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    <p>It’s possible to add formal properties to a class via <code>class_addProperty()</code>:</p> <pre><code>BOOL class_addProperty(Class cls, const char *name, const objc_property_attribute_t *attributes, unsigned int attributeCount) </code></pre> <p>The first two parameters are self-explanatory. The third parameter is an array of property attributes, and each property attribute is a name-value pair which follow Objective-C <a href="http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtTypeEncodings.html" rel="noreferrer">type encodings</a> for <a href="http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtPropertyIntrospection.html" rel="noreferrer">declared properties</a>. Note that the documentation still mentions the comma-separated string for the encoding of property attributes. Each segment in the comma-separated string is represented by one <code>objc_property_attribute_t</code> instance. Furthermore, <code>objc_property_attribute_t</code> accepts class names besides the generic <code>@</code> type encoding of <code>id</code>.</p> <p>Here’s a first draft of a program that dynamically adds a property called <code>name</code> to a class that already has an instance variable called <code>_privateName</code>:</p> <pre><code>#include &lt;objc/runtime.h&gt; #import &lt;Foundation/Foundation.h&gt; @interface SomeClass : NSObject { NSString *_privateName; } @end @implementation SomeClass - (id)init { self = [super init]; if (self) _privateName = @"Steve"; return self; } @end NSString *nameGetter(id self, SEL _cmd) { Ivar ivar = class_getInstanceVariable([SomeClass class], "_privateName"); return object_getIvar(self, ivar); } void nameSetter(id self, SEL _cmd, NSString *newName) { Ivar ivar = class_getInstanceVariable([SomeClass class], "_privateName"); id oldName = object_getIvar(self, ivar); if (oldName != newName) object_setIvar(self, ivar, [newName copy]); } int main(void) { @autoreleasepool { objc_property_attribute_t type = { "T", "@\"NSString\"" }; objc_property_attribute_t ownership = { "C", "" }; // C = copy objc_property_attribute_t backingivar = { "V", "_privateName" }; objc_property_attribute_t attrs[] = { type, ownership, backingivar }; class_addProperty([SomeClass class], "name", attrs, 3); class_addMethod([SomeClass class], @selector(name), (IMP)nameGetter, "@@:"); class_addMethod([SomeClass class], @selector(setName:), (IMP)nameSetter, "v@:@"); id o = [SomeClass new]; NSLog(@"%@", [o name]); [o setName:@"Jobs"]; NSLog(@"%@", [o name]); } } </code></pre> <p>Its (trimmed) output:</p> <pre><code>Steve Jobs </code></pre> <p>The getter and setter methods should be written more carefully but this should be enough as an example of how to dynamically add a formal property at runtime.</p>
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    1. POHow can I add properties to an object at runtime?
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    1. COclass_addProperty returns true, but class_getInstanceVariable always returns nil. I've tried putting the property name instead of the ivar name, but still no luck. Any idea what could be the issue?
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    2. CO@Bavarious, how did you fool complier? I mean [o name] results in compilation error 'No known instance method for selector 'name''.
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      1. USHitesh Savaliya
 

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