# Freezing a Custom Component

When a custom component decorated by @ComponentV2 is inactive, it can be frozen so that its state variable does not respond to updates. That is, the @Monitor decorated method is not called, and the node associated with the state variable is not re-rendered. You can use the **freezeWhenInactive** attribute to specify whether to freeze a custom component. If no parameter is passed in, the feature is disabled. This feature works in following scenarios: page routing, **TabContent**, and **Navigation**.

Before reading this topic, you are advised to read [\@ComponentV2](./arkts-new-componentV2.md).

> **NOTE**
>
> @ComponentV2 decorated custom component freezing is supported since API version 12.
>
> Different from freezing the @Component decorated components, custom components decorated by @ComponentV2 do not support freezing the cached list items in the **LazyForEach** scenario.


## Use Scenarios

### Page Routing

> **NOTE**
>
> This example uses router for page redirection but you are advised to use the **Navigation** component instead, because **Navigation** provides more functions and more flexible customization capabilities. For details, see the use cases of [Navigation](#navigation).

- When page 1 calls the **router.pushUrl** API to jump to page 2, page 1 is hidden and invisible. In this case, if the state variable on page 1 is updated, page 1 is not re-rendered.
For details, see the following.

![freezeInPage](./figures/freezeInPage.png)

Page 1

```ts
import { router } from '@kit.ArkUI';

@ObservedV2
export class Book {
  @Trace name: string = "100";

  constructor(page: string) {
    this.name = page;
  }
}

@Entry
@ComponentV2({ freezeWhenInactive: true })
export struct Page1 {
  @Local bookTest: Book = new Book("A Midsummer Night's Dream");

  @Monitor("bookTest.name")
  onMessageChange(monitor: IMonitor) {
    console.log(`The book name change from ${monitor.value()?.before} to ${monitor.value()?.now}`);
  }

  build() {
    Column() {
      Text(`Book name is ${this.bookTest.name}`).fontSize(25)
      Button('changeBookName').fontSize(25)
        .onClick(() => {
          this.bookTest.name = "The Old Man and the Sea";
        })
      Button('go to next page').fontSize(25)
        .onClick(() => {
          router.pushUrl({ url: 'pages/Page2' });
          setTimeout(() => {
            this.bookTest = new Book("Jane Austen oPride and Prejudice");
          }, 1000)
        })
    }
  }
}
```

Page 2

```ts
import { router } from '@kit.ArkUI';

@Entry
@ComponentV2
struct Page2 {
  build() {
    Column() {
      Text(`This is the page2`).fontSize(25)
      Button('Back')
        .onClick(() => {
          router.back();
        })
    }
  }
}
```

In the preceding example:

1. Click the **changeBookName** button on page 1. The **name** attribute of the **bookTest** variable is changed, and the **onMessageChange** method registered in @Monitor is called.

2. Click the **go to next page** button on page 1 to redirect to page 2, and then update the state variable **bookTest** 1s later. When **bookTest** is updated, page 2 is displayed and Page 1 is in the inactive state. The state variable @Local bookTest does not respond to the update. Therefore, the @Monitor is not called, and the node associated with the state variable is not updated.
The trace diagram is as follows.

![Example Image](./figures/freeze1.png)


3. Click **Back**. Page 2 is destroyed, and the state of page 1 changes from inactive to active. The update of the state variable **bookTest** is observed, the **onMessageChange** method registered in @Monitor is called, and the corresponding text content is changed.

![freezeV2Page](./figures/freezeV2page.gif)


### TabContent

- You can freeze invisible **TabContent** components in the **Tabs** container so that they do not trigger UI re-rendering.

- During initial rendering, only the **TabContent** component that is being displayed is created. All **TabContent** components are created only after all of them have been switched to.

For details, see the following.
![freezeWithTab](./figures/freezewithTabs.png)


```ts
@Entry
@ComponentV2
struct TabContentTest {
  @Local message: number = 0;
  @Local data: number[] = [0, 1];

  build() {
    Row() {
      Column() {
        Button('change message').onClick(() => {
          this.message++;
        })

        Tabs() {
          ForEach(this.data, (item: number) => {
            TabContent() {
              FreezeChild({ message: this.message, index: item })
            }.tabBar(`tab${item}`)
          }, (item: number) => item.toString())
        }
      }
      .width('100%')
    }
    .height('100%')
  }
}

@ComponentV2({ freezeWhenInactive: true })
struct FreezeChild {
  @Param message: number = 0;
  @Param index: number = 0;

  @Monitor('message') onMessageUpdated(mon: IMonitor) {
    console.info(`FreezeChild message callback func ${this.message}, index: ${this.index}`);
  }

  build() {
    Text("message" + `${this.message}, index: ${this.index}`)
      .fontSize(50)
      .fontWeight(FontWeight.Bold)
  }
}
```

In the preceding example:

1. When **change message** is clicked, the value of **message** changes, and the @Monitor decorated **onMessageUpdated** method of the **TabContent** component being displayed is called.

2. When **tab1** in **TabBar** is clicked to switch to another **TabContent** component, the component switches from inactive to active, and the corresponding @Monitor decorated **onMessageUpdated** method is called.

3. When **change message** is clicked again, the value of **message** changes, and only the @Monitor decorated **onMessageUpdated** method of the **TabContent** component being displayed is called. Other inactive **TabContent** components do not trigger @Monitor.

![TabContent.gif](figures/TabContent.gif)


### Navigation

- You can freeze an invisible page so that it does not trigger UI re-rendering. When the user returns to this page, a re-render is triggered through an @Monitor decorated callback.

```ts
@Entry
@ComponentV2
struct MyNavigationTestStack {
  @Provider('pageInfo') pageInfo: NavPathStack = new NavPathStack();
  @Local message: number = 0;

  @Monitor('message') info() {
    console.info(`freeze-test MyNavigation message callback ${this.message}`);
  }

  @Builder
  PageMap(name: string) {
    if (name === 'pageOne') {
      pageOneStack({ message: this.message })
    } else if (name === 'pageTwo') {
      pageTwoStack({ message: this.message })
    } else if (name === 'pageThree') {
      pageThreeStack({ message: this.message })
    }
  }

  build() {
    Column() {
      Button('change message')
        .onClick(() => {
          this.message++;
        })
      Navigation(this.pageInfo) {
        Column() {
          Button('Next Page', { stateEffect: true, type: ButtonType.Capsule })
            .onClick(() => {
              this.pageInfo.pushPath({ name: 'pageOne' }); // Push the navigation destination page specified by name to the navigation stack.
            })
        }
      }.title('NavIndex')
      .navDestination(this.PageMap)
      .mode(NavigationMode.Stack)
    }
  }
}

@ComponentV2
struct pageOneStack {
  @Consumer('pageInfo') pageInfo: NavPathStack = new NavPathStack();
  @Local index: number = 1;
  @Param message: number = 0;

  build() {
    NavDestination() {
      Column() {
        NavigationContentMsgStack({ message: this.message, index: this.index })
        Text("cur stack size:" + `${this.pageInfo.size()}`)
          .fontSize(30)
        Button('Next Page', { stateEffect: true, type: ButtonType.Capsule })
          .onClick(() => {
            this.pageInfo.pushPathByName('pageTwo', null);
          })
        Button('Back Page', { stateEffect: true, type: ButtonType.Capsule })
          .onClick(() => {
            this.pageInfo.pop();
          })
      }.width('100%').height('100%')
    }.title('pageOne')
    .onBackPressed(() => {
      this.pageInfo.pop();
      return true;
    })
  }
}

@ComponentV2
struct pageTwoStack {
  @Consumer('pageInfo') pageInfo: NavPathStack = new NavPathStack();
  @Local index: number = 2;
  @Param message: number = 0;

  build() {
    NavDestination() {
      Column() {
        NavigationContentMsgStack({ message: this.message, index: this.index })
        Text("cur stack size:" + `${this.pageInfo.size()}`)
          .fontSize(30)
        Button('Next Page', { stateEffect: true, type: ButtonType.Capsule })
          .onClick(() => {
            this.pageInfo.pushPathByName('pageThree', null);
          })
        Button('Back Page', { stateEffect: true, type: ButtonType.Capsule })
          .onClick(() => {
            this.pageInfo.pop();
          })
      }
    }.title('pageTwo')
    .onBackPressed(() => {
      this.pageInfo.pop();
      return true;
    })
  }
}

@ComponentV2
struct pageThreeStack {
  @Consumer('pageInfo') pageInfo: NavPathStack = new NavPathStack();
  @Local index: number = 3;
  @Param message: number = 0;

  build() {
    NavDestination() {
      Column() {
        NavigationContentMsgStack({ message: this.message, index: this.index })
        Text("cur stack size:" + `${this.pageInfo.size()}`)
          .fontSize(30)
        Button('Next Page', { stateEffect: true, type: ButtonType.Capsule })
          .height(40)
          .onClick(() => {
            this.pageInfo.pushPathByName('pageOne', null);
          })
        Button('Back Page', { stateEffect: true, type: ButtonType.Capsule })
          .height(40)
          .onClick(() => {
            this.pageInfo.pop();
          })
      }
    }.title('pageThree')
    .onBackPressed(() => {
      this.pageInfo.pop();
      return true;
    })
  }
}

@ComponentV2({ freezeWhenInactive: true })
struct NavigationContentMsgStack {
  @Param message: number = 0;
  @Param index: number = 0;

  @Monitor('message') info() {
    console.info(`freeze-test NavigationContent message callback ${this.message}`);
    console.info(`freeze-test ---- called by content ${this.index}`);
  }

  build() {
    Column() {
      Text("msg:" + `${this.message}`)
        .fontSize(30)
    }
  }
}
```

In the preceding example:

1. When **change message** is clicked, the value of **message** changes, and the @Monitor decorated **info** method of the **MyNavigationTestStack** component being displayed is called.

2. When **Next Page** is clicked, the page is switched to **PageOne** and the **pageOneStack** node is created.

3. When **change message** is clicked again, the value of **message** changes, and only the @Monitor decorated **info** method of the **NavigationContentMsgStack** child component in **pageOneStack** is called.

4. When **Next Page** is clicked, the page is switched to **PageTwo** and the **pageTwoStack** node is created. The state of the **pageOneStack** node changes from active to inactive.

5. When **change message** is clicked again, the value of **message** changes, and only the @Monitor decorated **info** method of the **NavigationContentMsgStack** child component in **pageTwoStack** is called. The child custom component in **NavDestination** that is not at the top of the navigation routing stack is in the inactive state. The @Monitor method is not triggered.

6. When **Next Page** is clicked, the page is switched to **PageThree** and the **pageThreeStack** node is created. The state of the **pageTwoStack** node changes from active to inactive.

7. When **change message** is clicked again, the value of **message** changes, and only the @Monitor decorated **info** method of the **NavigationContentMsgStack** child component in **pageThreeStack** is called. The child custom component in **NavDestination** that is not at the top of the navigation routing stack is in the inactive state. The @Monitor method is not triggered.

8. Click **Back Page** to return to **PageTwo**. The state of the **pageTwoStack** node changes from inactive to active, and the **info** method registered in @Monitor of the **NavigationContentMsgStack** child component is triggered.

9. Click **Back Page** again to return to **PageOne**. The state of the **pageOneStack** node changes from inactive to active, and the **info** method registered in @Monitor of the **NavigationContentMsgStack** child component is triggered.

10. When **Back Page** is clicked, the page is switched to the initial page.

![navigation-freeze.gif](figures/navigation-freeze.gif)

## Constraints
As shown in the following example, the custom node [BuilderNode](../reference/apis-arkui/js-apis-arkui-builderNode.md) is used in **FreezeBuildNode**. **BuilderNode **can dynamically mount components using commands and component freezing strongly depends on the parent-child relationship to determine whether it is enabled. In this case, if the parent component is frozen and **BuilderNode** is enabled at the middle level of the component tree, the child component of the **BuilderNode** cannot be frozen.

```
import { BuilderNode, FrameNode, NodeController, UIContext } from '@kit.ArkUI';

// Define a Params class to pass parameters.
@ObservedV2
class Params {
  // Singleton mode. Ensure that there is only one Params instance.
  static singleton_: Params;

  // Method for obtaining the Params instance.
  static instance() {
    if (!Params.singleton_) {
      Params.singleton_ = new Params(0);
    }
    return Params.singleton_;
  }

  // Use the @Trace decorator to decorate the message attribute so that its changes are observable.
  @Trace message: string = "Hello";
  index: number = 0;

  constructor(index: number) {
    this.index = index;
  }
}

// Define a buildNodeChild component that contains a message attribute and an index attribute.
@ComponentV2
struct buildNodeChild {
  // Use the Params instance as the storage attribute.
  storage: Params = Params.instance();
  @Param index: number = 0;

  // Use the @Monitor decorator to listen for the changes of storage.message.
  @Monitor("storage.message")
  onMessageChange(monitor: IMonitor) {
    console.log(`FreezeBuildNode buildNodeChild message callback func ${this.storage.message}, index:${this.index}`);
  }

  build() {
    Text(`buildNode Child message: ${this.storage.message}`).fontSize(30)
  }
}

// Define a buildText function that receives a Params parameter and constructs a Column component.
@Builder
function buildText(params: Params) {
  Column() {
    buildNodeChild({ index: params.index })
  }
}

class TextNodeController extends NodeController {
  private textNode: BuilderNode<[Params]> | null = null;
  private index: number = 0;
  
  // The constructor receives an index parameter.
  constructor(index: number) {
    super();
    this.index = index;
  }

  // Create and return a FrameNode.
  makeNode(context: UIContext): FrameNode | null {
    this.textNode = new BuilderNode(context);
    this.textNode.build(wrapBuilder<[Params]>(buildText), new Params(this.index));
    return this.textNode.getFrameNode();
  }
}

// Define an index component that contains a message attribute and a data array.
@Entry
@ComponentV2
struct Index {
  // Use the Params instance as the storage attribute.
  storage: Params = Params.instance();
  private data: number[] = [0, 1];

  build() {
    Row() {
      Column() {
        Button("change").fontSize(30)
          .onClick(() => {
            this.storage.message += 'a';
          })

        Tabs() {
          // Use Repeat to repeatedly render the TabContent component.
          Repeat<number>(this.data)
            .each((obj: RepeatItem<number>) => {
              TabContent() {
                FreezeBuildNode({ index: obj.item })
                  .margin({ top: 20 })
              }.tabBar(`tab${obj.item}`)
            })
            .key((item: number) => item.toString())
        }
      }
    }
    .width('100%')
    .height('100%')
  }
}

// Define a FreezeBuildNode component that contains a message attribute and an index attribute.
@ComponentV2({ freezeWhenInactive: true })
struct FreezeBuildNode {
  // Use the Params instance as the storage attribute.
  storage: Params = Params.instance();
  @Param index: number = 0;

  // Use the @Monitor decorator to listen for the changes of storage.message.
  @Monitor("storage.message")
  onMessageChange(monitor: IMonitor) {
    console.log(`FreezeBuildNode message callback func ${this.storage.message}, index: ${this.index}`);
  }

  build() {
    NodeContainer(new TextNodeController(this.index))
      .width('100%')
      .height('100%')
      .backgroundColor('#FFF0F0F0')
  }
}
```

Click **Button("change")** to change the value of **message**. The **onMessageUpdated** method registered in @Watch of the **TabContent** component that is being displayed is triggered, and that under the **BuilderNode** node of **TabContent** that is not displayed is also triggered.

![builderNode.gif](figures/builderNode.gif)