文档
画布
画布是一个用 HTML Canvas 2D API 的操作绘制的视图:图表、进度环、仪表盘、签名、自定义图标,凡是画出来比用视图拼出来更容易的东西都适合。在 Rust 中,canvas(..) 显示一个 Painter 录制的内容;在 TSX 中,nsh 的 Canvas 接收一个 draw 函数,一次性录制整幅绘制。平台以原生方式回放(iOS 上是 Core Graphics,Android 上是 android.graphics.Canvas),按屏幕分辨率绘制,所以在任何像素密度下都很清晰。
import { Canvas, type GestureResponderEvent, mount, Path2D, signal, Text, View } from "nsh";
import { createAnimatedValue, Easing } from "nsh/animated";
import { desktop, Inspector, Panel, size, tint, ui, wide } from "./palette";
const TAU = Math.PI * 2;
const heart = new Path2D("M12 21s-7-4.35-9.5-8.5C.5 8.5 3 4 7 4c2 0 3.5 1 5 3 1.5-2 3-3 5-3 4 0 6.5 4.5 4.5 8.5C19 16.65 12 21 12 21z");
function Button(props: { label: string; onPress: () => void }) {
return (
<View
onPress={props.onPress}
activeStyle={{ opacity: 0.6 }}
style={
desktop
? { ...ui().raised, alignSelf: "flex-start", height: size.control, justifyContent: "center", paddingHorizontal: 12, borderRadius: 8 }
: { ...ui().raised, paddingHorizontal: 14, paddingVertical: 9, borderRadius: 10 }
}
>
<Text style={{ color: ui().text, fontWeight: "600", fontSize: desktop ? 13 : undefined }}>{props.label}</Text>
</View>
);
}
function App() {
// The ring's sweep is an animated value: animating it redraws without running `draw`.
const progress = createAnimatedValue(0.35);
const target = signal(0.35);
const go = () => {
const to = Math.round(Math.random() * 100) / 100;
target.set(to);
progress.timing(to, { duration: 800, easing: Easing.inOut(Easing.cubic) });
};
// The chart reads a signal: changing it runs `draw` again.
const data = signal([12, 30, 22, 48, 36, 60, 52, 75]);
// Finger drawing through the responder system.
const strokes = signal<[number, number][][]>([]);
const point = (e: GestureResponderEvent): [number, number] => [e.locationX, e.locationY];
const ring = () => (
<Canvas
style={{ width: 110, height: 110 }}
draw={(cx, { width, height }) => {
const centerX = width / 2;
const cy = height / 2;
const r = width / 2 - 8;
cx.lineWidth = 10;
cx.lineCap = "round";
cx.strokeStyle = ui().border;
cx.beginPath();
cx.arc(centerX, cy, r, 0, TAU);
cx.stroke();
const g = cx.createConicGradient(-Math.PI / 2, centerX, cy);
g.addColorStop(0, ui().up);
g.addColorStop(1, ui().accent);
cx.strokeStyle = g;
cx.beginPath();
cx.arc(centerX, cy, r, -Math.PI / 2, progress.mul(TAU).sub(Math.PI / 2));
cx.stroke();
cx.fillStyle = ui().text;
cx.font = "600 18px system-ui";
cx.textAlign = "center";
cx.textBaseline = "middle";
cx.fillText(`${Math.round(target() * 100)}%`, centerX, cy);
}}
/>
);
const heartCanvas = () => (
<Canvas
style={{ width: 48, height: 48 }}
draw={(cx) => {
cx.scale(2, 2);
cx.shadowColor = tint(ui().down, 0.45);
cx.shadowBlur = 4;
cx.fillStyle = ui().down;
cx.fill(heart);
}}
/>
);
const chart = (height: number) => (
<Canvas
style={{ ...ui().inset, height, borderRadius: 12 }}
draw={(cx, { width, height }) => {
const values = data();
const pad = 16;
const x = (i: number) => pad + (i / (values.length - 1)) * (width - 2 * pad);
const y = (v: number) => height - pad - (v / 100) * (height - 2 * pad);
const area = cx.createLinearGradient(0, pad, 0, height - pad);
area.addColorStop(0, tint(ui().accent, 0.28));
area.addColorStop(1, tint(ui().accent, 0));
cx.beginPath();
values.forEach((v, i) => (i === 0 ? cx.moveTo(x(i), y(v)) : cx.lineTo(x(i), y(v))));
cx.lineTo(x(values.length - 1), height - pad);
cx.lineTo(x(0), height - pad);
cx.closePath();
cx.fillStyle = area;
cx.fill();
cx.beginPath();
values.forEach((v, i) => (i === 0 ? cx.moveTo(x(i), y(v)) : cx.lineTo(x(i), y(v))));
cx.strokeStyle = ui().accent;
cx.lineWidth = 2.5;
cx.lineJoin = "round";
cx.stroke();
}}
/>
);
const newData = () => <Button label="New data" onPress={() => data.set(data().map(() => Math.round(10 + Math.random() * 85)))} />;
const pad = () => (
<Canvas
style={{ ...ui().inset, flex: 1, borderRadius: 12 }}
onStartShouldSetResponder={() => true}
onResponderGrant={(e) => {
strokes.set([...strokes(), [point(e)]]);
return true;
}}
onResponderMove={(e) => {
const all = strokes();
strokes.set([...all.slice(0, -1), [...all[all.length - 1], point(e)]]);
}}
draw={(cx, { width }) => {
cx.fillStyle = ui().faint;
cx.font = "13px system-ui";
cx.textAlign = "center";
cx.fillText("Draw here", width / 2, 20);
cx.strokeStyle = ui().text;
cx.lineWidth = 3;
cx.lineCap = "round";
cx.lineJoin = "round";
for (const s of strokes()) {
cx.beginPath();
s.forEach(([px, py], i) => (i === 0 ? cx.moveTo(px, py) : cx.lineTo(px, py)));
cx.stroke();
}
}}
/>
);
return (
<View style={{ flex: 1, backgroundColor: ui().page, padding: 16, paddingTop: 24, gap: 14 }}>
{() =>
wide() ? (
<View style={{ flex: 1, flexDirection: "row", gap: 16 }}>
<View style={{ flex: 1, minWidth: 0, gap: 14 }}>
{chart(200)}
{pad()}
</View>
<Inspector>
<Panel title="An animated value in draw">
<View style={{ gap: 10 }}>
<View style={{ flexDirection: "row", alignItems: "center", gap: 16 }}>
{ring()}
{heartCanvas()}
</View>
<Button label="Animate the ring" onPress={go} />
</View>
</Panel>
<Panel title="A signal in draw">
<View style={{ gap: 10 }}>
<Text style={{ fontSize: 13, color: ui().text, fontVariant: ["tabular-nums"] }}>[{data().join(", ")}]</Text>
{newData()}
</View>
</Panel>
<Panel title="Responder events">
<View style={{ gap: 10 }}>
<Text style={{ fontSize: 13, color: ui().text, fontVariant: ["tabular-nums"] }}>
{strokes().length} strokes · {strokes().reduce((n, s) => n + s.length, 0)} points
</Text>
<Button label="Clear" onPress={() => strokes.set([])} />
</View>
</Panel>
</Inspector>
</View>
) : (
<>
<View style={{ flexDirection: "row", alignItems: "center", gap: 16 }}>
{ring()}
<View style={{ gap: 10 }}>
<Button label="Animate the ring" onPress={go} />
{heartCanvas()}
</View>
</View>
{chart(150)}
{newData()}
{pad()}
</>
)
}
</View>
);
}
mount(App);use std::f32::consts::TAU;
use nsh_core::ui::prelude::*;
#[component]
pub fn App() -> impl IntoElement {
let area = signal((0.0_f32, 0.0_f32));
view().flex_1().p_6().pt(px(60.0)).child(
canvas(move || {
let (w, h) = area.get();
Painter::new()
.fill_color(rgb(0x0969da))
.begin_path()
.arc(w / 2.0, h / 2.0, 60.0, 0.0, TAU)
.fill()
.stroke_color(rgb(0x1f2328))
.line_width(4.0)
.stroke_rect(bounds(point(px(20.0), px(20.0)), size(px(w - 40.0), px(h - 40.0))))
})
.h(px(200.0))
.rounded(px(12.0))
.bg(white())
.on_layout(move |frame: &Bounds<Points>| area.set((frame.size.width.0, frame.size.height.0))),
)
}import { Canvas, mount, View } from "nsh";
function App() {
return (
<View style={{ flex: 1, padding: 24, paddingTop: 60 }}>
<Canvas
style={{ height: 200, borderRadius: 12, backgroundColor: "#ffffff" }}
draw={(cx, { width, height }) => {
cx.fillStyle = "#0969da";
cx.beginPath();
cx.arc(width / 2, height / 2, 60, 0, Math.PI * 2);
cx.fill();
cx.strokeStyle = "#1f2328";
cx.lineWidth = 4;
cx.strokeRect(20, 20, width - 40, height - 40);
}}
/>
</View>
);
}
mount(App);画布接受视图的一切(样式、类、触摸事件、无障碍、ref),再加上它的绘制。它需要从样式或布局中得到尺寸;它的内容就是绘制结果。坐标以点为单位,原点在左上角。
#绘制何时生成
在 Rust 中,canvas(move || ..) 在绑定中录制:闭包运行一次,它读取过的 signal 改变时再运行。依赖画布尺寸的绘制把 .on_layout(..) 得到的尺寸存在 signal 中并读取它,如上例所示。在 TSX 中,draw(cx, { width, height }) 会在以下时机运行:
- 画布尺寸确定后(第一次布局之后);
- 画布尺寸改变时;
- 它读取过的 signal 改变时。
每次运行都会替换整幅绘制。在绘制时读取 signal,画布就能跟随你的数据:
let values = signal(vec![12.0_f32, 30.0, 22.0, 48.0, 36.0, 60.0]);
let width = signal(0.0_f32);
let height = 160.0;
canvas(move || {
let w = width.get();
values.with(|v| {
let x = |i: usize| i as f32 / (v.len() - 1) as f32 * w;
let y = |n: f32| height - n / 100.0 * height;
let mut p = Painter::new().begin_path().move_to(x(0), y(v[0]));
for (i, &n) in v.iter().enumerate().skip(1) {
p = p.line_to(x(i), y(n));
}
p.stroke_color(rgb(0x3b82f6)).line_width(2.5).line_join(LineJoin::Round).stroke()
})
})
.h(px(height))
.on_layout(move |frame: &Bounds<Points>| width.set(frame.size.width.0))const values = signal([12, 30, 22, 48, 36, 60]);
<Canvas
style={{ height: 160 }}
draw={(cx, { width, height }) => {
const v = values();
const x = (i: number) => (i / (v.length - 1)) * width;
const y = (n: number) => height - (n / 100) * height;
cx.beginPath();
v.forEach((n, i) => (i === 0 ? cx.moveTo(x(i), y(n)) : cx.lineTo(x(i), y(n))));
cx.strokeStyle = "#3b82f6";
cx.lineWidth = 2.5;
cx.lineJoin = "round";
cx.stroke();
}}
/>绘制调用只是录制:录制完成后由平台进行绘制,所以不会读回像素。录制结果作为一个操作发送给平台。
#2D 操作
这些操作是 HTML Canvas 2D API 的一个子集:TSX 中是 CanvasRenderingContext2D,Rust 中是 Painter 的方法,使用 Rust 风格的名称(begin_path、fill_rect、line_width、set_transform、draw_image 等;颜色通过 fill_color / stroke_color,渐变通过 fill_style / stroke_style)。支持的内容:
状态
| 成员 | Rust | 说明 |
|---|---|---|
save()、restore() | .save()、.restore() | 压入和弹出绘制状态(样式、变换、裁剪)。 |
fillStyle、strokeStyle | .fill_color(..)、.fill_style(..)、.stroke_color(..)、.stroke_style(..) | CSS 颜色、渐变,或插值为颜色的动画值。 |
lineWidth、lineCap、lineJoin、miterLimit | .line_width(..)、.line_cap(..)、.line_join(..)、.miter_limit(..) | 描边形状。 |
setLineDash(segments)、getLineDash()、lineDashOffset | .line_dash(..)、.line_dash_offset(..) | 虚线。 |
globalAlpha | .global_alpha(..) | 后续绘制的不透明度,0..1。 |
globalCompositeOperation | .composite(Composite::DestinationOut) | 合成方式:"source-over"、"destination-out"、"multiply"、"screen" 等。 |
shadowColor、shadowBlur、shadowOffsetX、shadowOffsetY | .shadow(color, blur, offset) | 阴影。 |
font | .font(..) | CSS font 简写:"600 14px Inter"(尺寸可用 px、pt 或 rem);small-caps 和 tabular-nums、oldstyle-nums 等数字形式可以写在尺寸之前。 |
fontVariantNumeric | .font_variant_numeric(..) | "tabular-nums" 让所有数字等宽,变化的数字(计数器、价格、坐标轴标签)不会跳动;还有 "proportional-nums"、"oldstyle-nums"、"lining-nums"、"normal"。这是 NSH 新增的(即 CSS 的 font-variant-numeric);设置 font 时会保留它。浏览器会忽略它。 |
textAlign、textBaseline | .text_align(..)、.text_baseline(..) | 文字位置。 |
变换:translate(x, y)、scale(x, y)、rotate(angle)(弧度,顺时针)、transform(a, b, c, d, e, f)、setTransform(a, b, c, d, e, f)、resetTransform()。
路径:beginPath()、moveTo、lineTo、quadraticCurveTo、bezierCurveTo、arc、arcTo、ellipse、rect、roundRect(x, y, w, h, radii)、closePath();然后是 fill(rule?)、stroke() 和 clip(rule?),它们也都接受一条路径(Rust 中的 fill_path、stroke_path、clip_path)。填充规则为 "nonzero" 和 "evenodd"。
矩形:fillRect、strokeRect、clearRect。
文字:fillText(text, x, y, maxWidth?)、strokeText(...)、measureText(text)(由平台的文字引擎按当前 font 同步测量;只有 width)。在 Rust 中,measure_text(font, text) 给出宽度。
命中测试:isPointInPath(path?, x, y, rule?) 和 isPointInStroke(path?, x, y)(Painter 上的 path_contains、stroke_contains、is_point_in_path 和 is_point_in_stroke)同步给出结果,依据录制的几何信息(当前变换、路径、lineWidth 和 lineCap)计算。保留绘制和画过的路径,就能找出点击位置下的形状:
let slices: Rc<Vec<Path>> = Rc::new(vec![svg_path("M50 50 L100 50 A50 50 0 0 1 50 100 Z") /* , ... */]);
let draw = {
let slices = slices.clone();
move || slices.iter().fold(Painter::new(), |p, s| p.fill_path(s, FillRule::NonZero))
};
canvas(draw.clone())
.size(px(100.0))
.claim_touches(TouchClaim::on_start())
.on_responder_release(move |e: &ResponderEvent| {
let drawing = draw();
if let Some(hit) = slices.iter().position(|s| drawing.path_contains(s, e.location, FillRule::NonZero)) {
selected.set(Some(hit));
}
})let cx: CanvasRenderingContext2D | undefined;
const slices = [new Path2D("M50 50 L100 50 A50 50 0 0 1 50 100 Z"), /* ... */];
<Canvas
style={{ width: 100, height: 100 }}
onStartShouldSetResponder={() => true}
onResponderRelease={(e) => {
const hit = slices.findIndex((p) => cx?.isPointInPath(p, e.locationX, e.locationY));
if (hit >= 0) select(hit);
}}
draw={(c) => {
cx = c;
slices.forEach((p) => c.fill(p));
}}
/>描边按圆角连接、不带虚线进行测试;以动画值给出的坐标按 0 计算。
渐变:createLinearGradient、createRadialGradient、createConicGradient,都通过 addColorStop(offset, color) 添加色标(Rust 中的 CanvasGradient::linear、radial、conic 和 add_color_stop)。
图片:drawImage(image, dx, dy)、drawImage(image, dx, dy, dw, dh)、drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)(Rust 中的 draw_image(url, dest) 和 draw_image_part(..))。
在 TSX 中,cx.canvas.width 和 cx.canvas.height 给出画布以点为单位的尺寸。
支持的范围:绘制操作。像素由平台写入,因此 getImageData / putImageData 等像素访问不在其中,filter 和在 drawImage 中使用 SVG 图片也不在其中。与 HTML Canvas 一样,无效的颜色或字体只会让那一个操作被丢弃。
#路径与 SVG 路径数据
构建一次的路径可以多次填充、描边或裁剪:Rust 中是 svg_path(d),TSX 中是 Path2D。两者都接受 SVG 路径数据,在原生端解析,用来画图标很方便:
use nsh_core::canvas::{FillRule, svg_path};
const HEART: &str = "M12 21s-7-4.35-9.5-8.5C.5 8.5 3 4 7 4c2 0 3.5 1 5 3 1.5-2 3-3 5-3 4 0 6.5 4.5 4.5 8.5C19 16.65 12 21 12 21z";
fn heart(size: f32, color: Rgba) -> impl IntoElement {
let path = svg_path(HEART);
canvas(
Painter::new()
.scale(size / 24.0, size / 24.0) // 路径画在 24 点的网格上
.fill_color(color)
.fill_path(&path, FillRule::NonZero),
)
.size(px(size))
}import { Canvas, Path2D } from "nsh";
const heart = new Path2D("M12 21s-7-4.35-9.5-8.5C.5 8.5 3 4 7 4c2 0 3.5 1 5 3 1.5-2 3-3 5-3 4 0 6.5 4.5 4.5 8.5C19 16.65 12 21 12 21z");
function Heart(props: { size: number; color: string }) {
return (
<Canvas
style={{ width: props.size, height: props.size }}
draw={(cx) => {
cx.scale(props.size / 24, props.size / 24); // 路径画在 24 点的网格上
cx.fillStyle = props.color;
cx.fill(heart);
}}
/>
);
}Path2D 拥有与上下文相同的路径方法(moveTo、arc、roundRect 等)以及 addPath(other)。new Path2D(other) 复制一条路径。
#渐变
canvas(move || {
let (w, h) = (width.get(), 120.0);
let linear = CanvasGradient::linear(point(px(0.0), px(0.0)), point(px(w), px(0.0)))
.add_color_stop(percentage(0.0), rgb(0x22d3ee))
.add_color_stop(percentage(1.0), rgb(0x6366f1));
let center = point(px(w / 2.0), px(h * 0.75));
let radial = CanvasGradient::radial(center, px(4.0), center, px(40.0))
.add_color_stop(percentage(0.0), rgb(0xfde68a))
.add_color_stop(percentage(1.0), rgb(0xf97316));
Painter::new()
.fill_style(linear)
.fill_rect(0.0, 0.0, w, h / 2.0)
.fill_style(radial)
.begin_path()
.arc(w / 2.0, h * 0.75, 28.0, 0.0, TAU)
.fill()
})
.h(px(120.0))
.on_layout(move |frame: &Bounds<Points>| width.set(frame.size.width.0))<Canvas
style={{ height: 120 }}
draw={(cx, { width, height }) => {
const linear = cx.createLinearGradient(0, 0, width, 0);
linear.addColorStop(0, "#22d3ee");
linear.addColorStop(1, "#6366f1");
cx.fillStyle = linear;
cx.fillRect(0, 0, width, height / 2);
const radial = cx.createRadialGradient(width / 2, height * 0.75, 4, width / 2, height * 0.75, 40);
radial.addColorStop(0, "#fde68a");
radial.addColorStop(1, "#f97316");
cx.fillStyle = radial;
cx.beginPath();
cx.arc(width / 2, height * 0.75, 28, 0, Math.PI * 2);
cx.fill();
}}
/>锥形渐变(createConicGradient(startAngle, x, y)、CanvasGradient::conic(start_angle, center))让颜色绕一个点旋转过渡;起始角以弧度表示,从 x 轴顺时针计算。把锥形渐变用在描边的圆弧上,就是渐变色的进度环。
#文字
Painter::new()
.font("600 20px system-ui")
.fill_color(rgb(0x1f2328))
.text_align(CanvasTextAlign::Center)
.text_baseline(CanvasTextBaseline::Middle)
.fill_text("72%", width / 2.0, height / 2.0)
.font("11px system-ui")
.fill_text("100", 32.0 - 6.0 - measure_text("11px system-ui", "100").0, 16.0)cx.font = "600 20px system-ui";
cx.fillStyle = "#1f2328";
cx.textAlign = "center";
cx.textBaseline = "middle";
cx.fillText("72%", width / 2, height / 2);
cx.font = "11px system-ui";
const label = "100";
cx.fillText(label, 32 - 6 - cx.measureText(label).width, 16);字体族与样式中的相同:系统字体族以及应用发布的字体名称(见字体)。
#图片
在 Rust 中,绘制图片接受 URL(资源的 URL 是 LOGO.url);在 TSX 中接受 URL 或导入的资源(import logo from "./logo.png")。图片在原生端加载,加载完成后画布会重新绘制,你不需要自己跟踪加载状态。远程 URL 受应用的网络能力声明约束。
iOS、macOS、Android、Windows、Linux 和 GPUI 上的画布绘制位图(PNG、JPEG、GIF、WebP 等):在这些平台上,SVG 来源按加载失败的图片处理,日志会指出它。SVG 图片请用 <Image> 显示,或把它们的形状作为 Path2D 路径绘制。在 Web 上,画布绘制浏览器能绘制的内容,包括 SVG。
const LOGO: ImageAsset = asset!("logo.png");
canvas(
Painter::new()
.save()
.begin_path()
.arc(60.0, 60.0, 56.0, 0.0, TAU)
.clip() // 圆形头像
.draw_image(LOGO.url, bounds(point(px(4.0), px(4.0)), size(px(112.0), px(112.0))))
.restore(),
)
.size(px(120.0))import { Canvas } from "nsh";
import logo from "./logo.png";
<Canvas
style={{ width: 120, height: 120 }}
draw={(cx) => {
cx.save();
cx.beginPath();
cx.arc(60, 60, 56, 0, Math.PI * 2);
cx.clip(); // 圆形头像
cx.drawImage(logo, 4, 4, 112, 112);
cx.restore();
}}
/>九个参数的形式(draw_image_part)中,源矩形以图片的像素为单位。
#用动画值驱动绘制
绘制调用中的任何数字参数,以及填充和描边的颜料,都可以是动画值或由它派生的节点。这类参数在平台绘制时才被读取;当动画值在动画中时,平台会自行逐帧重绘,无需重新录制绘制,也不运行你的任何代码。
use std::f32::consts::{FRAC_PI_2, TAU};
use nsh_core::{
canvas::{CanvasTextAlign, CanvasTextBaseline, LineCap},
ui::prelude::*,
};
pub fn ring() -> impl IntoElement {
let progress = animated_value(0.3);
let label = signal(30_u32);
let go = move |to: f32| {
label.set((to * 100.0).round() as u32);
progress.timing(to, 900.0, Easing::Curve(Curve::Cubic, EaseMode::InOut));
};
let (center_x, cy, r) = (70.0, 70.0, 60.0);
view()
.flex_1()
.items_center()
.justify_center()
.gap_4()
.child(
canvas(move || {
// 终止角在每一帧读取动画值。
let end = progress.mul(TAU).sub(FRAC_PI_2);
Painter::new()
.line_width(12.0)
.line_cap(LineCap::Round)
.stroke_color(rgb(0xe5e7eb))
.begin_path()
.arc(center_x, cy, r, 0.0, TAU)
.stroke()
.stroke_color(rgb(0x6366f1))
.begin_path()
.arc(center_x, cy, r, -FRAC_PI_2, &end)
.stroke()
.fill_color(rgb(0x1f2328))
.font("600 22px system-ui")
.text_align(CanvasTextAlign::Center)
.text_baseline(CanvasTextBaseline::Middle)
.fill_text(&format!("{}%", label.get()), center_x, cy)
})
.size(px(140.0)),
)
.child(
text("Animate")
.text_color(rgb(0x0969da))
.on_press(move || go(if label.get_untracked() < 50 { 0.85 } else { 0.2 })),
)
}import { Canvas, mount, signal, Text, View } from "nsh";
import { createAnimatedValue, Easing } from "nsh/animated";
function Ring() {
const progress = createAnimatedValue(0.3);
const label = signal(30);
const go = (to: number) => {
label.set(Math.round(to * 100));
progress.timing(to, { duration: 900, easing: Easing.inOut(Easing.cubic) });
};
return (
<View style={{ flex: 1, alignItems: "center", justifyContent: "center", gap: 16 }}>
<Canvas
style={{ width: 140, height: 140 }}
draw={(cx, { width, height }) => {
const centerX = width / 2, cy = height / 2, r = width / 2 - 10;
cx.lineWidth = 12;
cx.lineCap = "round";
cx.strokeStyle = "#e5e7eb";
cx.beginPath();
cx.arc(centerX, cy, r, 0, Math.PI * 2);
cx.stroke();
cx.strokeStyle = "#6366f1";
cx.beginPath();
// 终止角在每一帧读取动画值。
cx.arc(centerX, cy, r, -Math.PI / 2, progress.mul(Math.PI * 2).sub(Math.PI / 2));
cx.stroke();
cx.fillStyle = "#1f2328";
cx.font = "600 22px system-ui";
cx.textAlign = "center";
cx.textBaseline = "middle";
cx.fillText(`${label()}%`, centerX, cy);
}}
/>
<View onPress={() => go(Math.random())}>
<Text style={{ color: "#0969da" }}>Animate</Text>
</View>
</View>
);
}
mount(Ring);圆弧的终止角是 progress.mul(2π).sub(π/2):它是一个节点,所以进度环会在 900 毫秒内以原生方式扫过,而绘制只录制了一次(标签读取了 signal,因此再录制一次)。时钟的秒针也是同样的做法:rotate(seconds.mul(π / 30)),其中 seconds 做线性动画。
颜色也可以这样动起来:Rust 中为 .fill_style(&t.interpolate_colors(&[0.0, 1.0], &[rgb(0x54aeff), rgb(0xcf222e)])),TSX 中为 cx.fillStyle = t.interpolate({ inputRange: [0, 1], outputRange: ["#54aeff", "#cf222e"] })。
#画布上的触摸
画布是一个视图,因此接受按压事件和手势响应者系统。手写绘图就是把点记录到 signal 中再画出来:
let strokes = signal(Vec::<Vec<(f32, f32)>>::new());
canvas(move || {
strokes.with(|strokes| {
let mut p = Painter::new().line_width(3.0).line_cap(LineCap::Round).line_join(LineJoin::Round);
for stroke in strokes {
p = p.begin_path();
for (i, &(x, y)) in stroke.iter().enumerate() {
p = if i == 0 { p.move_to(x, y) } else { p.line_to(x, y) };
}
p = p.stroke();
}
p
})
})
.h(px(240.0))
.claim_touches(TouchClaim::on_start())
.keep_touches(true) // 外层的滚动视图不能接管触摸
.on_responder_grant(move |e: &ResponderEvent| {
strokes.update(|s| s.push(vec![(e.location.x.0, e.location.y.0)]))
})
.on_responder_move(move |e: &ResponderEvent| {
strokes.update(|s| {
if let Some(stroke) = s.last_mut() {
stroke.push((e.location.x.0, e.location.y.0));
}
})
})const strokes = signal<[number, number][][]>([]);
<Canvas
style={{ height: 240 }}
onStartShouldSetResponder={() => true}
onResponderGrant={(e) => {
strokes.set([...strokes(), [[e.locationX, e.locationY]]]);
return true; // 外层的滚动视图不能接管触摸
}}
onResponderMove={(e) => {
const all = strokes();
strokes.set([...all.slice(0, -1), [...all[all.length - 1], [e.locationX, e.locationY]]]);
}}
draw={(cx) => {
cx.lineWidth = 3;
cx.lineCap = "round";
cx.lineJoin = "round";
for (const s of strokes()) {
cx.beginPath();
s.forEach(([x, y], i) => (i === 0 ? cx.moveTo(x, y) : cx.lineTo(x, y)));
cx.stroke();
}
}}
/>#性能
- 绘制在 Rust 中录制(在 TSX 中是 JS 调用上下文时),并作为一个操作发送给平台;不存在每次调用都跨越一次桥的开销。
- 每次绘制都重新录制全部内容。让工作量与显示的内容相称,把每帧都变化的数据放在动画值而不是 signal 中。
- 如果一幅绘制既有静态部分又有频繁变化的部分,把它拆成上下叠放的两个画布,这样只有一个需要重绘。
- 在可缩放的滚动视图中,缩放结束后画布会按缩放后的分辨率重新绘制。