UIControlEventTouchDragExit triggers when 100 pixels away from UIButton

问题

At present, the UIControlEventTouchDragExit only triggers when I drag 100 pixels away from the button. I'd like to customize this behavior and bring that range in to around 25 pixels, but I'm relatively new to programming and have never needed to override / customize an in-built method like this.

I've read in some other posts here that I'd need to subclass the UIButton (or perhaps even UIControl?), and override -(BOOL) beginTrackingWithTouch: (UITouch *) touch withEvent: (UIEvent *) event and related methods, but I don't really know where to begin doing so.

Could anyone kindly offer some advice as to how I might achieve this? Much appreciated! ^_^

回答1:

Override continueTrackingWithTouch:withEvent: like this to send DragExit/DragOutside events inside of the default gutter:

- (BOOL)continueTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event
{
    CGFloat boundsExtension = 25.0f;
    CGRect outerBounds = CGRectInset(self.bounds, -1 * boundsExtension, -1 * boundsExtension);

    BOOL touchOutside = !CGRectContainsPoint(outerBounds, [touch locationInView:self]);
    if(touchOutside)
    {
        BOOL previousTouchInside = CGRectContainsPoint(outerBounds, [touch previousLocationInView:self]);
        if(previousTouchInside)
        {
            NSLog(@"Sending UIControlEventTouchDragExit");
            [self sendActionsForControlEvents:UIControlEventTouchDragExit];
        }
        else
        {
            NSLog(@"Sending UIControlEventTouchDragOutside");
            [self sendActionsForControlEvents:UIControlEventTouchDragOutside];
        }
    }
    return [super continueTrackingWithTouch:touch withEvent:event];
}

回答2:

I found the accepted answer has two problems.

1. The registered actions would be called twice：
   • the first time is when finger leaves the control 25 pixels, which is set in the overwritten method.
   • the second time is called when finger leaves the control about 70 pixels, which is the default behavior of the UIControl.
2. The second problem is the position retrieved from event parameter is (0, 0), it's not initialized correctly.

I found another way to accomplish this purpose based on this answer, the basic idea is to handle the event in the callbacks rather than overwrite that method. There are two steps:

Register the actions:

    // to get the touch up event
    [btn addTarget:self action:@selector(btnTouchUp:withEvent:) forControlEvents:UIControlEventTouchUpInside];
    [btn addTarget:self action:@selector(btnTouchUp:withEvent:) forControlEvents:UIControlEventTouchUpOutside];
    // to get the drag event
    [btn addTarget:self action:@selector(btnDragged:withEvent:) forControlEvents:UIControlEventTouchDragInside];
    [btn addTarget:self action:@selector(btnDragged:withEvent:) forControlEvents:UIControlEventTouchDragOutside];

Handle the events:

- (void)btnTouchUp:(UIButton *)sender withEvent:(UIEvent *)event {
    UITouch *touch = [[event allTouches] anyObject];
    CGFloat boundsExtension = 25.0f;
    CGRect outerBounds = CGRectInset(sender.bounds, -1 * boundsExtension, -1 * boundsExtension);
    BOOL touchOutside = !CGRectContainsPoint(outerBounds, [touch locationInView:sender]);
    if (touchOutside) {
        // UIControlEventTouchUpOutside
    } else {
        // UIControlEventTouchUpInside
    }
}

- (void)btnDragged:(UIButton *)sender withEvent:(UIEvent *)event {
    UITouch *touch = [[event allTouches] anyObject];
    CGFloat boundsExtension = 25.0f;
    CGRect outerBounds = CGRectInset(sender.bounds, -1 * boundsExtension, -1 * boundsExtension);
    BOOL touchOutside = !CGRectContainsPoint(outerBounds, [touch locationInView:sender]);
    if (touchOutside) {
        BOOL previewTouchInside = CGRectContainsPoint(outerBounds, [touch previousLocationInView:sender]);
        if (previewTouchInside) {
            // UIControlEventTouchDragExit
        } else {
            // UIControlEventTouchDragOutside
        }
    } else {
        BOOL previewTouchOutside = !CGRectContainsPoint(outerBounds, [touch previousLocationInView:sender]);
        if (previewTouchOutside) {
            // UIControlEventTouchDragEnter
        } else {
            // UIControlEventTouchDragInside
        }
    }    
}

Now all the six events could be handled with the bounds extension of 25 pixel, you could of course set this value to other as you wish.

回答3:

According to what I read here

Apple puts 100 pixels to account for the inaccuracy of using a finger. You can overide the methods using:

• -(BOOL)beginTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event
• -(BOOL)continueTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event

And other methods related methods. Apple Documentation.

来源：https://stackoverflow.com/questions/14340122/uicontroleventtouchdragexit-triggers-when-100-pixels-away-from-uibutton