, operations on pause state might still continue even
+// after the thread unpauses.
+//
+// The methods below can be used to compare an old context with the current one
+// and see if the operation is now invalid and should be abandoned. Actions can
+// also include a 'cx' Context property, which will be checked by the context
+// middleware. If the action fails validateContextAction() then it will not be
+// dispatched.
+//
+// Context can additionally be used as a shortcut to access the main properties
+// of the pause state.
+
+// A normal Context is invalidated if the target navigates.
+export type NavigateContext = {|
+  // Counter reflecting how many times the debugger has navigated to a new page
+  // and reset most of its state.
+  +navigateCounter: number,
+|};
+
+// A ThreadContext is invalidated if the target navigates, or if the current
+// thread changes, pauses, or resumes.
+export type ThreadContext = {|
+  +navigateCounter: number,
+
+  // The currently selected thread.
+  +thread: ThreadId,
+
+  // Counter reflecting how many times the selected thread has paused or
+  // resumed.
+  +pauseCounter: number,
+
+  // Whether the current thread is paused. This is determined from the other
+  // Context properties and is here for convenient access.
+  +isPaused: boolean,
+|};
+
+export type Context = NavigateContext | ThreadContext;
+
+export class ContextError extends Error {}
+
+export function validateNavigateContext(state: State, cx: Context): void {
+  const newcx = getThreadContext(state);
+
+  if (newcx.navigateCounter != cx.navigateCounter) {
+    throw new ContextError("Page has navigated");
+  }
+}
+
+export function validateThreadContext(state: State, cx: ThreadContext): void {
+  const newcx = getThreadContext(state);
+
+  if (cx.thread != newcx.thread) {
+    throw new ContextError("Current thread has changed");
+  }
+
+  if (cx.pauseCounter != newcx.pauseCounter) {
+    throw new ContextError("Current thread has paused or resumed");
+  }
+}
+
+export function validateContext(state: State, cx: Context): void {
+  validateNavigateContext(state, cx);
+
+  if ("thread" in cx) {
+    validateThreadContext(state, (cx: any));
+  }
+}
+
+export function isValidThreadContext(state: State, cx: ThreadContext): boolean {
+  const newcx = getThreadContext(state);
+  return cx.thread == newcx.thread && cx.pauseCounter == newcx.pauseCounter;
+}