Delphi 12多设备表单ListView添加图片不显示问题求助

问题说明 #

在Delphi 12多设备表单中使用ListView加载网络JPG图片时，仅显示空白区域。当前代码为了判空两次调用图片下载函数，尝试过表格、ImageList存储图片的示例但未解决问题。

现有代码 #

主逻辑代码 #

var
  jsa: TJSONArray;
  jso : TJsonObject;
  i: Integer;
  Name: String;
  LItem: TListViewItem;
begin

if Length(S) > 10 then
begin
  jso := TJSonObject.ParseJSONValue(S) as TJsonObject;
  if jso <> nil then
    Try
      jsa := jso.GetValue('results') as TJSONArray;
      if Jsa.Count > 0 then
      begin
        for I := 0 to jsa.Count-1 do
        begin                             
          LItem := ListView1.Items.Add;
          if GetImageFromURL(jsa[i].GetValue<string>('image')) <> nil then
            LItem.Bitmap := GetImageFromURL(jsa[i].GetValue<string>('image'));
          LItem.Detail := jsa[i].GetValue<string>('text');
          LItem.Text := jsa[i].GetValue<string>('expire');
        end;
      end;
    Finally
      jso.Free;
    End;

end;

获取网络图片函数 #

function TPromoForm.GetImageFromURL(const S: String): TBitmap;
var
  ms       : TMemoryStream;
  httpCli  : TNetHTTPClient;
  resp     : IHTTPResponse;
  url      : String;
  Image: TBitmap;
begin
result := nil;
httpCli := TNetHTTPClient.Create(nil);
try
  httpCli.UserAgent := 'Mozilla/5.0 (Windows NT 10.0; WOW64; rv:44.0) Gecko/20100101 Firefox/44.0';
  ms := TMemoryStream.Create();
  Image  := TBitmap.Create;
  Image := nil;
  try
    url  := S;
    resp := httpCli.Get(url, ms);
    if resp.StatusCode <> 200 then
      Showmessage(Format('HTTP Error=%d %s', [resp.StatusCode, resp.StatusText]))
    else
    begin
      try
        Image  := TBitmap.Create;
        ms.Position := 0;
        Image.LoadFromStream(ms);
        result := Image;
      finally
        Image.Free;
      end;
    end;
  finally
    ms.Free;
  end;
finally
  httpCli.Free;
end;
end;

问题分析与修复方案 #

1. 核心内存错误导致图片丢失 #

在GetImageFromURL函数中，加载好的Image赋值给result后，立刻在finally块执行Image.Free，导致返回的TBitmap指针指向已释放的内存，ListView无法正常显示图片。

修复后的GetImageFromURL函数：

function TPromoForm.GetImageFromURL(const S: String): TBitmap;
var
  ms       : TMemoryStream;
  httpCli  : TNetHTTPClient;
  resp     : IHTTPResponse;
  url      : String;
begin
  Result := nil;
  httpCli := TNetHTTPClient.Create(nil);
  try
    httpCli.UserAgent := 'Mozilla/5.0 (Windows NT 10.0; WOW64; rv:44.0) Gecko/20100101 Firefox/44.0';
    ms := TMemoryStream.Create();
    try
      url := S;
      resp := httpCli.Get(url, ms);
      if resp.StatusCode = 200 then
      begin
        Result := TBitmap.Create;
        try
          ms.Position := 0;
          Result.LoadFromStream(ms);
        except
          Result.Free;
          Result := nil;
          raise;
        end;
      end
      else
      begin
        Showmessage(Format('HTTP Error=%d %s', [resp.StatusCode, resp.StatusText]));
      end;
    finally
      ms.Free;
    end;
  finally
    httpCli.Free;
  end;
end;

2. 避免重复下载图片 #

原代码两次调用GetImageFromURL会重复发起HTTP请求，浪费资源且降低效率。修改主逻辑，仅调用一次并缓存结果：

var
  jsa: TJSONArray;
  jso : TJsonObject;
  i: Integer;
  LItem: TListViewItem;
  imgBitmap: TBitmap;
begin
if Length(S) > 10 then
begin
  jso := TJSonObject.ParseJSONValue(S) as TJsonObject;
  if jso <> nil then
    Try
      jsa := jso.GetValue('results') as TJSONArray;
      if Jsa.Count > 0 then
      begin
        for I := 0 to jsa.Count-1 do
        begin                             
          LItem := ListView1.Items.Add;
          imgBitmap := GetImageFromURL(jsa[i].GetValue<string>('image'));
          if imgBitmap <> nil then
            LItem.Bitmap := imgBitmap
          else
            LItem.Bitmap := nil; // 可替换为默认占位图
          LItem.Detail := jsa[i].GetValue<string>('text');
          LItem.Text := jsa[i].GetValue<string>('expire');
        end;
      end;
    Finally
      jso.Free;
    End;
end;

3. 多设备下额外注意事项 #

• 确保ListView的ItemAppearance设置为包含图片的样式（如DynamicAppearance并添加Image项）
• 跨平台环境下需引用Vcl.Imaging.jpeg单元以支持JPG格式加载
• 若图片尺寸过大，可在加载后缩放至ListView适配大小，避免显示异常

内容的提问来源于stack exchange，提问作者Kim HJ