> For the complete documentation index, see [llms.txt](https://docs.decentraland.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.decentraland.org/creator/content-creator-zh/chang-jing-sdk7/bian-cheng-mo-shi/mutable-data.md).

# 可变数据

了解如何处理来自组件的只读数据和可变数据

当引用来自某个 [组件](/creator/content-creator-zh/chang-jing-sdk7/jia-gou/entities-components.md)时，你可以获取可变版本或只读（不可变）版本。

在可能的情况下，你应始终处理数据的只读版本。与始终处理同一数据的可变版本相比，这种做法可以显著提升场景性能。

该 `.get()` 组件中的 function 会返回该组件的只读（不可变）版本。你只能读取其值，不能更改其上的任何属性。

该 `.getMutable()` function 会返回一个允许你更改其值的组件版本。只有在你计划更改组件时才使用可变版本，否则始终使用 `get()`.

```ts
// 获取只读（不可变）版本
const immutableTransform = Transform.get(myEntity)

// 以下代码不起作用：
// immutableTransform.position.y = 2

const mutableTransform = Transform.getMutable(myEntity)

// 下面这一行确实会更改实体的位置
mutableTransform.position.y = 2
```

一种好的做法是遍历只读组件以检查值，然后仅在需要更改时获取某个单独组件的可变版本。

```ts
// 硬编码的最大高度
const MAX_HEIGHT = 10

// 定义系统
function HeightLimitSystem(dt: number) {
	// 遍历所有具有 Transform 组件的实体
	for (const [entity] of engine.getEntitiesWith(Transform)) {
		// 获取只读值
		const currentHeight = Transform.get(entity).position.y

		// 比较值
		if (currentHeight > MAX_HEIGHT) {
			// 获取可变版本以进行更改
			const mutableTransform = Transform.getMutable(entity)

			// 更改变换
			mutableTransform.position.y = MAX_HEIGHT
		}
	}
}

// 将系统添加到引擎
engine.addSystem(HeightLimitSystem)
```

在上面的示例中，系统会检查实体的只读 `Transform` 组件的值。每个 tick 都会检查位置的 *y* 是否高于一个硬编码的最大高度。如果 Transform 上的高度恰好高于这个限制，那么且只有在这时我们才会获取 Transform 的可变版本。这对场景来说看起来像是额外工作，但在一个我们每个游戏循环 tick 都要检查值、只偶尔进行更改的场景中，这会带来巨大的性能提升。

这种做法遵循 [面向数据编程](/creator/content-creator-zh/chang-jing-sdk7/jia-gou/data-oriented-programming.md)。由于它带来的提升非常明显，它也正在逐渐被游戏行业采纳为行业标准实践。

{% hint style="warning" %}
**📔 注意**：在旧版 SDK（6.x 或更早版本）中，组件始终被视为可变的。这种模式可能更容易理解一些，但运行效率要低得多。
{% endhint %}


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