gfx 各個類的原始碼解讀（6） WebGL2CommandBuffer
WebGL2CommandBuffer 在 webgl2-command-buffer.ts
對WebGL2CmdPackage類命令填充和狀態儲存,
WebGL2CommandAllocator 負責cmd的alloc，
上面兩個類例項在成員變數裡，不是全域性的
成員變數：

    protected _cmdAllocator: WebGL2CommandAllocator = new WebGL2CommandAllocator();
    protected _cmdAllocator: WebGL2CommandAllocator = new WebGL2CommandAllocator();
 

狀態儲存成員變數（後面用到會詳細解讀）

    protected _curGPUPipelineState: IWebGL2GPUPipelineState | null = null;
    protected _curGPUDescriptorSets: IWebGL2GPUDescriptorSet[] = [];
    protected _curGPUInputAssembler: IWebGL2GPUInputAssembler | null = null;
    protected _curDynamicOffsets: number[] = Array(8).fill(0);
    protected _curDynamicStates: DynamicStates = new DynamicStates();
    protected _isStateInvalied = false;

 

比如對WebGL2CmdBeginRenderPass命令的填充

    public beginRenderPass (
        renderPass: RenderPass,
        framebuffer: Framebuffer,
        renderArea: Rect,
        clearColors: Color[],
        clearDepth: number,
        clearStencil: number,
    ) {
        //pool alloc cmd
        const cmd = this._cmdAllocator.beginRenderPassCmdPool.alloc(WebGL2CmdBeginRenderPass);  
        
        cmd.gpuRenderPass = (renderPass as WebGL2RenderPass).gpuRenderPass;
        cmd.gpuFramebuffer = (framebuffer as WebGL2Framebuffer).gpuFramebuffer;
        cmd.renderArea = renderArea;
        for (let i = 0; i < clearColors.length; ++i) {
            cmd.clearColors[i] = clearColors[i];
        }
        cmd.clearDepth = clearDepth;
        cmd.clearStencil = clearStencil;
        //cmdPackage裝入cmd
        this.cmdPackage裝入cmd.beginRenderPassCmds.push(cmd);
        this.cmdPackage.cmds.push(WebGL2Cmd.BEGIN_RENDER_PASS);
        this._isInRenderPass = true;
    }