com.unity.uiwidgets/Runtime/ui/renderer/cmdbufferCanvas/rendering/canvas_shader_initializer.cs

using System;
using Unity.UIWidgets.foundation;
using UnityEngine;
using UnityEngine.Rendering;

namespace Unity.UIWidgets.ui {
    enum InitStage {
        NotPrepared,
        Prepared,
        Ready
    }

    static partial class CanvasShader {
        static InitStage initStage = InitStage.NotPrepared;
        static int initialFrameCount;
        static Shader testShader;
        const string testShaderName = "UIWidgets/canvas_convexFill_cb";

        static bool OnNotPrepared() {
            initStage = InitStage.Prepared;

            initialFrameCount = Time.frameCount;
            testShader = GetShader(testShaderName);
            var material = new Material(testShader);
            //for Unity 2018 or below, shader is compiled after Shader.Find() call immediately,
            //therefore we can just skip the manually preload if the compilation fails
            if (!material.shader.isSupported) {
                ObjectUtils.SafeDestroy(material);
                return OnPrepared(true);
            }

            using (var cmdBuf = new CommandBuffer()) {
                var renderTarget = new RenderTexture(1, 1, 1);
                cmdBuf.SetRenderTarget(renderTarget);

                var mesh = new Mesh {
                    vertices = new[] {new Vector3(0, 0, 0), new Vector3(0, 1, 0), new Vector3(1, 1, 0)},
                    uv = new[] {new Vector2(0, 0), new Vector2(0, 1), new Vector2(1, 1)},
                    triangles = new[] {0, 1, 2}
                };
                cmdBuf.DrawMesh(mesh, Matrix4x4.identity, material);
                cmdBuf.DisableScissorRect();
                Graphics.ExecuteCommandBuffer(cmdBuf);

                ObjectUtils.SafeDestroy(renderTarget);
                ObjectUtils.SafeDestroy(mesh);
            }

            ObjectUtils.SafeDestroy(material);

            return false;
        }

        static bool OnPrepared(bool forceReady = false) {
            D.assert(initStage == InitStage.Prepared);
            if (!forceReady && initialFrameCount >= Time.frameCount) {
                return false;
            }

            initStage = InitStage.Ready;
            DoPrepare();
            return true;
        }

        static void DoPrepare() {
            D.assert(testShader != null);
            var isShaderSupported = testShader.isSupported;
            testShader = null;
            supportComputeBuffer = enableComputeBuffer && SystemInfo.supportsComputeShaders && isShaderSupported;

            if (!supportComputeBuffer) {
                var convexFillShader = GetShader("UIWidgets/canvas_convexFill");
                var fill0Shader = GetShader("UIWidgets/canvas_fill0");
                var fill1Shader = GetShader("UIWidgets/canvas_fill1");
                var stroke0Shader = GetShader("UIWidgets/canvas_stroke0");
                var stroke1Shader = GetShader("UIWidgets/canvas_stroke1");
                var texShader = GetShader("UIWidgets/canvas_tex");
                var stencilShader = GetShader("UIWidgets/canvas_stencil");
                var filterShader = GetShader("UIWidgets/canvas_filter");
                var shadowBoxShader = GetShader("UIWidgets/ShadowBox");
                var shadowRBoxShader = GetShader("UIWidgets/ShadowRBox");
                var strokeAlphaShader = GetShader("UIWidgets/canvas_strokeAlpha");

                _convexFillMat = new MaterialByBlendModeStencilComp(convexFillShader);
                _fill0Mat = new MaterialByStencilComp(fill0Shader);
                _fill1Mat = new MaterialByBlendMode(fill1Shader);
                _stroke0Mat = new MaterialByBlendModeStencilComp(stroke0Shader);
                _stroke1Mat = new Material(stroke1Shader) {hideFlags = HideFlags.HideAndDontSave};
                _strokeAlphaMat = new MaterialByBlendModeStencilComp(strokeAlphaShader);
                _texMat = new MaterialByBlendModeStencilComp(texShader);
                _stencilMat = new Material(stencilShader) {hideFlags = HideFlags.HideAndDontSave};
                _filterMat = new Material(filterShader) {hideFlags = HideFlags.HideAndDontSave};
                _shadowBox = new Material(shadowBoxShader) {hideFlags = HideFlags.HideAndDontSave};
                _shadowRBox = new Material(shadowRBoxShader) {hideFlags = HideFlags.HideAndDontSave};
            }
            else {
                var convexFillShaderCompute = GetShader("UIWidgets/canvas_convexFill_cb");
                var fill0ShaderCompute = GetShader("UIWidgets/canvas_fill0_cb");
                var fill1ShaderCompute = GetShader("UIWidgets/canvas_fill1_cb");
                var stroke0ShaderCompute = GetShader("UIWidgets/canvas_stroke0_cb");
                var stroke1ShaderCompute = GetShader("UIWidgets/canvas_stroke1_cb");
                var texShaderCompute = GetShader("UIWidgets/canvas_tex_cb");
                var stencilShaderCompute = GetShader("UIWidgets/canvas_stencil_cb");
                var filterShaderCompute = GetShader("UIWidgets/canvas_filter_cb");
                var shadowBoxShaderCompute = GetShader("UIWidgets/ShadowBox_cb");
                var shadowRBoxShaderCompute = GetShader("UIWidgets/ShadowRBox_cb");
                var strokeAlphaShaderCompute = GetShader("UIWidgets/canvas_strokeAlpha_cb");

                _convexFillMat = new MaterialByBlendModeStencilComp(convexFillShaderCompute);
                _fill0Mat = new MaterialByStencilComp(fill0ShaderCompute);
                _fill1Mat = new MaterialByBlendMode(fill1ShaderCompute);
                _stroke0Mat = new MaterialByBlendModeStencilComp(stroke0ShaderCompute);
                _stroke1Mat = new Material(stroke1ShaderCompute) {hideFlags = HideFlags.HideAndDontSave};
                _strokeAlphaMat = new MaterialByBlendModeStencilComp(strokeAlphaShaderCompute);
                _texMat = new MaterialByBlendModeStencilComp(texShaderCompute);
                _stencilMat = new Material(stencilShaderCompute) {hideFlags = HideFlags.HideAndDontSave};
                _filterMat = new Material(filterShaderCompute) {hideFlags = HideFlags.HideAndDontSave};
                _shadowBox = new Material(shadowBoxShaderCompute) {hideFlags = HideFlags.HideAndDontSave};
                _shadowRBox = new Material(shadowRBoxShaderCompute) {hideFlags = HideFlags.HideAndDontSave};
            }
        }

        public static bool isReady() {
            switch (initStage) {
                case InitStage.NotPrepared:
                    return OnNotPrepared();
                case InitStage.Prepared:
                    return OnPrepared();
                case InitStage.Ready:
                    return true;
                default:
                    throw new ArgumentOutOfRangeException();
            }
        }
    }
}