“could not resolve <mx:…> to a component implementation”

If you have ever tried to add mx.core.ToolTip (or certain other classes) as a mxml-tag to your Flex project then you might’ve run into the following compiler-error:

“Could not resolve <mx:ToolTip> to a component implementation.”

So why does Flex (or rather MXMLC, the compiler) let you write <mx:Button> and <mx:Canvas> and all those other Flex components, but not <mx:ToolTip> — it’s a native Flex class and it’s in the mx package just like all of the others, right?

Fact is, it’s not the mx class-package that defines whether or not you can add a component by saying <mx:...>, but rather the Flex component manifest file — it describes which of the native Flex classes fall under the mx xml-namespace (or actually any namespace you want to use). I might write a post about manifest-files in Flex somewhere in the near future, but for now you’ll have to read up in the docs if you want to know more.

It so happens that the ToolTip class isn’t included in the native Flex component manifest, and consequently the compiler won’t be able to find it if you just write “<mx:ToolTip />”. You can work around this by specifying a seperate xml-namespace for the mx.controls package:


Note that in the above example we can’t change the value of the xmlns:mx attribute so that it would point to the mx.controls class-package, because if we would then the compiler wouldn’t recognize the <mx:Application> tag anymore (since the Application class isn’t in the mx.controls package).

Also, for the naysayers who completely miss the point and start talking about why on earth you would want to instantiate a ToolTip through mxml: To tell the truth I was wondering about this myself, but if you think about it, tooltips in Flex aren’t restricted to just having whatever you put into UIComponent.toolTip automatically pop up when you mouse-over. In fact you may want to create a custom mxml-component subclass of ToolTip and manually add it to your application using the ToolTipManager class during runtime.

That having been said, ToolTip isn’t the only Flex-class not included in the Flex component manifest, naturally what I wrote in this post also applies to any other class in the Flex framework that extends UIComponent but isn’t included in the manifest.

8 thoughts on ““could not resolve <mx:…> to a component implementation”

  1. Hi!
    I have this problem (‘Could not resolve to a component implementation’) when I’m truing to Run your application with that book. What can be wrong here.

  2. I’m having the same issue:

    “Could not resolve to a component implementation.”

    Here’s what I did (Using Windows Vista):
    1) Created an empty Flex project in Eclipse v 3.4.2 (w/ Flex plugin)
    2) Copied the com and org folders to the project’s root directory
    3) Refreshed the project (folders showed up ok)
    4) Added the xmlns:rs=”com.rubenswieringa.book.*” namespace attribute to the mx:Application tag

    At this point, nothing big and I could compile and run.

    5) Added the rs:Book tags from the simple.mxml file

    Then I received the error. Does this have something to do with Windows permissions on the folders maybe?

  3. “Could not resolve to a component implementation.”

    Ok, my bad: I copied the com and org folders into the root for the project, rather that the src folder.

  4. I am new to this field.
    I copied an example from the book and pasted it in Flash Builder.
    Here is the code.


    I am getting the Error;
    Could not resolve to a component implementation.

    So please let me what is going wrong.
    I am stuck in this problem form last 3 days.
    I searched a lot but didn’t got any response.
    So please Help me out.
    I am using Flah Builder 4
    SDK 4
    Flash Player 10.2

  5. The error is not shown in the above comment
    Here i am again posting the error
    mx:Script Could not resolve to a component implementation.

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