MessageBox是异步的。您可以使用回呼，例如：

...
...
    checkFile: function (file, successCallBack, successCallBackScope) {
        var result = /.pdf$/i.test(file.name);
        var fileOk;
        var maxSizeMo = this.MAX_SIZE / (1024 * 1024);
        var cancelUpload;

        if (result) {
            if (file.size < this.ALERTE_SIZE) {
                fileOk = true;
            } else if (file.size > this.ALERTE_SIZE && file.size < this.MAX_SIZE) {
                fileOk = true;

                Ext.MessageBox.show({
                    title: 'Warning',
                    msg: 'Some msg',
                    width: 330,
                    buttons: Ext.MessageBox.OKCANCEL,
                    icon: Ext.MessageBox.WARNING,
                    closable: false,
                    fn: function (btn) {
                        if (btn != 'ok') {
                            successCallBack.call(successCallBackScope); 
                        } else {
                            cancelUpload = false;
                            return;
                        }
                    }
                });

            } else {
                fileOk = false;
                cancelUpload = true;
            }
        } else {
            fileOk = false;
            cancelUpload = true;
        }
        return {
            isTestOk: fileOk,
            isCancelled: cancelUpload
        };
    },
    uploadFile: function (fileField, value, eOpts) {
        var file = fileField.fileInputEl.dom.files[0];
        var resultCheck = this.checkFile(file, this.successCallBack, this);

        if (resultCheck.isTestOk) {
            if (!resultCheck.isCancelled) {} else {
                PopupHelper.alerte("Upload cancelled by the user");
            }
        }
    },

    successCallBack: function () {
        this.loadMask.show();
        app.FileUploader.uploadFile(file, this, function (success, fileName, fileReference, exception) {
            try {
                if (success === true) {
                    var recordConfig = [{
                        label: fileName,
                        value: fileReference,
                        supprimer: false,
                        context: null
                    }];
                    this.addPJAndSelect(recordConfig);
                    fileField.reset();
                } else if (exception) {}
            } finally {
                this.loadMask.hide();
            }
        });

    }
...
...

代码不可用..

如果你使用async/await，你可以创建自己的异步方法并等待它。下面是一个示例实现。

Ext.override(Ext.window.MessageBox, {
  showAsync: function(config) {
    var me = this;
    var deferred = Ext.create('Ext.Deferred');
    me.show(config);
    /* We're overriding the userCallback below, but we want to
     * restore it after we're done, in case we decide to not use
     * showAsync next time */
    var frameworkCallback = me.userCallback;
    me.userCallback = function(buttonId, value, cfg) {
      // Restore framework's callback
      me.userCallback = frameworkCallback;
      deferred.resolve({
        buttonId: buttonId,
        value: value,
        cfg: cfg
      });
    };
    return deferred;
  }
});