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| <a name="Forwarding-hook"></a> |
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| <h4 class="subsection">8.10.2 Forwarding hook</h4> |
| |
| <p>The GNU Objective-C runtime provides a hook, called |
| <code>__objc_msg_forward2</code>, which is called by |
| <code>objc_msg_lookup()</code> when it can't find a method implementation in |
| the runtime tables and after calling <code>+resolveInstanceMethod:</code> |
| and <code>+resolveClassMethod:</code> has been attempted and did not succeed |
| in dynamically registering the method. |
| |
| <p>To configure the hook, you set the global variable |
| <code>__objc_msg_forward2</code> to a function with the same argument and |
| return types of <code>objc_msg_lookup()</code>. When |
| <code>objc_msg_lookup()</code> can not find a method implementation, it |
| invokes the hook function you provided to get a method implementation |
| to return. So, in practice <code>__objc_msg_forward2</code> allows you to |
| extend <code>objc_msg_lookup()</code> by adding some custom code that is |
| called to do a further lookup when no standard method implementation |
| can be found using the normal lookup. |
| |
| <p>This hook is generally reserved for “Foundation” libraries such as |
| GNUstep Base, which use it to implement their high-level method |
| forwarding API, typically based around the <code>forwardInvocation:</code> |
| method. So, unless you are implementing your own “Foundation” |
| library, you should not set this hook. |
| |
| <p>In a typical forwarding implementation, the <code>__objc_msg_forward2</code> |
| hook function determines the argument and return type of the method |
| that is being looked up, and then creates a function that takes these |
| arguments and has that return type, and returns it to the caller. |
| Creating this function is non-trivial and is typically performed using |
| a dedicated library such as <code>libffi</code>. |
| |
| <p>The forwarding method implementation thus created is returned by |
| <code>objc_msg_lookup()</code> and is executed as if it was a normal method |
| implementation. When the forwarding method implementation is called, |
| it is usually expected to pack all arguments into some sort of object |
| (typically, an <code>NSInvocation</code> in a “Foundation” library), and |
| hand it over to the programmer (<code>forwardInvocation:</code>) who is then |
| allowed to manipulate the method invocation using a high-level API |
| provided by the “Foundation” library. For example, the programmer |
| may want to examine the method invocation arguments and name and |
| potentially change them before forwarding the method invocation to one |
| or more local objects (<code>performInvocation:</code>) or even to remote |
| objects (by using Distributed Objects or some other mechanism). When |
| all this completes, the return value is passed back and must be |
| returned correctly to the original caller. |
| |
| <p>Note that the GNU Objective-C runtime currently provides no support |
| for method forwarding or method invocations other than the |
| <code>__objc_msg_forward2</code> hook. |
| |
| <p>If the forwarding hook does not exist or returns <code>NULL</code>, the |
| runtime currently attempts forwarding using an older, deprecated API, |
| and if that fails, it aborts the program. In future versions of the |
| GNU Objective-C runtime, the runtime will immediately abort. |
| |
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