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POIs it possible to redirect system API in Objective-C?
 

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    <p>Due to some reasons I have to keep original code without modification, but try to redirect system API into my code and then call back to the original one. For example, I would like to do something more in <code>[NSString stringWithFormat:]</code>. </p> <p>Now I try to use method swizzling. But it seems that <code>NSString</code> is not loaded during the runtime of main, I move swizzling method to <code>MyAppDelegate</code>. After that <code>class_getInstanceMethod([NSString class], @selector(stringWithFormat:)</code>) is not nil. However, the swizzling method is still not working because <code>class_getInstanceMethod([NSString class], @selector(override_stringWithFormat:))</code> is still nil, how should I fix that?</p> <p>Thanks, clu</p> <pre><code>@interface NSString (MyNSString) + (id)stringWithFormat:(NSString *)format, ...; @end @implementation NSString (MyNSString) + (id)stringWithFormat:(NSString *)format, ... { //do something... [NSString stringWithFormat:format]; } @end </code></pre> <p>Here is the code in MyAppDelegate</p> <pre><code>#import "MyNSString.h" -(void) MethodSwizzle:(Class)c replaceOrig:(SEL) origSEL withNew:(SEL) overrideSEL { Method origMethod = class_getInstanceMethod(c, origSEL); Method overrideMethod = class_getInstanceMethod(c, overrideSEL); if(class_addMethod(c, origSEL, method_getImplementation(overrideMethod), method_getTypeEncoding(overrideMethod))) class_replaceMethod(c, overrideSEL, method_getImplementation(origMethod), method_getTypeEncoding(origMethod)); else method_exchangeImplementations(origMethod, overrideMethod); } - (BOOL) application:(UIApplication*) application didFinishLaunchingWithOptions:(NSDictionary *) options { ... //Unit test NSString *a=[NSString override_stringWithFormat:@"Test"]; //returned something Method b = class_getInstanceMethod([NSString class], @selector(override_stringWithFormat:)); //return nil; //do something... [self MethodSwizzle:[NSString class] replaceOrig:@selector(stringWithFormat:) withNew:@selector(override_stringWithFormat:)]; } </code></pre>
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