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114 lines
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114 lines
5.7 KiB
HTML
4 years ago
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<link rel="up" href="Messaging-with-the-GNU-Objective_002dC-runtime.html#Messaging-with-the-GNU-Objective_002dC-runtime" title="Messaging with the GNU Objective-C runtime">
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<a name="Forwarding-hook"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Dynamically-registering-methods.html#Dynamically-registering-methods">Dynamically registering methods</a>,
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Up: <a rel="up" accesskey="u" href="Messaging-with-the-GNU-Objective_002dC-runtime.html#Messaging-with-the-GNU-Objective_002dC-runtime">Messaging with the GNU Objective-C runtime</a>
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</div>
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<h4 class="subsection">8.10.2 Forwarding Hook</h4>
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<p>The GNU Objective-C runtime provides a hook, called
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<code>__objc_msg_forward2</code>, which is called by
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<code>objc_msg_lookup()</code> when it can't find a method implementation in
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the runtime tables and after calling <code>+resolveInstanceMethod:</code>
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and <code>+resolveClassMethod:</code> has been attempted and did not succeed
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in dynamically registering the method.
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<p>To configure the hook, you set the global variable
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<code>__objc_msg_forward2</code> to a function with the same argument and
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return types of <code>objc_msg_lookup()</code>. When
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<code>objc_msg_lookup()</code> can not find a method implementation, it
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invokes the hook function you provided to get a method implementation
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to return. So, in practice <code>__objc_msg_forward2</code> allows you to
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extend <code>objc_msg_lookup()</code> by adding some custom code that is
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called to do a further lookup when no standard method implementation
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can be found using the normal lookup.
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<p>This hook is generally reserved for “Foundation” libraries such as
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GNUstep Base, which use it to implement their high-level method
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forwarding API, typically based around the <code>forwardInvocation:</code>
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method. So, unless you are implementing your own “Foundation”
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library, you should not set this hook.
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<p>In a typical forwarding implementation, the <code>__objc_msg_forward2</code>
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hook function determines the argument and return type of the method
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that is being looked up, and then creates a function that takes these
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arguments and has that return type, and returns it to the caller.
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Creating this function is non-trivial and is typically performed using
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a dedicated library such as <code>libffi</code>.
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<p>The forwarding method implementation thus created is returned by
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<code>objc_msg_lookup()</code> and is executed as if it was a normal method
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implementation. When the forwarding method implementation is called,
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it is usually expected to pack all arguments into some sort of object
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(typically, an <code>NSInvocation</code> in a “Foundation” library), and
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hand it over to the programmer (<code>forwardInvocation:</code>) who is then
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allowed to manipulate the method invocation using a high-level API
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provided by the “Foundation” library. For example, the programmer
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may want to examine the method invocation arguments and name and
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potentially change them before forwarding the method invocation to one
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or more local objects (<code>performInvocation:</code>) or even to remote
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objects (by using Distributed Objects or some other mechanism). When
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all this completes, the return value is passed back and must be
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returned correctly to the original caller.
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<p>Note that the GNU Objective-C runtime currently provides no support
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for method forwarding or method invocations other than the
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<code>__objc_msg_forward2</code> hook.
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<p>If the forwarding hook does not exist or returns <code>NULL</code>, the
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runtime currently attempts forwarding using an older, deprecated API,
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and if that fails, it aborts the program. In future versions of the
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GNU Objective-C runtime, the runtime will immediately abort.
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<!-- Copyright (C) 2002-2015 Free Software Foundation, Inc. -->
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<!-- This is part of the GCC manual. -->
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