如何繼續drawRect：當手指在屏幕上時

Question

我有當前代碼：

- (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];
}

我的問題是我要讓物體一直跟隨您的手指，直到您將手指從屏幕上移開。 如果我的手指在移動，它只會跟隨。 touchesEnded僅在我將手指從屏幕上touchesEnded時才有效，所以那也不是我想要的。 如何啟用可以解決我問題的功能？

Answer 1

如果您想觸摸屏幕的一部分，並且想要在不放開手指的情況下沿該方向移動繪制的對象，則有兩種方法。

進場方法是使用某種形式的計時器，這種方法會在用戶將手指按住在屏幕上時重復調用一種方法（因為，正如您所指出的，移動時您只會獲得touchesMoved更新）。 雖然NSTimer是您遇到的最常見的計時器，但在這種情況下，您需要使用一種稱為顯示鏈接的專用計時器，即CADisplayLink ，它可以在執行屏幕更新時觸發。 因此，您將：

• 在touchesBegan ，捕獲用戶在屏幕上觸摸的位置並啟動CADisplayLink ；

• 在touchesMoved ，您將更新用戶的觸摸位置（但只有在他們移動手指時才會調用）；

• 在touchesEnded ，您可能會停止顯示鏈接； 和

• 在CADisplayLink處理程序中，您將更新位置（並且您需要知道其移動速度）。

因此，它看起來像：

- (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];
}

並使用它，您需要定義一些屬性：

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

Answer 2

如果要用手指拖動它，則需要：

• 在touchesBegan ，保存起始locationInView以及要拖動的對象的“原始位置”；

• 在touchesMoved ，獲取新的locationInView ，計算該locationInView與原始locationInView之間的增量（“平移”），將其添加到視圖的已保存“原始位置”，然后使用該值來更新視圖。

這樣，當您在屏幕上拖動對象時，該對象將用手指跟蹤100％。

---

例如，您可能會：

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    self.touchBeganLocation = [[touches anyObject] locationInView:self];
    self.originalObjectPoint = self.objectPoint;
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    CGPoint location = [[touches anyObject] locationInView:self];
    CGPoint translation = CGPointMake(location.x - self.touchBeganLocation.x, location.y - self.touchBeganLocation.y);
    self.objectPoint = CGPointMake(self.originalObjectPoint.x + translation.x, self.originalObjectPoint.y + translation.y);
    [self setNeedsDisplay];
}

可能無需多說，您需要一些屬性來跟蹤這兩個新的CGPoint值：

@property (nonatomic) CGPoint originalObjectPoint;
@property (nonatomic) CGPoint touchBeganLocation;

坦白地說，我可能會使用手勢識別器，但這是使用touchesBegan和touchesMoved拖動的示例。