1// Copyright 2014 The Flutter Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5// This example shows how to use the ui.Canvas interface to draw various shapes
6// with gradients and transforms.
7
8import 'dart:math' as math;
9import 'dart:typed_data';
10import 'dart:ui' as ui;
11
12// The FlutterView into which this example will draw; set in the main method.
13late final ui.FlutterView view;
14
15ui.Picture paint(ui.Rect paintBounds) {
16 // First we create a PictureRecorder to record the commands we're going to
17 // feed in the canvas. The PictureRecorder will eventually produce a Picture,
18 // which is an immutable record of those commands.
19 final ui.PictureRecorder recorder = ui.PictureRecorder();
20
21 // Next, we create a canvas from the recorder. The canvas is an interface
22 // which can receive drawing commands. The canvas interface is modeled after
23 // the SkCanvas interface from Skia. The paintBounds establishes a "cull rect"
24 // for the canvas, which lets the implementation discard any commands that
25 // are entirely outside this rectangle.
26 final ui.Canvas canvas = ui.Canvas(recorder, paintBounds);
27
28 final ui.Paint paint = ui.Paint();
29 canvas.drawPaint(ui.Paint()..color = const ui.Color(0xFFFFFFFF));
30
31 final ui.Size size = paintBounds.size;
32 final ui.Offset mid = size.center(ui.Offset.zero);
33 final double radius = size.shortestSide / 2.0;
34
35 final double devicePixelRatio = view.devicePixelRatio;
36 final ui.Size logicalSize = view.physicalSize / devicePixelRatio;
37
38 // Saves a copy of current transform onto the save stack.
39 canvas.save();
40
41 // Transforms that occur after this point apply only to the
42 // yellow-bluish rectangle.
43
44 // This line will cause the transform to shift entirely outside the paint
45 // boundaries, which will cause the canvas interface to discard its
46 // commands. Comment it out to see it on screen.
47 canvas.translate(-mid.dx / 2.0, logicalSize.height * 2.0);
48
49 // Clips the current transform.
50 canvas.clipRect(
51 ui.Rect.fromLTRB(0, radius + 50, logicalSize.width, logicalSize.height),
52 clipOp: ui.ClipOp.difference,
53 );
54
55 // Shifts the coordinate space of and rotates the current transform.
56 canvas.translate(mid.dx, mid.dy);
57 canvas.rotate(math.pi / 4);
58
59 final ui.Gradient yellowBlue = ui.Gradient.linear(
60 ui.Offset(-radius, -radius),
61 ui.Offset.zero,
62 <ui.Color>[const ui.Color(0xFFFFFF00), const ui.Color(0xFF0000FF)],
63 );
64
65 // Draws a yellow-bluish rectangle.
66 canvas.drawRect(
67 ui.Rect.fromLTRB(-radius, -radius, radius, radius),
68 ui.Paint()..shader = yellowBlue,
69 );
70
71 // Transforms that occur after this point apply only to the
72 // yellow circle.
73
74 // Scale x and y by 0.5.
75 final Float64List scaleMatrix = Float64List.fromList(<double>[
76 0.5,
77 0.0,
78 0.0,
79 0.0,
80 0.0,
81 0.5,
82 0.0,
83 0.0,
84 0.0,
85 0.0,
86 1.0,
87 0.0,
88 0.0,
89 0.0,
90 0.0,
91 1.0,
92 ]);
93 canvas.transform(scaleMatrix);
94
95 // Sets paint to transparent yellow.
96 paint.color = const ui.Color.fromARGB(128, 0, 255, 0);
97
98 // Draws a transparent yellow circle.
99 canvas.drawCircle(ui.Offset.zero, radius, paint);
100
101 // Restores the transform from before `save` was called.
102 canvas.restore();
103
104 // Sets paint to transparent red.
105 paint.color = const ui.Color.fromARGB(128, 255, 0, 0);
106
107 // This circle is drawn on top of the previous layer that contains
108 // the rectangle and smaller circle.
109 canvas.drawCircle(const ui.Offset(150.0, 300.0), radius, paint);
110
111 // When we're done issuing painting commands, we end the recording and receive
112 // a Picture, which is an immutable record of the commands we've issued. You
113 // can draw a Picture into another canvas or include it as part of a
114 // composited scene.
115 return recorder.endRecording();
116}
117
118ui.Scene composite(ui.Picture picture, ui.Rect paintBounds) {
119 final double devicePixelRatio = view.devicePixelRatio;
120 final Float64List deviceTransform = Float64List(16)
121 ..[0] = devicePixelRatio
122 ..[5] = devicePixelRatio
123 ..[10] = 1.0
124 ..[15] = 1.0;
125 final ui.SceneBuilder sceneBuilder = ui.SceneBuilder()
126 ..pushTransform(deviceTransform)
127 ..addPicture(ui.Offset.zero, picture)
128 ..pop();
129 return sceneBuilder.build();
130}
131
132void beginFrame(Duration timeStamp) {
133 final ui.Rect paintBounds = ui.Offset.zero & (view.physicalSize / view.devicePixelRatio);
134 final ui.Picture picture = paint(paintBounds);
135 final ui.Scene scene = composite(picture, paintBounds);
136 view.render(scene);
137}
138
139void main() {
140 // TODO(goderbauer): Create a window if embedder doesn't provide an implicit view to draw into.
141 assert(ui.PlatformDispatcher.instance.implicitView != null);
142 view = ui.PlatformDispatcher.instance.implicitView!;
143
144 ui.PlatformDispatcher.instance
145 ..onBeginFrame = beginFrame
146 ..scheduleFrame();
147}
148