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The context stack

Mustache sections open new contexts

Mustache sections allow you digging inside an object:

      My pet is named {{name}}.

Suppose this template is provided this object:

{ person: { pet: { name: 'Plato' }}}

The person key will return a person.

This person becomes the context in the person section: the pet key will be looked in that person.

Finally, the name key will be looked in the pet.

Context stack and missing keys

GRMustache uses the standard Key-Value Coding valueForKey: method when performing key lookup.

GRMustache considers a key to be missing if and only if this method returns nil or throws an NSUndefinedKeyException.

When a key is missing, GRMustache looks for it in the enclosing contexts, the values that populated the enclosing sections, one after the other, until it finds a non-nil value.

For instance, when rendering the above template, the name key will be asked to the pet first. In case of failure, GRMustache will then check the person object. Eventually, when all previous objects have failed providing the key, the lookup will stop.

This is the context stack: it starts with the object initially provided, grows when GRMustache enters a section, and shrinks on section leaving.

A pratical use of this feature is the conditional rendering of a string:


The {{#title}} section renders only if the title is not empty. In the section, the current context is the title string itself. Since this string fails providing the title key, the key loopup hence goes on, and finds again the title in the enclosing context, so that it can be rendered.

Sections vs. Key paths

You should be aware that these three template snippets are quite similar, but not stricly equivalent:

  • ...{{#foo}}{{bar}}{{/foo}}...
  • ...{{#foo}}{{.bar}}{{/foo}}...
  • ...{{}}...

The first will look for bar anywhere in the context stack, starting with the foo object.

The two others are identical: they ensure the bar key comes from the foo object.

Missing keys in detail: NSUndefinedKeyException

GRMustache catches NSUndefinedKeyException

NSDictionary never complains when asked for an unknown key. However, the default NSObject implementation of valueForKey: invokes valueForUndefinedKey: when asked for an unknown key. valueForUndefinedKey:, in turn, raises an NSUndefinedKeyException in its default implementation.

GRMustache catches those exceptions.

For instance, if the pet above has to name property, it will raise an NSUndefinedKeyException that will be caught by GRMustache so that the key lookup can continue with the person object.


When debugging your project, those exceptions may become a real annoyance, because it's likely you've told your debugger to stop on every Objective-C exceptions.

You can avoid that: make sure you call before any GRMustache rendering the following method:

[GRMustache preventNSUndefinedKeyExceptionAttack];

You'll get a slight performance hit, so you'd probably make sure this call does not enter your Release configuration.

One way to achieve this is to add -DDEBUG to the "Other C Flags" setting of your development configuration, and to wrap the preventNSUndefinedKeyExceptionAttack method call in a #if block, like:

#ifdef DEBUG
[GRMustache preventNSUndefinedKeyExceptionAttack];

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