diff --git a/docs/docs/testing/intersection-observer-mock.mdx b/docs/docs/testing/intersection-observer-mock.mdx
index a713991..ea65fc7 100644
--- a/docs/docs/testing/intersection-observer-mock.mdx
+++ b/docs/docs/testing/intersection-observer-mock.mdx
@@ -30,3 +30,65 @@ observer.simulateIntersection([
   },
 ]);
 ```
+
+### Typical unit-test
+
+Here is an example of unit test with `IntersectionObserverMock`. For implementation `useIntersection` is used:
+
+```tsx
+import { IntersectionObserverContext, useIntersection } from '@krutoo/utils/react';
+import { IntersectionObserverMock } from '@krutoo/utils/testing';
+import { act, render } from '@testing-library/react';
+import { describe, expect, mock, test } from '#test-utils';
+
+function MyComponent({ onChange }) {
+  const ref = useRef(null);
+
+  useIntersection(ref, () => {
+    onChange?.();
+  });
+
+  return (
+    <div ref={ref} data-testid='my-component'>
+      Hello from my component
+    </div>
+  );
+}
+
+describe('MyComponent', () => {
+  test('should call onChange on intersection change', () => {
+    const observer = new ResizeObserverMock(() => {});
+
+    const context = {
+      getObserver(callback: IntersectionObserverCallback) {
+        return observer.addCallback(callback);
+      },
+    };
+
+    const spy = mock.fn();
+
+    const { getByTestId } = render(
+      <IntersectionObserverContext value={context}>
+        <MyComponent onChange={spy} />
+      </IntersectionObserverContext>,
+    );
+
+    // no calls after render
+    expect(spy.mock.callCount()).toBe(0);
+
+    // one call after intersection simulated
+    act(() => {
+      observer.simulateIntersection([
+        {
+          target: getByTestId('my-component'),
+          isIntersecting: true,
+        },
+      ]);
+    });
+    expect(spy.mock.callCount()).toBe(1);
+  });
+});
+```
+
+As you can see, mock includes chainable `addCallback` method.
+You can use it to provide one observer for multiple components during test.
diff --git a/docs/docs/testing/resize-observer-mock.mdx b/docs/docs/testing/resize-observer-mock.mdx
index 0e5e9ca..7d1d900 100644
--- a/docs/docs/testing/resize-observer-mock.mdx
+++ b/docs/docs/testing/resize-observer-mock.mdx
@@ -29,3 +29,64 @@ observer.simulateResize([
   },
 ]);
 ```
+
+### Typical unit-test
+
+Here is an example of unit test with `ResizeObserverMock`. For implementation `useResize` is used:
+
+```tsx
+import { ResizeObserverContext, useResize } from '@krutoo/utils/react';
+import { ResizeObserverMock } from '@krutoo/utils/testing';
+import { act, render } from '@testing-library/react';
+import { describe, expect, mock, test } from '#test-utils';
+
+function MyComponent({ onResize }) {
+  const ref = useRef(null);
+
+  useResize(ref, () => {
+    onResize?.();
+  });
+
+  return (
+    <div ref={ref} data-testid='my-component'>
+      Hello from my component
+    </div>
+  );
+}
+
+describe('MyComponent', () => {
+  test('should call onResize on resize', () => {
+    const observer = new ResizeObserverMock(() => {});
+
+    const context = {
+      getObserver(callback: ResizeObserverCallback) {
+        return observer.addCallback(callback);
+      },
+    };
+
+    const spy = mock.fn();
+
+    const { getByTestId } = render(
+      <ResizeObserverContext value={context}>
+        <MyComponent onResize={spy} />
+      </ResizeObserverContext>,
+    );
+
+    // no calls after render
+    expect(spy.mock.callCount()).toBe(0);
+
+    // one call after resize simulated
+    act(() => {
+      observer.simulateResize([
+        {
+          target: getByTestId('my-component'),
+        },
+      ]);
+    });
+    expect(spy.mock.callCount()).toBe(1);
+  });
+});
+```
+
+As you can see, mock includes chainable `addCallback` method.
+You can use it to provide one observer for multiple components during test.
diff --git a/src/testing/intersection-observer-mock.ts b/src/testing/intersection-observer-mock.ts
index 25a075e..3061a6c 100644
--- a/src/testing/intersection-observer-mock.ts
+++ b/src/testing/intersection-observer-mock.ts
@@ -17,8 +17,8 @@ export class IntersectionObserverMock implements IntersectionObserver {
   /** @inheritdoc */
   readonly thresholds: ReadonlyArray<number>;
 
-  /** Callback. */
-  protected readonly callback: IntersectionObserverCallback;
+  /** Callbacks. */
+  protected readonly callbacks: Set<IntersectionObserverCallback>;
 
   /** Observed elements. */
   protected readonly elements: Set<Element>;
@@ -28,7 +28,7 @@ export class IntersectionObserverMock implements IntersectionObserver {
     this.rootMargin = init?.rootMargin ?? '0px 0px 0px 0px';
     this.thresholds = Array.isArray(init?.threshold) ? init.threshold : [init?.threshold ?? 0];
 
-    this.callback = callback;
+    this.callbacks = new Set([callback]);
     this.elements = new Set();
   }
 
@@ -82,6 +82,18 @@ export class IntersectionObserverMock implements IntersectionObserver {
       });
     }
 
-    this.callback(readyEntries, this);
+    this.callbacks.forEach(fn => fn(readyEntries, this));
+  }
+
+  /**
+   * Adds callback.
+   * Useful to define single observer during tests and add callbacks to it.
+   * @param callback Callback.
+   * @returns This instance.
+   */
+  addCallback(callback: IntersectionObserverCallback): this {
+    this.callbacks.add(callback);
+
+    return this;
   }
 }
diff --git a/src/testing/resize-observer-mock.ts b/src/testing/resize-observer-mock.ts
index e9c079b..1bba8fe 100644
--- a/src/testing/resize-observer-mock.ts
+++ b/src/testing/resize-observer-mock.ts
@@ -8,14 +8,14 @@ export type PartialResizeObserverEntry = Pick<ResizeObserverEntry, 'target'> &
  * Useful for unit testing.
  */
 export class ResizeObserverMock implements ResizeObserver {
-  /** Callback. */
-  protected readonly callback: ResizeObserverCallback;
+  /** Callbacks. */
+  protected readonly callbacks: Set<ResizeObserverCallback>;
 
   /** Observed elements. */
   protected readonly elements: Set<Element>;
 
   constructor(callback: ResizeObserverCallback) {
-    this.callback = callback;
+    this.callbacks = new Set([callback]);
     this.elements = new Set();
   }
 
@@ -60,6 +60,18 @@ export class ResizeObserverMock implements ResizeObserver {
       });
     }
 
-    this.callback(readyEntries, this);
+    this.callbacks.forEach(fn => fn(readyEntries, this));
+  }
+
+  /**
+   * Adds callback.
+   * Useful to define single observer during tests and add callbacks to it.
+   * @param callback Callback.
+   * @returns This instance.
+   */
+  addCallback(callback: ResizeObserverCallback): this {
+    this.callbacks.add(callback);
+
+    return this;
   }
 }