CAShapeLayer文字动画

文字动画也是CAShapeLayer图形动画的一种，巧妙运用文字动画可以实现很不错的效果，比如半糖app的下拉动画。本文就主要讲一下其实现原理。

文字动画主要分为两部分：

• 将文字转化为CAShapeLayer
• 通过更改CAShapeLayer的StrokeEnd属性值生成动画

将文字转换为CAShapeLayer

将文字转换为CAShapeLayer的过程比较复杂，可以细分为以下几个步骤：

• 创建NSAttributedString并生成CTLineRef
• 使用CTLineRef生成CTRunRef数组
• 遍历CTRunRef数组，得到每个CTRunRef
• 遍历CTRunRef中每个长度为1的区间生成CGGlyph并转换为CGPath路径，将所有路径拼接起来
• 创建ShapeLayer并将生成的路径赋值给该ShapeLayer

以下是每个步骤的实现方式：

创建NSAttributedString并生成CTLineRef

// 定义字体属性
CTFontRef font = CTFontCreateWithName(CFSTR("HelveticaNeue-UltraLight"), fontSize, NULL);
NSDictionary *attrs = [NSDictionary dictionaryWithObjectsAndKeys:(__bridge id)font, kCTFontAttributeName,nil];
// 创建NSAttributedString
NSAttributedString *str = [[NSAttributedString alloc] initWithString:text attributes:attrs];
CTLineRef line = CTLineCreateWithAttributedString((CFAttributedStringRef)str);

使用CTLineRef生成CTRunRef数组

CTLineRef line = CTLineCreateWithAttributedString((CFAttributedStringRef)str);
CFArrayRef runArray = CTLineGetGlyphRuns(line);

遍历CTRunRef数组，得到每个CTRunRef

for (CFIndex runIndex = 0; runIndex < CFArrayGetCount(runArray); runIndex++) {
    // 
    CTRunRef run = (CTRunRef)CFArrayGetValueAtIndex(runArray, runIndex);
    CTFontRef runFont = CFDictionaryGetValue(CTRunGetAttributes(run), kCTFontAttributeName);
}

遍历CTRunRef中每个长度为1的区间生成CGGlyph并转换为CGPath路径，将所有路径拼接起来

for (CFIndex glyphIndex = 0; glyphIndex < CTRunGetGlyphCount(run); glyphIndex++) {
      
    CGGlyph glyph;
    CGPoint position;
    CFRange currentRange = CFRangeMake(glyphIndex, 1);
    CTRunGetGlyphs(run, currentRange, &glyph);
    CTRunGetPositions(run, currentRange, &position);
            
    CGPathRef letter = CTFontCreatePathForGlyph(runFont, glyph, NULL);
    CGAffineTransform t = CGAffineTransformMakeTranslation(position.x, position.y);
    CGPathAddPath(letters, &t, letter);
    CGPathRelease(letter);
}

创建ShapeLayer并将生成的路径赋值给该ShapeLayer

// 创建UIBezierPath
UIBezierPath *path = [UIBezierPath bezierPathWithCGPath:letters];
// 创建并配置CAShapeLayer
CAShapeLayer *pathLayer = [CAShapeLayer layer];
pathLayer.frame = self.bounds;
pathLayer.bounds = CGPathGetBoundingBox(path.CGPath);
pathLayer.geometryFlipped = YES;
pathLayer.path = path.CGPath;
pathLayer.strokeColor = fontColor.CGColor;
pathLayer.fillColor = nil;
pathLayer.lineWidth = 1.0f;
pathLayer.lineJoin = kCALineJoinBevel;
pathLayer.strokeStart = 0;
pathLayer.strokeEnd = 0;

最后，将ShapeLayer添加到动画图层中就行了

生成文字动画

通过改变ShapeLayer的StrokeEnd属性值就可以生成文字动画

添加Slider

- (void)setupSlider {
    CGFloat leftMargin = 20;
    UISlider *slider = [[UISlider alloc] initWithFrame:CGRectMake(leftMargin, kMainHeight - 30, kMainWidth - leftMargin * 2, 3)];
    slider.minimumValue = 0;
    slider.maximumValue = 1;
    [self.view addSubview:slider];
    [slider addTarget:self action:@selector(sliderValueChanged:) forControlEvents:UIControlEventValueChanged];
}

实现Slider的ValueChange方法

- (void)sliderValueChanged:(UISlider *)sender {
    self.pathLayer.strokeEnd = sender.value;
}

至此，一个文字动画就完成了

本文demo的github地址：https://github.com/zephyrw/TextAnimationDemo.git