我试图理解CompositionLocal实际上是如何隐含地设置值的，以及需要满足哪些要求才能使其发挥作用，但Android关于Locally scoped data with CompositionLocal的文档无济于事.
有一个示例，通过为color参数指定新值来更改Text的 colored颜色 .
// Some composable deep in the hierarchy of MaterialTheme
@Composable
fun SomeTextLabel(labelText: String) {
Text(
text = labelText,
// `primary` is obtained from MaterialTheme's
// LocalColors CompositionLocal
color = MaterialTheme.colors.primary
)
}
但如果您需要这样设置，这并不是隐含的！
然后他们展示了另一个例子，他们通过CompositionLocalProvider改变了ContentAlpha，这里Text suddenly可以隐式地使用新值，即使他们写的是CompositionLocal is what the Material theme uses under the hood.，那么为什么第一个例子不行呢？
@Composable
fun CompositionLocalExample() {
MaterialTheme { // MaterialTheme sets ContentAlpha.high as default
Column {
Text("Uses MaterialTheme's provided alpha")
CompositionLocalProvider(LocalContentAlpha provides ContentAlpha.medium) {
Text("Medium value provided for LocalContentAlpha")
Text("This Text also uses the medium value")
CompositionLocalProvider(LocalContentAlpha provides ContentAlpha.disabled) {
DescendantExample()
}
}
}
}
}
@Composable
fun DescendantExample() {
// CompositionLocalProviders also work across composable functions
Text("This Text uses the disabled alpha now")
}
还有第三个例子，他们展示了如何create your own CompositionLocal，但在这里，他们explicitly设置了Card‘S elevation参数！
@Composable
fun SomeComposable() {
// Access the globally defined LocalElevations variable to get the
// current Elevations in this part of the Composition
Card(elevation = LocalElevations.current.card) {
// Content
}
}
他们创造CompositionLocalProvider难道不是为了避免这样做吗？
// Bind elevation as the value for LocalElevations
CompositionLocalProvider(LocalElevations provides elevations) {
// ... Content goes here ...
// This part of Composition will see the `elevations` instance
// when accessing LocalElevations.current
}