Load Objective-C at runtime

When a program is only weakly dependent on a function , You can use dynamic libraries to organize dependent functions , And use the code to load the dynamic library and its functional modules in the main program , This method is called runtime loading .

about C/C++ Language , I believe many people know about runtime loading .

For example Win32 platform ：

auto module = LoadLibrary(L"A.dll");
// void test(int)
auto testFunc = (void(*)(int)) GetProcAddress(module, "test");

In the class unix platform ：

auto module = dlopen("A.so", RTLD_LAZY); //  stay  MacOSX  On ,A.dylib
// void test(int)
auto testFunc = (void(*)(int)) dlsym(module, "test");

When there are many functions , You usually need to use C++ Class encapsulation , Using pure virtual interface classes can facilitate runtime loading . Just export a function that creates objects .

class StreamBase
{
    virtual ~Base() = 0;
    virtual Open(int) = 0;
    virtual Read(char *, int) = 0;
    virtual White(char const *, int) = 0;
    virtual Close() = 0;
};

extern "C" StreamBase * CreateStream();

When we're in MacOSX platform , Use Objective-C When the function is realized , We can realize runtime loading more conveniently . There is no need to deliberately introduce a pure virtual interface class , There is no need to export a creator function . in fact ,Objective-C What is exported is the class object itself . With class object , Everything will be there .

Let's take an example ：

This is a player implementation , It is provided to the main program through the dynamic library

@interface Player : NSObject

+ (void) initialize;

- (instancetype) initWithDisplayLayer: (AVSampleBufferDisplayLayer*) layer;
- (instancetype) initWithImageView: (NSView*) view;
- (int) open: (char const *) url;
- (NSSize) videoSize;
- (int) play;
- (void) stop;
- (void) close;

@end

In the main program , After getting the class object , Everything works as usual

auto module = dlopen("libPlayer.dylib", RTLD_LAZY);
Class cls = (__bridge Class) dlsym(module, "OBJC_CLASS_$_Player");
if (cls == nullptr) {
    m_error = -2;
    return;
}
Player * player = [cls alloc];
[player initWithImageView: imageView];

Go and have a try ！