ios - 如何继续绘制Rect : when finger on screen

Question

我有当前代码:

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

    self.objectPoint = [[touches anyObject] locationInView:self];
    float x, y;
    if (self.objectPoint.x > self.objectPoint.x) {
        x = self.objectPoint.x + 1;
    }
    else x = self.objectPoint.x - 1;
    if (self.fingerPoint.y > self.objectPoint.y) {
        y = self.objectPoint.y + 1;
    }
    else y = self.minionPoint.y - 1;
    self.objectPoint = CGPointMake(x, y);

    [self setNeedsDisplay];
}

我的问题是我想让对象一直跟随您的手指，直到您将手指从屏幕上移开。只有当我的手指移动时它才会跟随。 touchesEnded 只有当我的手指离开屏幕时才有效，所以这也不是我想要的。如何启用可以解决我的问题的功能？

Answer 1

如果您想要触摸屏幕的一部分，并且想要在按住手指的情况下沿该方向移动绘制的对象，则有两种方法。

一种方法是使用某种形式的计时器，当用户将手指按住屏幕时，它会重复调用一个方法(因为，正如您所指出的，您只会获得 touchesMoved 当你移动时)。虽然 NSTimer 是您遇到的最常见的计时器，但在这种情况下，您需要使用一个称为显示链接的专用计时器，即 CADisplayLink，它会触发何时可以执行屏幕更新。所以，你会:

• 在touchesBegan中，捕获用户在屏幕上触摸的位置并启动CADisplayLink；

• 在 touchesMoved 中，您将更新用户的触摸位置(但仅在他们移动手指时调用)；

• 在 touchesEnded 中，您可能会停止显示链接；和

• 在您的 CADisplayLink 处理程序中，您需要更新位置(并且您需要知道您希望它移动的速度)。

所以，这看起来像:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    self.velocity = 100.0;     // 100 points per second
    self.touchLocation = [[touches anyObject] locationInView:self];

    [self startDisplayLink];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    self.touchLocation = [[touches anyObject] locationInView:self];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self stopDisplayLink];
}

- (void)startDisplayLink
{
    self.displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(handleDisplayLink:)];
    self.lastTimestamp = CACurrentMediaTime();   // initialize the `lastTimestamp`
    [self.displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
}

- (void)stopDisplayLink
{
    [self.displayLink invalidate];
    self.displayLink = nil;
}

- (void)handleDisplayLink:(CADisplayLink *)displayLink
{
    // figure out the time elapsed, and reset the `lastTimestamp`

    CFTimeInterval currentTimestamp = CACurrentMediaTime();
    CFTimeInterval elapsed = currentTimestamp - self.lastTimestamp;
    self.lastTimestamp = currentTimestamp;

    // figure out distance to touch and distance we'd move on basis of velocity and elapsed time

    CGFloat distanceToTouch  = hypotf(self.touchLocation.y - self.objectPoint.y, self.touchLocation.x - self.objectPoint.x);
    CGFloat distanceWillMove = self.velocity * elapsed;

    // this does the calculation of the angle between the touch location and
    // the current `self.objectPoint`, and then updates `self.objectPoint` on
    // the basis of (a) the angle; and (b) the desired velocity.

    if (distanceToTouch == 0.0)                  // if we're already at touchLocation, then just quit
        return;
    if (distanceToTouch < distanceWillMove) {    // if the distance to move is less than the target, just move to touchLocation
        self.objectPoint = self.touchLocation;
    } else {                                     // otherwise, calculate where we're going to move to
        CGFloat angle = atan2f(self.touchLocation.y - self.objectPoint.y, self.touchLocation.x - self.objectPoint.x);
        self.objectPoint = CGPointMake(self.objectPoint.x + cosf(angle) * distanceWillMove,
                                       self.objectPoint.y + sinf(angle) * distanceWillMove);
    }
    [self setNeedsDisplay];
}

要使用它，您需要定义一些属性:

@property (nonatomic) CGFloat velocity;
@property (nonatomic) CGPoint touchLocation;
@property (nonatomic, strong) CADisplayLink *displayLink;
@property (nonatomic) CFTimeInterval lastTimestamp;