MarqueeLabel原理探究

- (CGSize)subLabelSize { //获取当前文本的宽高

    // Calculate expected size 进行一些初始化设置

    CGSize expectedLabelSize = CGSizeZero;

    CGSize maximumLabelSize = CGSizeMake(CGFLOAT_MAX, CGFLOAT_MAX);//设置最大显示长度

    // Get size of subLabel  根据self.subLabel的文本获取当前最合适的尺寸

    expectedLabelSize = [self.subLabel sizeThatFits:maximumLabelSize];

#ifdef TARGET_OS_IOS

    // Sanitize width to 5461.0f (largest width a UILabel will draw on an iPhone 6S Plus)

    expectedLabelSize.width = MIN(expectedLabelSize.width, 5461.0f); //比较最大显示长度

#elif TARGET_OS_TV 

    // Sanitize width to 16384.0 (largest width a UILabel will draw on tvOS) //tvOS下的设置

    expectedLabelSize.width = MIN(expectedLabelSize.width, 16384.0f);

#endif

    // Adjust to own height (make text baseline match normal label)

    expectedLabelSize.height = self.bounds.size.height; //当前显示的高度

    return expectedLabelSize;

}

- (void)scrollAwayWithInterval:(NSTimeInterval)interval delayAmount:(NSTimeInterval)delayAmount shouldReturn:(BOOL)shouldReturn {
    // Check for conditions which would prevent scrolling
    if (![self labelReadyForScroll]) {
        return;
    }
    
    // Return labels to home (cancel any animations)
    [self returnLabelToOriginImmediately];
    
    // Call pre-animation method
    [self labelWillBeginScroll];
    
    // Animate 开启动画
    [CATransaction begin];
    
    // Set Duration
    [CATransaction setAnimationDuration:(!shouldReturn ? CGFLOAT_MAX : 2.0 * (delayAmount + interval))];
    
    // Create animation for gradient, if needed
    if (self.fadeLength != 0.0f) {
        CAKeyframeAnimation *gradAnim = [self keyFrameAnimationForGradientFadeLength:self.fadeLength
                                                                            interval:interval
                                                                               delay:delayAmount];
        [self.layer.mask addAnimation:gradAnim forKey:@"gradient"]; //更加self.fadeLength遮罩添加动画
    }
    
    __weak __typeof__(self) weakSelf = self;
    self.scrollCompletionBlock = ^(BOOL finished) {
        if (!finished || !weakSelf) {
            // Do not continue into the next loop
            return;
        }
        // Call returned home method
        [weakSelf labelReturnedToHome:YES];
        // Check to ensure that:
        // 1) We don‘t double fire if an animation already exists
        // 2) The instance is still attached to a window - this completion block is called for
        //    many reasons, including if the animation is removed due to the view being removed
        //    from the UIWindow (typically when the view controller is no longer the "top" view)
        if (self.window && ![weakSelf.subLabel.layer animationForKey:@"position"]) {
            // Begin again, if conditions met
            if (weakSelf.labelShouldScroll && !weakSelf.tapToScroll && !weakSelf.holdScrolling) {
                [weakSelf scrollAwayWithInterval:interval delayAmount:delayAmount shouldReturn:shouldReturn];
            }
        }
    };
    
    
    // Create animation for position
    CGPoint homeOrigin = self.homeLabelFrame.origin;
    CGPoint awayOrigin = MLOffsetCGPoint(self.homeLabelFrame.origin, self.awayOffset);
    
    NSArray *values = nil;
    switch (self.marqueeType) {
        case MLLeft:
        case MLRight:
            values = @[[NSValue valueWithCGPoint:homeOrigin],      // Initial location, home
                       [NSValue valueWithCGPoint:homeOrigin],      // Initial delay, at home
                       [NSValue valueWithCGPoint:awayOrigin],      // Animation to away
                       [NSValue valueWithCGPoint:awayOrigin]];     // Delay at away
            break;
            
        default:
            values = @[[NSValue valueWithCGPoint:homeOrigin],      // Initial location, home
                       [NSValue valueWithCGPoint:homeOrigin],      // Initial delay, at home
                       [NSValue valueWithCGPoint:awayOrigin],      // Animation to away
                       [NSValue valueWithCGPoint:awayOrigin],      // Delay at away
                       [NSValue valueWithCGPoint:homeOrigin]];     // Animation to home
            break;
    }
    
    CAKeyframeAnimation *awayAnim = [self keyFrameAnimationForProperty:@"position"
                                                                values:values
                                                              interval:interval
                                                                 delay:delayAmount];
    // Add completion block
    [awayAnim setValue:@(YES) forKey:kMarqueeLabelAnimationCompletionBlock];
    
    // Add animation
    [self.subLabel.layer addAnimation:awayAnim forKey:@"position"]; //根据position属性滚动动画
    
    [CATransaction commit];
}

// Configure replication //复制一份相同的图层，设置偏移值，每当前一个repliLayer滚出边界，则会重新生成一个layer，实现无限循环效果。
 self.repliLayer.instanceCount = 2; 
 self.repliLayer.instanceTransform = CATransform3DMakeTranslation(-self.awayOffset, 0.0, 0.0);

UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(0, 300, 70, 70)];
imageView.image = [UIImage imageNamed:@"bg"];
imageView.layer.backgroundColor = [UIColor blackColor].CGColor;
imageView.backgroundColor  = [UIColor blueColor];
[self.view addSubview:imageView];

CALayer* shapeLayer = [CALayer layer];
shapeLayer.frame = imageView.bounds;
shapeLayer.backgroundColor = [UIColor colorWithRed:1.0 green:1.0 blue:1.0 alpha:1.0].CGColor;//当设置alpha为0时，则图片出不来
imageView.layer.mask = shapeLayer;

UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(0, 300, 70, 70)];
imageView.image = [UIImage imageNamed:@"bg"];
imageView.layer.backgroundColor = [UIColor blackColor].CGColor;
imageView.backgroundColor  = [UIColor blueColor];
[self.view addSubview:imageView];

CALayer* shapeLayer = [CALayer layer];
shapeLayer.frame = imageView.bounds;
shapeLayer.contents = (id)[UIImage imageNamed:@"bgTransparent"].CGImage; //提示：注意图片的透明通道问题
imageView.layer.mask = shapeLayer;