iOS 绘图优化

  在iOS中，软件绘图通常是由Core Graphics框架完成来完成。但是，在一些必要的情况下，相比Core Animation和OpenGL，Core Graphics要慢了不少。
  软件绘图不仅效率低，还会消耗可观的内存。 CALayer 只需要一些与自己相关的内存：只有它的寄宿图会消耗一定的内存空间。即使直接赋给 contents 属性一张图片，也不需要增加额外的照片存储大小。如果相同的一张图片被多个图层作为 contents 属性，那么他们将会共用同一块内存，而不是复制内存块。
  但是一旦你实现了 CALayerDelegate 协议中的 -drawLayer:inContext: 方法或者 UIView 中的 -drawRect: 方法（其实就是前者的包装方法），图层就创建了一个绘制上下文，这个上下文需要的大小的内存可从这个算式得出：图层宽图层高4字节，宽高的单位均为像素。对于一个在Retina iPad上的全屏图层来说，这个内存量就是 204815264字节，相当于12MB内存，图层每次重绘的时候都需要重新抹掉内存然后重新分配。

1.用Core Graphics实现一个简单的绘图应用

#import "DrawingCoreGraphicsView.h"

@interface DrawingCoreGraphicsView()
@property (nonatomic, strong) UIBezierPath *path;
@end

@implementation DrawingCoreGraphicsView

- (instancetype)initWithFrame:(CGRect)frame{
    self = [super initWithFrame:frame];
    if (self) {
        [self initPath];
    }
    return self;
}

- (void)awakeFromNib{
    [super awakeFromNib];
    [self initPath];
}

- (void)initPath{
    self.path = [[UIBezierPath alloc] init];
    self.path.lineJoinStyle = kCGLineJoinRound;
    self.path.lineCapStyle = kCGLineCapRound;
    self.path.lineWidth = 5;
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    //get the starting point
    CGPoint point = [[touches anyObject] locationInView:self];
    //move the path drawing cursor to the starting point
    [self.path moveToPoint:point];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    //get the current point
    CGPoint point = [[touches anyObject] locationInView:self];
    //add a new line segment to our path
    [self.path addLineToPoint:point];
    //redraw the view
    [self setNeedsDisplay];
}

- (void)drawRect:(CGRect)rect
{
    //draw path
    [[UIColor clearColor] setFill];
    [[UIColor redColor] setStroke];
    [self.path stroke];
}

@end

绘制帧率

  这样实现的问题在于，我们画得越多，程序就会越慢。因为每次移动手指的时候都会重绘整个贝塞尔路径（ UIBezierPath ），随着路径越来越复杂，每次重绘的工作就会增加，直接导致了帧数的下降。

2.用 CAShapeLayer 重新实现绘图应用

#import "DrawingShapeLayerView.h"

@interface DrawingShapeLayerView()
@property (nonatomic, strong) UIBezierPath *path;
@end

@implementation DrawingShapeLayerView

+ (Class)layerClass
{
    //this makes our view create a CAShapeLayer
    //instead of a CALayer for its backing layer
    return [CAShapeLayer class];
}

- (instancetype)initWithFrame:(CGRect)frame{
    self = [super initWithFrame:frame];
    if (self) {
        self.path = [[UIBezierPath alloc] init];
        //configure the layer
        CAShapeLayer *shapeLayer = (CAShapeLayer *)self.layer;
        shapeLayer.strokeColor = [UIColor redColor].CGColor;
        shapeLayer.fillColor = [UIColor clearColor].CGColor;
        shapeLayer.lineJoin = kCALineJoinRound;
        shapeLayer.lineCap = kCALineCapRound;
        shapeLayer.lineWidth = 5;
    }
    return self;
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    //get the starting point
    CGPoint point = [[touches anyObject] locationInView:self];
    //move the path drawing cursor to the starting point
    [self.path moveToPoint:point];
}
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    //get the current point
    CGPoint point = [[touches anyObject] locationInView:self];
    //add a new line segment to our path
    [self.path addLineToPoint:point];
    //update the layer with a copy of the path
    
    ((CAShapeLayer *)self.layer).path = self.path.CGPath;
}

@end

绘制帧率2

  这种比CoreGraphics稍微好点，但绘制的点多了，掉帧一样很厉害。
  为了减少不必要的绘制，Mac OS和iOS设备将会把屏幕区分为需要重绘的区域和不需要重绘的区域。那些需要重绘的部分被称作『脏区域』当一个视图被改动过了，TA可能需要重绘。但是很多情况下，只是这个视图的一部分被改变了，所以重绘整个寄宿图就太浪费了。

3.下面是使用图片来绘制一个黑板檫的功能，用-setNeedsDisplayInRect: 来减少不必要的绘制

#define BRUSH_SIZE 32

@interface DrawingBlackBoardView()
@property (nonatomic, strong) NSMutableArray *strokes;
@end

@implementation DrawingBlackBoardView

- (instancetype)initWithFrame:(CGRect)frame{
    self = [super initWithFrame:frame];
    if (self) {
        self.strokes = [NSMutableArray array];
    }
    return self;
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    //get the starting point
    CGPoint point = [[touches anyObject] locationInView:self];
    //add brush stroke
    [self addBrushStrokeAtPoint:point];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    //get the touch point
    CGPoint point = [[touches anyObject] locationInView:self];
    //add brush stroke
    [self addBrushStrokeAtPoint:point];
}

- (void)addBrushStrokeAtPoint:(CGPoint)point
{
    //add brush stroke to array
    [self.strokes addObject:[NSValue valueWithCGPoint:point]];
    [self setNeedsDisplayInRect:[self brushRectForPoint:point]];
}

- (CGRect)brushRectForPoint:(CGPoint)point
{
    return CGRectMake(point.x - BRUSH_SIZE/2, point.y - BRUSH_SIZE/2, BRUSH_SIZE, BRUSH_SIZE);
}
     
-(void)drawRect:(CGRect)rect
{
    for (NSValue *value in self.strokes) {
        CGPoint point = [value CGPointValue];
        CGRect brushRect = [self brushRectForPoint:point];
        if (CGRectIntersectsRect(rect, brushRect)) {
            [[UIImage imageNamed:@"Chalk.png"] drawInRect:brushRect];
        }
    }
}
@end

帧率

  可以看到，即使到了最后面，帧率还能接近最大值。这种只绘制重绘区域，极大的优化了重绘功能。
总结:
1.第三种方式以极大的优化了绘制的性能，我们还可以使用异步绘制来优化，性能会更棒，这个就留读者自行思考了。
2.有任何问题欢迎留言评论。