Blazor 问题呈现从 Web Api 返回的数据

【问题标题】：Blazor problem rendering data returned from Web ApiBlazor 问题呈现从 Web Api 返回的数据
【发布时间】：2019-05-24 09:39:28
【问题描述】：

我希望有人能建议我如何找出这个潜在问题所在。我认为是序列化/反序列化而不是网格。

我正在尝试使用 Syncfusion Blazor 网格组件呈现从 ASP.NET Web Api 返回的数据。 JliffDocument 来自open source library。也许对象是由复合类型构成的事实是一个促成因素，但我可以在单元测试中成功地序列化和反序列化对象图。例如。 TextElement 实现一个接口，Segment 存储该接口的集合。

This Works（但 jlfDoc 是本地创建的数据）

<EjsGrid id="Grid" DataSource="@jlfDoc.Segments" Toolbar="@(new List<string>() { "Add", "Edit", "Delete", "Update", "Cancel" })" AllowPaging="true">
    <GridPageSettings PageSize="5"></GridPageSettings>
    <GridEditSettings AllowAdding="true" AllowDeleting="true" AllowEditing="true"></GridEditSettings>
    <GridColumns>
        <GridColumn Field="SourceText" HeaderText="Source"  IsPrimaryKey="true" TextAlign="@Syncfusion.EJ2.RazorComponents.Grids.TextAlign.Right" Width="120"></GridColumn>
        <GridColumn Field="TargetText" HeaderText="Target" Width="150"></GridColumn>
    </GridColumns>
</EjsGrid>

<hr/>
<p>@rawContent</p>

<h1>There are @segments Segments</h1>

<i>@oneTextString</i>


@functions {
    public object gridData { get; set; }
    public JliffDocument jlfDoc { get; set; }
    public string rawContent { get; set; }
    public int segments { get; set; }
    public string oneTextString { get; set; }
    protected override async Task OnInitAsync()
    {
        jlfDoc = new JliffDocument("en-US",
            "it-IT",
            new File("f1",
                new Unit("u1",
                    new Segment(
                        new TextElement("Hello"),
                        new TextElement("Ciao")),
                    new Segment(
                        new TextElement("Goodnight"),
                        new TextElement("Buonanotte")))));
    }
}

这也部分有效（rawContent、segments 和 oneTextString 变量 DO 如果 jlfDoc = jd; 被注释掉并且 @987654332 @被移除，说明jd的Api调用和反序列化成功。

<EjsGrid id="Grid" DataSource="@gridData" Toolbar="@(new List<string>() { "Add", "Edit", "Delete", "Update", "Cancel" })" AllowPaging="true">
    <GridPageSettings PageSize="5"></GridPageSettings>
    <GridEditSettings AllowAdding="true" AllowDeleting="true" AllowEditing="true"></GridEditSettings>
    <GridColumns>
        <GridColumn Field="SourceText" HeaderText="Source"  IsPrimaryKey="true" TextAlign="@Syncfusion.EJ2.RazorComponents.Grids.TextAlign.Right" Width="120"></GridColumn>
        <GridColumn Field="TargetText" HeaderText="Target" Width="150"></GridColumn>
    </GridColumns>
</EjsGrid>

<hr/>
<p>@rawContent</p>

<h1>There are @segments Segments</h1>

<i>@oneTextString</i>


@functions {
    public object gridData { get; set; }
    public JliffDocument jlfDoc { get; set; }
    public string rawContent { get; set; }
    public int segments { get; set; }
    public string oneTextString { get; set; }
    protected override async Task OnInitAsync()
    {
        string serverDoc = String.Empty;

        using (HttpClient client = new HttpClient())
        {
            serverDoc = await client.GetStringAsync("https://localhost:44381/api/document");
        }

        rawContent = serverDoc;

        JliffDocument jd;
        JSON.MakeInto(JSON.Load(serverDoc), out jd);
        segments = jd.Segments.Count;
        oneTextString = jd.Segments[0].TargetText;
        //jlfDoc = jd;
    }
}

这两行更改失败

====> DataSource reinstated |
                            |
<EjsGrid id="Grid" DataSource="@jlfDoc.Segments" Toolbar="@(new List<string>() { "Add", "Edit", "Delete", "Update", "Cancel" })" AllowPaging="true">
    <GridPageSettings PageSize="5"></GridPageSettings>
    <GridEditSettings AllowAdding="true" AllowDeleting="true" AllowEditing="true"></GridEditSettings>
    <GridColumns>
        <GridColumn Field="SourceText" HeaderText="Source"  IsPrimaryKey="true" TextAlign="@Syncfusion.EJ2.RazorComponents.Grids.TextAlign.Right" Width="120"></GridColumn>
        <GridColumn Field="TargetText" HeaderText="Target" Width="150"></GridColumn>
    </GridColumns>
</EjsGrid>

<hr/>
<p>@rawContent</p>

<h1>There are @segments Segments</h1>

<i>@oneTextString</i>


@functions {
    public object gridData { get; set; }
    public JliffDocument jlfDoc { get; set; }
    public string rawContent { get; set; }
    public int segments { get; set; }
    public string oneTextString { get; set; }
    protected override async Task OnInitAsync()
    {
        string serverDoc = String.Empty;

        using (HttpClient client = new HttpClient())
        {
            serverDoc = await client.GetStringAsync("https://localhost:44381/api/document");
        }

        rawContent = serverDoc;

        JliffDocument jd;
        JSON.MakeInto(JSON.Load(serverDoc), out jd);
        segments = jd.Segments.Count;
        oneTextString = jd.Segments[0].TargetText;
        jlfDoc = jd;                                  <==== UNCOMMENTED
    }
}

浏览器工具控制台的错误是：

WASM: Unhandled exception rendering component:
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM: System.NullReferenceException: Object reference not set to an instance of an object.
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Blazorgrid.Pages.Index.BuildRenderTree (Microsoft.AspNetCore.Components.RenderTree.RenderTreeBuilder builder) [0x00035] in <1cc78bbb60324b879905902bf1175ac2>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.ComponentBase.<.ctor>b__5_0 (Microsoft.AspNetCore.Components.RenderTree.RenderTreeBuilder builder) <0x20a7d38 + 0x00020> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at (wrapper delegate-invoke) <Module>.invoke_void_RenderTreeBuilder(Microsoft.AspNetCore.Components.RenderTree.RenderTreeBuilder)
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.Rendering.ComponentState.RenderIntoBatch (Microsoft.AspNetCore.Components.Rendering.RenderBatchBuilder batchBuilder, Microsoft.AspNetCore.Components.RenderFragment renderFragment) <0x20a7190 + 0x00092> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.Rendering.Renderer.RenderInExistingBatch (Microsoft.AspNetCore.Components.Rendering.RenderQueueEntry renderQueueEntry) <0x20a6e48 + 0x00036> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.Rendering.Renderer.ProcessRenderQueue () <0x20a3eb8 + 0x00056> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM: Unhandled exception rendering component:
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM: Microsoft.JSInterop.JSException: No element is currently associated with component 6
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM: Error: No element is currently associated with component 6
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:     at e.updateComponent (http://localhost:53239/_framework/blazor.webassembly.js:1:13495)
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:     at Object.t.renderBatch (http://localhost:53239/_framework/blazor.webassembly.js:1:1686)
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:     at window.Blazor._internal.renderBatch (http://localhost:53239/_framework/blazor.webassembly.js:1:30422)
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:     at _mono_wasm_invoke_js_unmarshalled (http://localhost:53239/_framework/wasm/mono.js:1:170240)
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:     at wasm-function[3762]:35
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:     at wasm-function[3770]:572
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:     at wasm-function[3771]:211
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:     at wasm-function[2977]:241
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:     at wasm-function[601]:22615
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:     at wasm-function[601]:21413
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Mono.WebAssembly.Interop.MonoWebAssemblyJSRuntime.InvokeUnmarshalled[T0,T1,T2,TRes] (System.String identifier, T0 arg0, T1 arg1, T2 arg2) <0x236bc00 + 0x00044> in <0544639430f948ad82c54a3effb7ea00>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Mono.WebAssembly.Interop.MonoWebAssemblyJSRuntime.InvokeUnmarshalled[T0,T1,TRes] (System.String identifier, T0 arg0, T1 arg1) <0x236bb28 + 0x00016> in <0544639430f948ad82c54a3effb7ea00>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Blazor.Rendering.WebAssemblyRenderer.UpdateDisplayAsync (Microsoft.AspNetCore.Components.Rendering.RenderBatch& batch) <0x236ba30 + 0x0001e> in <91b564433c704544acd7894656c3aff9>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.Rendering.Renderer.ProcessRenderQueue () <0x20a3eb8 + 0x000be> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM: Unhandled exception rendering component:
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM: System.NullReferenceException: Object reference not set to an instance of an object.
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.RenderTree.RenderTreeFrame..ctor (System.Int32 sequence, System.Type componentType, System.Int32 subtreeLength, Microsoft.AspNetCore.Components.Rendering.ComponentState componentState) <0x21001d8 + 0x0001c> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.RenderTree.RenderTreeFrame.WithComponent (Microsoft.AspNetCore.Components.Rendering.ComponentState componentState) <0x2100178 + 0x0001c> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder.UpdateRetainedChildComponent (Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder+DiffContext& diffContext, System.Int32 oldComponentIndex, System.Int32 newComponentIndex) <0x24c9160 + 0x0004c> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder.AppendDiffEntriesForFramesWithSameSequence (Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder+DiffContext& diffContext, System.Int32 oldFrameIndex, System.Int32 newFrameIndex) <0x24b9610 + 0x00368> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder.AppendDiffEntriesForRange (Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder+DiffContext& diffContext, System.Int32 oldStartIndex, System.Int32 oldEndIndexExcl, System.Int32 newStartIndex, System.Int32 newEndIndexExcl) <0x20a9950 + 0x000ae> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder.ComputeDiff (Microsoft.AspNetCore.Components.Rendering.Renderer renderer, Microsoft.AspNetCore.Components.Rendering.RenderBatchBuilder batchBuilder, System.Int32 componentId, Microsoft.AspNetCore.Components.RenderTree.ArrayRange`1[T] oldTree, Microsoft.AspNetCore.Components.RenderTree.ArrayRange`1[T] newTree) <0x20a9558 + 0x0009e> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.Rendering.ComponentState.RenderIntoBatch (Microsoft.AspNetCore.Components.Rendering.RenderBatchBuilder batchBuilder, Microsoft.AspNetCore.Components.RenderFragment renderFragment) <0x20a7190 + 0x0013a> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.Rendering.Renderer.RenderInExistingBatch (Microsoft.AspNetCore.Components.Rendering.RenderQueueEntry renderQueueEntry) <0x20a6e48 + 0x00036> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.Rendering.Renderer.ProcessRenderQueue () <0x20a3eb8 + 0x00056> in <13ab8f8dacb6489b93c9655168c56037>:0 
d.printErr @ blazor.webassembly.js:1

【问题讨论】：

  • 如果您使用的是 Blazor 客户端，为什么要在 using block 中实例化 HttpClient ？ HttpClient 由框架作为服务提供，您应该将其注入到您的组件中。它被配置为在 SPA 应用程序上使用，包括设置基地址。为什么要使用这样的 url：“localhost:44381/api/document”。尝试相对网址。
  • 在 Blazor 服务器端，using (httpClient = ...) 与 wrong 一样。
  • 您可能应该查看 FetchData 演示页面并使用相同的方法：@if(jlfDoc != null) { &lt;Grid&gt; ... &lt;/Grid&gt; }
  • @Issac，是的，我知道IHttpClientFactory，但这不是我的问题的核心。这些天，我在我的大部分代码中都使用了类型化的 http 客户端。
  • @HenkHolterman，太棒了！我知道我被一些简单而明显的事情蒙蔽了双眼。我应该早点从代码中退后一步。请提交您的空检查评论作为答案，我会这样标记。

标签： syncfusion blazor


【解决方案1】：

我们怀疑您正在尝试使用未初始化对象的引用（“jlfDoc”可能为空）。因此，我们建议您在访问其属性之前添加条件以检查对象是否为空，以解决@HenkHolterman 在评论部分中提到的问题。

【讨论】：

    【解决方案2】：

    此外，如果您忘记在要从组件外部访问的组件属性上使用 [Parameter] 属性，您可能会收到“当前没有与组件关联的元素”错误。

    【讨论】：

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