我创建了Xamarin项目,并为其添加了几个.Net标准类库(每层一个:数据访问,服务层等)。然后,在ServiceLayer项目中,我实现了一个方法,它从我的Web API(外部ASP Net Core项目)中获取数据。说到httpClient.GetAsync(),android app崩溃了。更重要的是,当我剪切这段代码并将其粘贴到默认的xamarin .Net标准库中时,一切正常。有任何想法吗?
代码在这里:
HttpClient httpClient = new HttpClient();
var responseMessage = await httpClient.GetStringAsync(uri);
更新:
在Viewmodel中:
constructor(IServiceLayerService service){
_ServiceLayerService = service;
GetTestCommand = new DelegateCommand(async () => await GetTests());
}
public async Task GetTests()
{
TestObservableCollection = new ObservableCollection<List<Test>>(await _ServiceLayerService.GetTestModels());
}
更新2:我已经按照第一个答案中提供的方式更改了我的异步方法调用。现在,当我尝试执行代码时,应用程序也会崩溃,但我收到错误消息:
07-05 14:39:04.518 F/ (25383): /Users/builder/jenkins/workspace/xamarin-android-d15-7/xamarin-android/external/mono/mono/mini/debugger-agent.c:4897: Could not execute the method because the containing type is not fully instantiated. assembly:<unknown assembly> type:<unknown type> member:(null) signature:<none>
07-05 14:39:04.518 F/libc (25383): Fatal signal 6 (SIGABRT), code -6 in tid 25383 (com.companyname), pid 25383 (com.companyname)
也许我在Unity依赖注入方面做错了,所以这里是在App.xaml.cs中注册服务层类
protected override void RegisterTypes(IContainerRegistry containerRegistry)
{
containerRegistry.RegisterForNavigation<NavigationPage>();
containerRegistry.RegisterForNavigation<TestsListPage, TestsListViewModel>();
containerRegistry.Register<IServiceLayerService, ServiceLayerService>();
}
问题出在async命令委托上
GetTestCommand = new DelegateCommand(async () => await GetTests());
命令委托导致async void
,它不会允许异常被捕获,因为它们被认为是火和忘记方法
参考Async/Await - Best Practices in Asynchronous Programming
创建一个事件和处理程序来管理异步调用
private event EventHandler gettingTests = delegate { };
private async void OnGettingTests(object sender, EventArgs args) {
try {
await GetTests();
} catch (Exception ex) {
//...handle exception
}
}
请注意,事件处理程序是允许async void
的规则的一个例外
在构造函数中订阅事件并在命令委托中引发事件
constructor (IServiceLayerService service) {
_ServiceLayerService = service;
this.gettingTests += OnGettingTests;
GetTestCommand = new DelegateCommand(() => gettingTests(this, EventArgs.Empty));
}
所以现在当调用该命令时,它将引发事件,异步处理程序可以正确地进行异步调用。