你什么时候需要 Modifier.composed { ... }? [英] When do you need Modifier.composed { ... }?

问题描述

Modifier.composed { ... } 什么时候有用?如果我可以简单地对 Modifier.composed { PaddingModifier(...) } 执行 Modifier.padding() ，为什么我需要它?

When is Modifier.composed { ... } useful? Why would I need it if I could simply do Modifier.padding() over Modifier.composed { PaddingModifier(...) }?

推荐答案

Modifier.composed 允许创建一个对具体化 实例特定、有状态 修饰符.来自文档:

Modifier.composed allows the creation of a composition-aware modifier-factory that's useful for materializing instance-specific, stateful modifiers. From the docs:

声明一个修改器的即时组合，它将为它修改的每个元素组合.组合可用于实现有状态修饰符，这些修饰符对每个修改过的元素都有特定于实例的状态，允许同一个修饰符实例安全地重用于多个元素，同时保持元素特定的状态.

Declare a just-in-time composition of a Modifier that will be composed for each element it modifies. composed may be used to implement stateful modifiers that have instance-specific state for each modified element, allowing the same Modifier instance to be safely reused for multiple elements while maintaining element-specific state.

换句话说，它允许您将提升状态注入特定于元素的 Modifier 并使用 remember、DisposableEffect、环境等，例如:

In other words, it allows you to inject hoisted state into an element-specific Modifier and use remember, DisposableEffect, Ambient, etc. For instance:

fun Modifier.fancyModifier(
    enabled: Boolean = false,
    onClick: () -> Unit = {}
) = composed(inspectorInfo = debugInspectorInfo {
    name = "fancyModifier"
    value = enabled
}) {
    var paddingValue by remember { mutableStateOf(0.dp) }
    onCommit(enabled) {
        paddingValue = if (enabled) 16.dp else 0.dp
    }
    fillMaxWidth()
        .clickable { onClick() }
        .padding(paddingValue)
}

LazyColumnFor(items = List(size = 10) { "$it" }) {
    var enabled by remember { mutableStateOf(false) }
    Text(
        text = "fancy modifier",
        modifier = Modifier.fancyModifier(enabled) {
            enabled = !enabled
        }
    )
}

您还可以声明 InspectorInfo 以帮助使用 debugInspectorInfo 进行调试.来自文档:

You can also declare InspectorInfo to help with debugging by using debugInspectorInfo. From the docs:

如果指定了inspectorInfo，则该修饰符将在开发期间对工具可见.指定原始修饰符的名称和参数.以及可选地声明 InspectorInfo 以帮助调试.

If inspectorInfo is specified this modifier will be visible to tools during development. Specify the name and arguments of the original modifier. as well as optionally declare InspectorInfo to help with debugging.

如果您想要跟踪的值不止一个，您可以使用 properties 字段而不是 value.

You can use the properties field instead of value if you have more than value you'd like to track.

注意:debugInspectorInfo lambda 从 release 构建中删除.

Note: debugInspectorInfo lambdas are removed from release builds.

class FancyModifierTest {

    @Before
    fun setup() {
        isDebugInspectorInfoEnabled = true
    }

    @After
    fun teardown() {
        isDebugInspectorInfoEnabled = false
    }

    @Test
    fun testFancyModifierInspectableValue() {
        val modifier = Modifier.fancyModifier() as InspectableValue
        assertEquals(modifier.nameFallback, "fancyModifier")
        assertEquals(modifier.valueOverride, false)
        assertEquals(modifier.inspectableElements.toList().size, 0)
    }

}

这里有一些更实际的例子:

Here are some more practical examples:

• JetSnack 渐变
• JetCaster 渐变
• InspectorInfo Samples
• cs.android.com

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