Velocity: Is a any way to check if variable is defined

问题

I want to include one template nested into others cont1, cont2, cont3. And nested template should be hide one specific control for cont1 only. Before inclusion into cont1 I would like to assign value to some flag variable $hideMyControl.

And inside nested template I would like to check if $hideMyControl is assigned value.

How to perform such check?

回答1:

You can do this using

  #if($!{$articleLeader})
      // Perform your operation or the template part you want to show.
  #end

For more info, see the 'formal reference' section of the Apache Velocity Reference Manual.

回答2:

#if($hideMyControl)
    // your code
#end

If $hideMyControl is defined, your code will execute

回答3:

#if($!{hideMyControl} != "")
## do something if $hideMyControl is defined
#end

This works for me in AWS API Gateway Body Mapping Templates. Please refer to Quiet Reference Notation in Velocity User Guide for more information.

回答4:

I was using

#if ($hideMyControl) 
    //do something 
#end

since a few months ago, however today its not working anymore.

I came here to find help, and noticed a new way of writing it :

#if($!{$hideMyControl})
   // do something
#end

this code works!

回答5:

According to the docs for Strict Reference Mode it is possible to several constructions to check if variable is defined.

#if ($foo)#end                  ## False
#if ( ! $foo)#end               ## True
#if ($foo && $foo.bar)#end      ## False and $foo.bar will not be evaluated
#if ($foo && $foo == "bar")#end ## False and $foo == "bar" wil not be evaluated
#if ($foo1 || $foo2)#end        ## False $foo1 and $foo2 are not defined

So this code works in my case.

#if( !$value )
  // Perform your operation or the template part you want to show.
#end

回答6:

To check if $hideMyControl is in Velocity context and IS NOT boolean 'true' value (or 'false' as well):

#if ($hideMyControl && $hideMyControl != true)
    ##do stuff
#end

Sure, if you really use your $hideMyControl variable as boolean type, you don't need second part of condition.

来源：https://stackoverflow.com/questions/4825636/velocity-is-a-any-way-to-check-if-variable-is-defined

标签

variables

velocity

template-engine