更新：

发现错误

while (msg.message != WM_QUIT)
    {
        if (PeekMessage(&msg, nullptr, 0, 0, PM_REMOVE)) {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
        rend.DrawLine();
    }

效果：

仍是黑色背景

原文

现象如图

我的代码（其他代码都是vs新建项目自带的，只把menu、快捷键删掉了）

Renderer.h

#pragma once

#include <d2d1.h>
#pragma comment(lib, "d2d1.lib")

class Renderer {
private:
    ID2D1Factory *fac;
    ID2D1HwndRenderTarget *rt;
    ID2D1SolidColorBrush *bru;
    RECT rc;
public:
    Renderer(HWND hwnd) {
        D2D1CreateFactory(
            D2D1_FACTORY_TYPE_SINGLE_THREADED,
            &fac
        );

        GetClientRect(hwnd, &rc);
        fac->CreateHwndRenderTarget(
            D2D1::RenderTargetProperties(),
            D2D1::HwndRenderTargetProperties(
                hwnd,
                D2D1::SizeU(
                    rc.right - rc.left,
                    rc.bottom - rc.top
                )
            ),
            &rt
        );

        rt->CreateSolidColorBrush(
            D2D1::ColorF(D2D1::ColorF::Blue),
            &bru
        );
    }

    void DrawLine() {
        rt->BeginDraw();
        
        rt->DrawLine(
            D2D1::Point2F(20, 20),
            D2D1::Point2F(200, 200),
            bru
        );

        rt->EndDraw();
    }
};

主循环的修改内容

Renderer rend(hWnd);

    MSG msg;
    PeekMessage(&msg, nullptr, 0, 0, PM_REMOVE);
    TranslateMessage(&msg);
    DispatchMessage(&msg);
    // 主消息循环:
    while (msg.message != WM_QUIT)
    {
        PeekMessage(&msg, nullptr, 0, 0, PM_REMOVE);
        TranslateMessage(&msg);
        DispatchMessage(&msg);
        rend.DrawLine();
    }

完全按照microsoft这个教程写的

https://docs.microsoft.com/zh-cn/windows/win32/direct2d/getting-started-with-direct2d

我想要达到的结果

白色背景，一条蓝色的线