构建实时进度环
将画布和状态组合成一个控件。
构建实时进度环 是 CoddyKit 上的免费 Jetpack Compose Academy 课时。 这是第 4 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 Jetpack Compose Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Jetpack Compose Academy 课程共包含 4 节课。
本课时的部分内容尚未翻译，以英文显示。
The Goal
Time to build something real: a progress ring that fills as a value grows. It blends canvas drawing with live Compose state.
An Arc, Not a Circle
A ring is really an arc. We draw a faint full circle as a track, then a bright arc on top showing the progress so far.
Hold the Progress
Store the value in state so the ring redraws when it changes. A float from 0f to 1f maps cleanly to zero through full.
var progress by remember { mutableStateOf(0.25f) }Draw the Track
First paint the background ring with drawCircle using a Stroke. A soft gray track shows the full path the progress will fill.
drawCircle(
color = Color.LightGray,
style = Stroke(width = 24f)
)Meet drawArc
The hero is drawArc. It draws a slice of a circle defined by a start angle and a sweep angle, both measured in degrees.
drawArc(
color = Color.Blue,
startAngle = -90f,
sweepAngle = 90f,
useCenter = false
)Start at the Top
Angle 0 points right, so we use a startAngle of -90f to begin at the top. That feels natural for a progress indicator.
Sweep by Progress
A full circle is 360 degrees, so the sweepAngle is just progress times 360. At 0.5f the arc covers exactly half the ring.
val sweep = progress * 360f
drawArc(color = Color.Blue, startAngle = -90f, sweepAngle = sweep, useCenter = false)useCenter Stays False
Keep useCenter false so you draw a ring slice, not a filled pie wedge. With a Stroke style it becomes a clean rounded band.
drawArc(color = Color.Blue, startAngle = -90f, sweepAngle = sweep, useCenter = false, style = Stroke(width = 24f, cap = StrokeCap.Round))Animate the Value
Wrap the value in animateFloatAsState so changes glide instead of jumping. The ring then smoothly grows toward each new target.
val animated by animateFloatAsState(targetValue = progress)Add a Gradient
Swap the flat color for a sweepGradient brush and the ring gets a vivid, modern look as the colors rotate around it.
drawArc(brush = Brush.sweepGradient(listOf(Color.Cyan, Color.Magenta)), startAngle = -90f, sweepAngle = sweep, useCenter = false, style = Stroke(24f))Layer the Label
Place a Text showing the percentage in a Box centered over the Canvas. Now your widget reads as a complete, polished control.
Quick Check
How do you turn a 0f-to-1f progress value into the arc's sweep?
Recap
You built a live ring: a track circle, a drawArc swept by progress times 360, animated state, and a gradient stroke. Beautiful work! 🌀
常见问题解答
「构建实时进度环」课时是免费的吗？
是的 — 「构建实时进度环」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Jetpack Compose Academy 课程的其余内容，请升级到 CoddyKit PRO。 Jetpack Compose Academy 课程共包含 4 节课。
「构建实时进度环」这节课中我会学到什么？
将画布和状态组合成一个控件。 你通过在浏览器中直接运行的动手代码来练习 Jetpack Compose Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。
学习 Jetpack Compose Academy 需要有经验吗？
无需任何先前经验。CoddyKit 上的 Jetpack Compose Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 4 节课，共 4 节。
「构建实时进度环」课时需要多长时间？
大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。
我能在这节 Jetpack Compose Academy 课中编写并运行代码吗？
能。每节 Jetpack Compose Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。