IOS在视频上添加字幕效果的基本思路是：

1. 使用自定义的CATextLayer文字图层或者CAShapeLayer文字图层，添加到视频的Layer上创建用户自定义的字幕效果。这两者的区别是：CATextLayer支持设置简单的文字效果，包括文字的内容、字体、字号大小、对其方式、文字颜色、背景颜色等基本的属性；CAShapeLayer功能更强大，提供了CATextLayer没有的边框大小、边框颜色等设置，如果需要更高级的文字内容展示，需要使用CATextLayer配合UIBezierPath来定制自定义的文字内容。

2. 通过设置Layer图层的动画来控制字幕的时间点和时间长度，这里有一个坑如果单独设置CATextLayer或者CAShapeLayer的动画不能控制开始的时间，需要额外添加一个CALayer图层，把文字图层CATextLayer或者CAShapeLayer添加到父CALayer图层中，在文字图层CATextLayer或者CAShapeLayer上设置开始的动画

    NSTimeInterval animatedInStartTime = startTime + initAnimationDuration;CABasicAnimation *fadeInAnimation = [CABasicAnimation animationWithKeyPath:@"opacity"];fadeInAnimation.fromValue = @0.0f;fadeInAnimation.toValue = @1.0f;fadeInAnimation.additive = NO;fadeInAnimation.removedOnCompletion = NO;fadeInAnimation.beginTime = animatedInStartTime;fadeInAnimation.duration = animationDuration;fadeInAnimation.autoreverses = NO;fadeInAnimation.fillMode = kCAFillModeBoth;[textLayer addAnimation:fadeInAnimation forKey:@"opacity"];
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在父CALayer图层上设置结束的动画，这样设置才能实现用户自定义的时间点和时间长度

NSTimeInterval animatedOutStartTime = startTime + duration - animationDuration;CABasicAnimation *fadeOutAnimation = [CABasicAnimation animationWithKeyPath:@"opacity"];fadeOutAnimation.fromValue = @1.0f;fadeOutAnimation.toValue = @0.0f;fadeOutAnimation.additive = NO;fadeOutAnimation.removedOnCompletion = NO;fadeOutAnimation.beginTime = animatedOutStartTime;fadeOutAnimation.duration = animationDuration;fadeOutAnimation.autoreverses = NO;fadeOutAnimation.fillMode = kCAFillModeBoth;[animatedTitleLayer addAnimation:fadeOutAnimation forKey:@"opacity"];
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完整的代码

- (CALayer *)buildLayerbuildTxt:(NSString*)texttextSize:(CGFloat)textSizetextColor:(UIColor*)textColorstrokeColor:(UIColor*)strokeColoropacity:(CGFloat)opacitytextRect:(CGRect)textRectfontPath:(NSString*)fontPathviewBounds:(CGSize)viewBoundsstartTime:(NSTimeInterval)startTimeduration:(NSTimeInterval)duration
{if (!text || [text isEqualToString:@""]){return nil;}// Create a layer for the overall title animation.CALayer *animatedTitleLayer = [CALayer layer];// 1. Create a layer for the text of the title.CATextLayer *titleLayer = [CATextLayer layer];titleLayer.string = text;titleLayer.font = (__bridge CFTypeRef)(@"Helvetica");titleLayer.fontSize = textSize;titleLayer.alignmentMode = kCAAlignmentCenter;titleLayer.bounds = CGRectMake(0, 0, textRect.size.width, textRect.size.height);titleLayer.foregroundColor = textColor.CGColor;titleLayer.backgroundColor = [UIColor clearColor].CGColor;// [animatedTitleLayer addSublayer:titleLayer];// 添加文字以及边框效果UIFont *font = nil;if ((fontPath != nil) && (fontPath.length > 0)) {font = [[FLVideoEditFontManager sharedFLVideoEditFontManager] fontWithPath:fontPath size:textSize];titleLayer.font = CGFontCreateWithFontName((__bridge CFStringRef)font.fontName);}if (font == nil) {titleLayer.font = (__bridge CFTypeRef)(@"Helvetica");}UIBezierPath *path = nil;if (font) {path = [FLLayerBuilderTool createPathForText:text fontHeight:textSize fontName:(__bridge CFStringRef)(font.fontName)];}else{path = [FLLayerBuilderTool createPathForText:text fontHeight:textSize fontName:CFSTR("Helvetica")];}CGRect rectPath = CGPathGetBoundingBox(path.CGPath);CAShapeLayer *textLayer = [CAShapeLayer layer];textLayer.path = path.CGPath;textLayer.lineWidth = 1;if (strokeColor != nil) {textLayer.strokeColor = strokeColor.CGColor;}if (textColor != nil) {textLayer.fillColor = textColor.CGColor;}textLayer.lineJoin = kCALineJoinRound;textLayer.lineCap = kCALineCapRound;textLayer.geometryFlipped = NO;textLayer.opacity = opacity;textLayer.bounds = CGRectMake(0, 0, rectPath.size.width, textSize+10);[animatedTitleLayer addSublayer:textLayer];// 动画图层位置animatedTitleLayer.position = CGPointMake(textRect.origin.x+textRect.size.width/2, viewBounds.height - textRect.size.height/2 - textRect.origin.y);NSTimeInterval initAnimationDuration = 0.1f;NSTimeInterval animationDuration = 0.1f;// 3.显示动画NSTimeInterval animatedInStartTime = startTime + initAnimationDuration;CABasicAnimation *fadeInAnimation = [CABasicAnimation animationWithKeyPath:@"opacity"];fadeInAnimation.fromValue = @0.0f;fadeInAnimation.toValue = @1.0f;fadeInAnimation.additive = NO;fadeInAnimation.removedOnCompletion = NO;fadeInAnimation.beginTime = animatedInStartTime;fadeInAnimation.duration = animationDuration;fadeInAnimation.autoreverses = NO;fadeInAnimation.fillMode = kCAFillModeBoth;[textLayer addAnimation:fadeInAnimation forKey:@"opacity"];NSTimeInterval animatedOutStartTime = startTime + duration - animationDuration;CABasicAnimation *fadeOutAnimation = [CABasicAnimation animationWithKeyPath:@"opacity"];fadeOutAnimation.fromValue = @1.0f;fadeOutAnimation.toValue = @0.0f;fadeOutAnimation.additive = NO;fadeOutAnimation.removedOnCompletion = NO;fadeOutAnimation.beginTime = animatedOutStartTime;fadeOutAnimation.duration = animationDuration;fadeOutAnimation.autoreverses = NO;fadeOutAnimation.fillMode = kCAFillModeBoth;[animatedTitleLayer addAnimation:fadeOutAnimation forKey:@"opacity"];return animatedTitleLayer;
}
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依赖的工具类FLLayerBuilderTool.m文件：

#import "FLLayerBuilderTool.h"
#import <CoreText/CoreText.h>@implementation FLLayerBuilderTool+ (UIBezierPath*) createPathForText:(NSString*)string fontHeight:(CGFloat)height fontName:(CFStringRef)fontName
{if ([string length] < 1)return nil;UIBezierPath *combinedGlyphsPath = nil;CGMutablePathRef letters = CGPathCreateMutable();CTFontRef font = CTFontCreateWithName(fontName, height, NULL);if (font == nil) {font = (__bridge CFTypeRef)(@"Helvetica");}NSDictionary *attrs = [NSDictionary dictionaryWithObjectsAndKeys:(__bridge id)font, kCTFontAttributeName,nil];NSAttributedString *attrString = [[NSAttributedString alloc] initWithString:stringattributes:attrs];CTLineRef line = CTLineCreateWithAttributedString((CFAttributedStringRef)attrString);CFArrayRef runArray = CTLineGetGlyphRuns(line);// for each RUNfor (CFIndex runIndex = 0; runIndex < CFArrayGetCount(runArray); runIndex++){// Get FONT for this runCTRunRef run = (CTRunRef)CFArrayGetValueAtIndex(runArray, runIndex);CTFontRef runFont = CFDictionaryGetValue(CTRunGetAttributes(run), kCTFontAttributeName);// for each GLYPH in runfor (CFIndex runGlyphIndex = 0; runGlyphIndex < CTRunGetGlyphCount(run); runGlyphIndex++){// get Glyph & Glyph-dataCFRange thisGlyphRange = CFRangeMake(runGlyphIndex, 1);CGGlyph glyph;CGPoint position;CTRunGetGlyphs(run, thisGlyphRange, &glyph);CTRunGetPositions(run, thisGlyphRange, &position);// Get PATH of outline{CGPathRef letter = CTFontCreatePathForGlyph(runFont, glyph, NULL);CGAffineTransform t = CGAffineTransformMakeTranslation(position.x, position.y);CGPathAddPath(letters, &t, letter);CGPathRelease(letter);}}}CFRelease(line);combinedGlyphsPath = [UIBezierPath bezierPath];[combinedGlyphsPath moveToPoint:CGPointZero];[combinedGlyphsPath appendPath:[UIBezierPath bezierPathWithCGPath:letters]];CGPathRelease(letters);CFRelease(font);if (attrString){attrString = nil;}return combinedGlyphsPath;
}@end
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参考资料：
视频特效制作：如何给视频添加边框、水印、动画以及3D效果
视频特效制作2
AVFoundation Tutorial: Adding Overlays and Animations to Videos