拖動查看，從你觸摸它的點iPhone

Question

我知道如何拖動一個視圖或對象：

- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{ 

    UITouch *touch = [[event allTouches] anyObject]; 


    if([touch view] == ViewMain) 
    { 
     CGPoint location = [touch locationInView:self.view]; 
     ViewMain.center = location; 

    } 


} 

，但我想開始拖動該視圖或圖像從我感動的地步。例如，如果我拖動（I強調視圖並置於藍色上的光標）：

我開始，如果我拖動鼠標20像素到例如右然後我想視圖拖動力矩也拖累20像素，而不是：

也許我不得不改變看法的地步的，我的拳頭觸摸它的中心。我怎麼能這樣做？

換句話說，我希望做一些事情，當你拖動iPad上的應用程序，如：

Answer 1

float startX = 0; 
float startY = 0; 
-(void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event 
{ 
    UITouch *touch = [[event allTouches] anyObject]; 

    if([touch view] == ViewMain) 
    { 
     CGPoint location = [touch locationInView:self.view]; 
     startX = location.x - ViewMain.center.x;   
     startY = ViewMain.center.y; 
    } 
} 
- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{ 
    UITouch *touch = [[event allTouches] anyObject]; 
    if([touch view] == ViewMain) 
    { 
     CGPoint location = [touch locationInView:self.view]; 
     location.x =location.x - startX; 
     location.y = startY; 
     ViewMain.center = location; 
    } 
}