Objective-C blocks

Block is equivalent to the function pointer in C or C ++, and is equivalent to the delegate in. Net or Java.

// Block Declaration

Double (^ blockname) (double, double) =

^ Double (double firstparam, double secondparam ){

Return firstparam * secondparam;

}

 

// Block call

Double result = blockname (2, 4 );

 

Block will form a closure

-(Void) testmethod {

Int I = 42;

 

Void (^ testblock) (void) = ^ {

Nslog (@ "% I", I );

};

 

I = 84;

Testblock ();

}

In this Code, the value 42 of block I is testblock capture. Even if the I value is modified before testblock is called, the execution result of testblock is still 42. in addition, even if I is modified in testblock, the original I value will not be affected.

 

You can use the _ block keyword to share the variables defined outside the block with the code in the block.

_ Block int I = 42;

 

Void (^ testblock) (void) = ^ {

Nslog (@ "% I", I );

I = 100;

}

 

Testblock ();

Nslog (@ "% I", I );

In this case, the output result is 42,100.

 

Block is usually passed to the method as a parameter and serves as a callback function. Block parameters are generally placed at the end of the method parameter list.

-(Void) testmethod: (nsstring *) STR callback: (void (^) (void) testblock;

 

Use typedef to simplify block syntax

Typedef void (^ nothinblock) (void );

In this way, you can create nothinblock instances to create blocks with the same signature.

Nothinblock callback = ^ {

...

};

 

If you define a block type attribute in a class, and this block captures self, it may cause a strongly referenced loop.

@ Interface nothin: nsobject

@ Property (copy) void (^ block) (void );

@ End

 

@ Implementation nothin

-(Void) configureblock {

Self. Block = ^ {

[Self dosomething];

}

}

@ End

 

Because self is a strong reference to the block, and the block has capture self, this will cause a strong reference loop. To avoid this situation, we need to use weak references to self in the block.

-(Void) configureblock {

Nothin * _ weak weakself = self;

Self. Block = ^ {

[Weakself dosomething];

}

}

In this case, when the block is executed, the reference to self is removed.