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我是 GWT/GAE 的新手。要使用 GAE Blobstore 服务，必须从服务器端返回一个上传 url，如本博客中的教程。

但似乎可以在用户浏览文件时调用此获取上传 url 服务，因此我需要在用户按下选择文件按钮时找到要处理的事件，并在那里设置表单面板的操作属性。

fileupload 本身具有附加/分离或更改事件，但它似乎不是我想要的。

我可以上传到 blobstore 的代码如下： 上传表单是：

import java.util.Arrays;
import java.util.Date;
import java.util.List;
import com.google.gwt.cell.client.DateCell;
import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.core.client.GWT;
import com.google.gwt.event.dom.client.ClickEvent;
import com.google.gwt.event.dom.client.ClickHandler;
import com.google.gwt.user.client.Window;
import com.google.gwt.user.client.rpc.AsyncCallback;
import com.google.gwt.user.client.ui.Button;
import com.google.gwt.user.client.ui.FileUpload;
import com.google.gwt.user.client.ui.FormPanel;
import com.google.gwt.user.client.ui.RootPanel;
import com.google.gwt.user.client.ui.VerticalPanel;
import com.google.gwt.user.client.ui.FormPanel.SubmitCompleteHandler;
import com.google.gwt.user.client.ui.FormPanel.SubmitCompleteEvent;
import com.google.gwt.user.client.ui.FlowPanel;
import com.google.gwt.user.client.ui.TextArea;

public class UploadFilePanel implements EntryPoint {
    private FileServiceAsync upls = GWT.create(FileService.class);
    AsyncCallback<String> callback;
    FormPanel form;
    String upload_url;

public void onModuleLoad() {

    get_upload_url();

    RootPanel rootPanel = RootPanel.get();

    FlowPanel flowPanel = new FlowPanel();
    rootPanel.add(flowPanel, 0, 0);

    add_submit_form(flowPanel);
}

private void add_submit_form(FlowPanel flowPanel) {
    VerticalPanel panel = new VerticalPanel();
    flowPanel.add(panel);

    // Create a FormPanel and point it at a service.
    form = new FormPanel();
    panel.add(form);
    form.setEncoding(FormPanel.ENCODING_MULTIPART);
    form.setMethod(FormPanel.METHOD_POST);

    form.addSubmitCompleteHandler(new SubmitCompleteHandler() {
        @Override
        public void onSubmitComplete(SubmitCompleteEvent event) {
            RootPanel.get().clear();
            RootPanel.get().getElement()
                    .setInnerHTML("upload complete " + event.toString());
        }
    });

    form.setSize("222px", "52px");

    // Create a FileUpload widget.
    FileUpload upload = new FileUpload();
    form.setWidget(upload);
    upload.setSize("100%", "100%");
    upload.setName("myFile");

    TextArea textArea = new TextArea();
    panel.add(textArea);
    textArea.setWidth("100%");

    // Add a 'submit' button.
    Button submitbtn = new Button("Submit");
    panel.add(submitbtn);
    submitbtn.addClickHandler(new ClickHandler() {
        @Override
        public void onClick(ClickEvent event) {
            upls.get_upload_url("/stmtgen/upload", callback);
        }
    });
}

private void get_upload_url() {
    callback = new AsyncCallback<String>() {
        @Override
        public void onSuccess(String result) {
            form.setAction(result);
            upload_url = result;
            form.submit();

        }

        @Override
        public void onFailure(Throwable caught) {
            RootPanel.get().clear();
            RootPanel.get().getElement()
                    .setInnerHTML("Can't get upload url");
        }
    };
    if (upls == null)
        upls = GWT.create(FileService.class);
}
}

我用来获取上传网址的异步服务是这样的：

import com.google.gwt.user.client.rpc.AsyncCallback;
public interface FileServiceAsync {
void get_upload_url(String up, AsyncCallback<String> callback);
}

问题是当用户点击提交按钮时，有两个服务调用会减慢响应速度。

我尝试了 addHandler(Handler, Type) 如下，它不起作用。

FileUpload upload = new FileUpload();
upload.addHandler(new ClickHandler() {

                @Override
                public void onClick(ClickEvent event) {
                    //get upload url here
                }
            }, ClickEvent.getType()
);
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