ios 9.2 预测TouchesForTouch : No documentation

Question

WWDC 2015 - session 233 - iOS:
https://developer.apple.com/videos/play/wwdc2015/233/?time=74

我可以找到 Apple 演示代码，但我终生无法在 [UIEvent predictedTouchesForTouch:] 上找到任何实际文档.

它应该是一个 iOS 9 API。这个 API 是活跃的，只是还没有记录吗？我可以看到他们添加到 Swift 的 API 差异，但仍然没有文档。

Answer 1

是的，预测触摸的文档(很像合并触摸的文档)很少。在这一点上(Xcode 7.2 和 7.3 Beta 4)，我看到的唯一引用(除了您引用的 WWDC 视频和其他散布在网络上的演示)在标题中，它描述了预测的接触:

An array of auxiliary UITouch’s for touch events that are predicted to occur for a given main touch. These predictions may not exactly match the real behavior of the touch as it moves, so they should be interpreted as an estimate.

但它的工作原理与您引用的 WWDC 视频中的描述完全相同，包括 Swift 和 Objective-C。

请注意，您可能不会在模拟器上看到预测和合并的触摸，但如果您使用功能强大的设备，您会看到。我在使用我的 iPhone 6+ 时偶尔会看到预期的触摸，但在 iPad Pro 上始终可以看到它们(如果您在 iPad Pro 上使用 Apple Pencil 甚至更多)。

只是为了说明合并和预测的触摸，这里是一个 Objective-C 演示:

@interface ViewController ()

@property (nonatomic, strong) UIBezierPath *path;
@property (nonatomic, weak) CAShapeLayer *pathLayer;
@property (nonatomic, weak) CAShapeLayer *predictedPathLayer;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    // set up layer for the main (non-predicted) path; line blue line

    CAShapeLayer *mainPathLayer = [CAShapeLayer layer];
    mainPathLayer.lineWidth = 1;
    mainPathLayer.strokeColor = [UIColor blueColor].CGColor;
    mainPathLayer.fillColor = [UIColor clearColor].CGColor;
    mainPathLayer.lineCap = kCALineCapRound;
    mainPathLayer.lineJoin = kCALineJoinRound;
    mainPathLayer.frame = self.view.layer.bounds;
    [self.view.layer addSublayer:mainPathLayer];
    self.pathLayer = mainPathLayer;

    // set up layer for the predicted path, if any; thicker red line (just so I can see it clearly)

    CAShapeLayer *predictedPathLayer = [CAShapeLayer layer];
    predictedPathLayer.lineWidth = 5;
    predictedPathLayer.strokeColor = [UIColor redColor].CGColor;
    predictedPathLayer.fillColor = [UIColor clearColor].CGColor;
    predictedPathLayer.lineCap = kCALineCapRound;
    predictedPathLayer.lineJoin = kCALineJoinRound;
    predictedPathLayer.frame = self.view.layer.bounds;
    [self.view.layer addSublayer:predictedPathLayer];
    self.predictedPathLayer = predictedPathLayer;
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    self.path = [UIBezierPath bezierPath];
    [self.path moveToPoint:[touch locationInView:self.view]];
}

- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint location = [touch locationInView:self.view];

    // handle coalesced touches

    NSArray<UITouch *> *coalescedTouches = [event coalescedTouchesForTouch:touch];
    for (UITouch *coalescedTouch in coalescedTouches) {
        location = [coalescedTouch locationInView:self.view];
        [self.path addLineToPoint:location];
    }

    self.pathLayer.path = self.path.CGPath;

    // now handle predicted touches

    UIBezierPath *predictedPath = [UIBezierPath bezierPath];
    [predictedPath moveToPoint:location];
    NSArray<UITouch *> *predictedTouches = [event predictedTouchesForTouch:touch];


    for (UITouch *predictedTouch in predictedTouches) {
        [predictedPath addLineToPoint:[predictedTouch locationInView:self.view]];
    }

    self.predictedPathLayer.path = predictedPath.CGPath;

    // for diagnostic purposes, let's log the count of each; do not do this in production app

    NSLog(@"%ld %ld", [coalescedTouches count], [predictedTouches count]);
}

- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    self.predictedPathLayer.path = NULL; // discard any predicted path, because that's no longer valid
}

@end

如果您需要支持 9 之前的 iOS 版本，请查看 respondsToSelector ， 也。

Swift 示例非常相似。请参阅这篇关于平滑曲线的无关帖子，它说明了如何在 Swift 中使用预测(和合并)触摸: Drawing class drawing straight lines instead of curved lines