Objective-C Memory Management

Objective-C Memory Management

Using Reference Counting

每一个从NSObject派生的对象都继承了对应的内存管理的行为。这些类的内部存在一个称为retain count的计数器。使用对应的调用，这个计数器会做自增或者自减。Objective-C runtime知道这个计数器何时变为0，此时会做deallocated。当对应deallocated时，它所占用的所有memory都会被释放。
有三种方法可以让retain count增加：

• 当你使用含有"alloc","create"的api来创建对象时，这个对象的retain count为1；
• 另外，当你通过一个包含"copy"的方法来得到一个对象时，这个对象的retain count为1；

• 你可以通过手动调用"retain"方法来增加retain count；
  当你要减小retain count时，你需要调用release。
  A standard allocation of an object

  Bar foo = [ [ Bar alloc ] init ];
  Retaining an object*

  Bar* foo = [ [ Bar alloc ] init ];
  [ foo retain ];

Using autorelease

Returning an autoreleased object

-( Foo* )getFoo {
    Foo* foo = [ [ Foo alloc ] init ];
    //do something with foo here...
    return [ [ foo autorelease ] ];
}  

The alloc/autorelease pattern

-( void )someMethod {
    Foo* foo = [ [ Foo alloc ] init ] autorelease ];

    //foo is still vaild here,
    //it won't be released until the method exists
    [ foo doSomething ];
}  

Using factory methods versus the traditional allocation pattern

-( void )usingFactories {
    NSMutableArray* array = [ NSMutableArray array ];    //nice, simple, autoreleased.
    NsMutableArray* array2 = [ [ NSMutableArray alloc ] init ];
    // do stuff with array and array2...

    // need to release this one.
    [ array2 release ];

    // [ array release ]; no need to release this,
    // it's already autoreleased
    // if you release it here, it will cause a crash
}  

Creating your own autorelease pool

-( void )inflateMemoryUsage {
    for( NSUInteger n = 0; n < 100000; ++n ) {
        NSAutoreleasePool *pool = [ [ NSAutoreleasePool alloc ] init ];
        //this object is autoreleased
        NSdata* data = [ self getBigBlobofData ];
        // do something with data...
        [ self doStuff:data ];
        [ pool release ];    // the autoreleased objects are deallocated here.
    }
    //nothing left over
}