性能优化之 UI渲染优化 - 异步渲染

使用displayLayer进行异步绘制

UIView 绘制流程图

@interface PWView : UIView
@property (nonatomic,strong) NSString *text;
@property (nonatomic,strong) UIColor *backgColor;
@end

#import "PWView.h"
#import <CoreText/CoreText.h>
@implementation PWView

// layer层的重写
-(void)displayLayer:(CALayer *)layer{
    CGSize size = self.bounds.size;
    CGFloat scale = [UIScreen mainScreen].scale;
    // 异步绘制 切换至子线程
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        UIGraphicsBeginImageContextWithOptions(size, NO, scale);
        // 获取当前上下文
        CGContextRef context = UIGraphicsGetCurrentContext();
        // 将坐标系反转
        CGContextSetTextMatrix(context, CGAffineTransformIdentity);
        // 文本沿着Y轴移动
        CGContextTranslateCTM(context, 0, size.height);
        // 文本反转成context坐标系
        CGContextScaleCTM(context, 1, -1);
        // 创建绘制区域
        CGMutablePathRef path = CGPathCreateMutable();
        CGPathAddRect(path, NULL, CGRectMake(0 , 0, size.width, size.height));
        // 背景色
        CGContextSetFillColorWithColor(context, self.backgColor.CGColor);
        CGContextAddRect(context, CGRectMake(0, 0, size.width * scale, size.height * scale));
        CGContextFillPath(context);
        // 创建需要绘制的文字
        NSMutableAttributedString * attStr = [[NSMutableAttributedString alloc]initWithString:self.text];
        // 根据attStr 生产CTFramesetterRef
        CTFramesetterRef frameSetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)attStr);
        CTFrameRef frame = CTFramesetterCreateFrame(frameSetter, CFRangeMake(0, attStr.length), path, NULL);
        // 将frame的内容绘制到context中
        CTFrameDraw(frame, context);
        
        UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();//结束绘制
        
        dispatch_async(dispatch_get_main_queue(), ^{
            self.layer.contents = (__bridge id _Nullable)(image.CGImage);
        });
    });
}

@end


调用 
    PWView *view = [[PWView alloc]initWithFrame:CGRectMake(100, 100, 200, 200)];
    view.text = @"BBBB";
    view.backgColor = [UIColor grayColor];
    [self.view addSubview:view];
    // 注意 需要调用layer层的setNeedsDisplay才会进行异步绘制
    [view.layer setNeedsDisplay];