为项目及第三方库重写NSLog方法

Great question! Let’s tackle this from two angles: overriding NSLog so your custom logic applies to third-party libraries, and alternative ways to intercept and modify all console output before it’s printed.

Since NSLog is a C function (not an Objective-C method), standard method swizzling won’t work here. The key is to hook the underlying system call that both your code and third-party libraries use when calling NSLog.

The most reliable way to do this is with fishhook, a lightweight library that lets you dynamically replace symbols in Mach-O binaries. This works for any code (including third-party libraries) that links against the system NSLog function dynamically.

Here’s a step-by-step implementation:

1. Add fishhook to your project (via CocoaPods or manual integration).
2. Write the hook logic to replace NSLog with your custom implementation:

#include <stdio.h>
#include <stdarg.h>
#include "fishhook.h"

// Store a pointer to the original NSLog function
void (*original_NSLog)(NSString *format, ...);

// Your custom NSLog implementation with string replacement logic
void custom_NSLog(NSString *format, ...) {
    va_list args;
    va_start(args, format);
    
    // Format the original log message
    NSString *originalMessage = [[NSString alloc] initWithFormat:format arguments:args];
    
    // Apply your custom rule: replace "dog" with 🐶
    NSString *modifiedMessage = [originalMessage stringByReplacingOccurrencesOfString:@"dog" withString:@"🐶"];
    
    // Pass the modified message to the original NSLog
    original_NSLog(@"%@", modifiedMessage);
    
    va_end(args);
}

// Set up the hook early in your app's lifecycle
void hookNSLog() {
    struct rebinding nslogRebinding = {
        "NSLog",
        custom_NSLog,
        (void *)&original_NSLog
    };
    rebind_symbols((struct rebinding[1]){nslogRebinding}, 1);
}

3. Call hookNSLog() at app startup—for example, in application:didFinishLaunchingWithOptions: or main.m before UIApplicationMain.

Important Notes:

• This works for almost all third-party libraries, as most dynamically link against NSLog. If a library statically links and inlines NSLog (a rare edge case), this method won’t catch those calls.
• Always handle variable arguments correctly with va_list to avoid crashes or malformed output.

If you want to capture all console output (not just NSLog—including printf, NSLogv, and system-level logs), you can redirect the standard error stream (stderr, which is where NSLog writes) to a pipe, process the output, then forward it to the original stream.

Here’s how to implement this with NSPipe:

#import <Foundation/Foundation.h>

void redirectConsoleOutput() {
    // Create a pipe to capture stderr output
    NSPipe *pipe = [NSPipe pipe];
    NSFileHandle *pipeReadHandle = [pipe fileHandleForReading];
    
    // Redirect stderr to the pipe's write end
    dup2([[pipe fileHandleForWriting] fileDescriptor], STDERR_FILENO);
    
    // Start a background thread to read and process the output
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        while (YES) {
            NSData *data = [pipeReadHandle readDataToEndOfFile];
            if (data.length == 0) continue;
            
            // Convert raw data to a string
            NSString *output = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
            
            // Apply your custom modification
            NSString *modifiedOutput = [output stringByReplacingOccurrencesOfString:@"dog" withString:@"🐶"];
            
            // Write the modified output back to the original stderr
            fprintf(stderr, "%s", modifiedOutput.UTF8String);
            fflush(stderr);
        }
    });
}

Call redirectConsoleOutput() early in your app’s lifecycle. This approach captures every bit of output sent to stderr, including logs from third-party libraries and system calls.

For iOS 10+, you could also use the os_log framework’s logging callbacks, but this requires more setup and specific entitlements for full log access. The pipe redirect method is simpler for most common use cases.

内容的提问来源于stack exchange，提问作者user16554276