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

问题描述

以下代码应该抛出一个错误#1009:无法访问空对象引用的属性或方法:

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;

但是，如果它位于由 MXML 数据绑定设置的 setter 内部，则不会:

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
}

要重现这种奇怪的行为，首先，通过扩展 s:Label 创建一个简单的自定义组件:

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
        }
    }
}

第二部分，将 BuggyLabel 添加到应用程序并绑定 buggySetter:

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}"/>

为什么这个应用会无声无息地失败?

推荐答案

这个问题的答案实际上相当简短:这是 Flex SDK 工程师做出的架构决策.如果您查看 Flex 源代码，您会看到一个 try ... catch 块吞下了 Binding 中抛出的大多数错误.

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:使绑定更容易使用，因为您不必考虑所有可能的错误状态

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

缺点:调试可能会更困难(尽管如果您知道这可能发生并且您有良好的单元测试，您可以将那方面的挫败感减少到几乎为零)

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)

我所说的源代码可以在mx.binding.Binding(在'framework'项目中)的wrapFunctionCall()方法中找到.这是相关部分:

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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