为什么MXML数据绑定吞下TypeErrors？ [英] Why does MXML data binding swallow TypeErrors?

问题描述

以下代码抛出错误＃1009：无法访问空对象引用的属性或方法：

  var label：Label; 
 label.text = value; 
  

但是，如果它位于由MXML数据绑定设置的setter内，则不会： / p>

  public function set buggySetter（value：String）：void {
 var label：Label; 
 label.text = value; //将静默失败
} 
  

要重现这个奇怪的行为，首先，创建一个简单的定制组件通过扩展s：标签：

  package {
 import spark.components.Label; 
 
 public class BuggyLabel extends Label {
 public function set buggySetter（value：String）：void {
 var label：Label; 
 label.text = value; //将默认失败
} 
} 
} 
  

Sectond将BuggyLabel添加到应用程序并绑定buggySetter：

 < fx：脚本> 
<！[CDATA [
 [Bindable] 
 public var foo：String ='NULL has no properties'; 
]]> 
< / fx：脚本> 
 
< local：BuggyLabel buggySetter ={foo}/> 
  

为什么这个应用程序默默地失败？

解决方案

这个问题的答案实际上是相当短的：它是由Flex SDK工程师做出的架构决策。如果您查看Flex源代码，您将看到一个 try ... catch 阻止吞噬大多数错误在Binding中抛出。

专业版：使您更容易使用绑定，因为您不必考虑所有可能的错误状态。

< con> Con ：它可能更难调试（虽然如果你知道这可能会发生，你有良好的单元测试，你可以减少从这方面的沮丧到接近零）

---

我在谈论的源代码可以在 mx.binding.Binding （in方法 wrapFunctionCall（）中的'框架'项目）。以下是相关的部分：

  try {
 ... 
} 
 catch ：错误）
 {
 //执行srcFunc或destFunc时某些错误正常，
 //所以我们吞下它们：
 //错误＃1006：尝试在对象不是一个函数。 
 //错误＃1009：null没有属性。 
 //错误＃1010：undefined没有属性。 
 //错误＃1055： - 没有属性。 
 //错误＃1069：属性 - 未找到 - 并且没有默认值
 //我们允许抛出任何其他错误。 
 if（（error.errorID！= 1006）&& 
（error.errorID！= 1009）&& 
（error.errorID！= 1010）& 
（error.errorID！= 1055）&& 
（error.errorID！= 1069））
 {
 throw error; 
} 
 else 
 {
 if（BindingManager.debugDestinationStrings [destString]）
 {
 trace（Binding：destString =+ destString +， error =+ error） 
} 
} 
} 
  

The following code shoud throw an Error #1009: Cannot access a property or method of a null object reference:

var label:Label;
label.text = value;

However, it doesn't if it's inside of a setter which is set by MXML data binding:

public function set buggySetter(value:String):void {
    var label:Label;
    label.text = value; //will fail silently
}

To reproduce this weird behaviour, first, create a simple custom component by extending s:Label:

package {
    import spark.components.Label;

    public class BuggyLabel extends Label {
        public function set buggySetter(value:String):void {
            var label:Label;
            label.text = value; //will fail silently
        }
    }
}

Sectond, add BuggyLabel to an Application and bind buggySetter:

<fx:Script>
    <![CDATA[
        [Bindable]
        public var foo:String = 'NULL has no properties';
    ]]>
</fx:Script>

<local:BuggyLabel buggySetter="{foo}"/>

Why does this app fail silently?

解决方案

The answer to that question is actually fairly short: it's an architectural decision made by the Flex SDK engineers. If you take a look at the Flex source code, you'll see a try ... catch block swallowing most errors thrown in a Binding.

Pro: makes it easier to use bindings, since you don't have to account for all possible faulty states

Con: it can be harder to debug (though if you know this can happen and you have good unit tests, you can reduce frustration from that side to nearly zero)

---

The source code I was talking about can be found in mx.binding.Binding (in the 'framework' project) in method wrapFunctionCall(). Here's the relevant part:

    try {
       ...
    }
    catch(error:Error)
    {
        // Certain errors are normal when executing a srcFunc or destFunc,
        // so we swallow them:
        //   Error #1006: Call attempted on an object that is not a function.
        //   Error #1009: null has no properties.
        //   Error #1010: undefined has no properties.
        //   Error #1055: - has no properties.
        //   Error #1069: Property - not found on - and there is no default value
        // We allow any other errors to be thrown.
        if ((error.errorID != 1006) &&
            (error.errorID != 1009) &&
            (error.errorID != 1010) &&
            (error.errorID != 1055) &&
            (error.errorID != 1069))
        {
            throw error;
        }
        else
        {
            if (BindingManager.debugDestinationStrings[destString])
            {
                trace("Binding: destString = " + destString + ", error = " + error);
            }
        }
    }

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