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CALayer

0.calyer的簡(jiǎn)介

1.使用calyer

layerView上添加一個(gè)藍(lán)色layer

#import "ViewController.h"
#import
@interface ViewController ()
@property (nonatomic, weak) IBOutlet UIView *layerView;
@end
@implementation ViewController
- (void)viewDidLoad
{
    [super viewDidLoad];
    //create sublayer
    CALayer *blueLayer = [CALayer layer];
    blueLayer.frame = CGRectMake(50.0f, 50.0f, 100.0f, 100.0f);
    blueLayer.backgroundColor = [UIColor blueColor].CGColor;
    //add it to our view
    [self.layerView.layer addSublayer:blueLayer];
} @end

2.contents屬性

用layer顯示一張圖片

@implementation ViewController
- (void)viewDidLoad
{
  [super viewDidLoad]; //load an image
  UIImage *image = [UIImage imageNamed:@"Snowman.png"];
  //add it directly to our view's layer
  self.layerView.layer.contents = (__bridge id)image.CGImage;
}
@end

3.contentGravity

CALayer 與 contentMode 相對(duì)應(yīng)的屬性叫 contentsGravity 但他是NSString類型，并不是枚舉。

kCAGravityCenter 
kCAGravityTop 
kCAGravityBottom
kCAGravityLeft
kCAGravityRight
kCAGravityTopLeft
kCAGravityTopRight
kCAGravityBottomLeft
kCAGravityBottomRight
kCAGravityResize
kCAGravityResizeAspect 等同于 UIViewContentModeScaleAspectFit
kCAGravityResizeAspectFill

self.layerView.layer.contentsGravity = kCAGravityResizeAspect;

4.contentsScale

contentsScale 屬性定義了寄宿圖的像素尺寸和視圖大小的比例，默認(rèn)為1.0f.這個(gè)屬性和屏幕分辨率有關(guān)系。如果為2.0f，那么就是Retina屏幕。這并不會(huì)對(duì)我們?cè)谑褂胟CAGravityResizeAspect時(shí)產(chǎn)生響因?yàn)樗褪抢旌蟮膱D片，已經(jīng)適應(yīng)圖層。根本不考慮分辨率問題。kCAGravityCenter 不會(huì)拉伸圖片。

@implementation ViewController
- (void)viewDidLoad
{
    [super viewDidLoad]; //load an image
    UIImage *image = [UIImage imageNamed:@"Snowman.png"]; //add     it directly to our view's layer
    self.layerView.layer.contents = (__bridge   id)image.CGImage; //center the image
    self.layerView.layer.contentsGravity = kCAGravityCenter;
    //set the contentsScale to match image
    self.layerView.layer.contentsScale = image.scale;
}
@end

layer.contentsScale = [UIScreen mainScreen].scale;

5.maskToBounds

UIView有一個(gè)叫做clipsToBounds的屬性可以用來決定是否顯示超出邊界的內(nèi)容,CALayer對(duì)應(yīng)的屬性叫做masksToBounds,把它設(shè)置為YES,超出的部分就不會(huì)顯示啦。

layer.masksToBounds = YES;

6.contentsRect

CALayer 的 contentsRect 允許我們?cè)趫D層邊框里顯示寄宿圖的一個(gè)子域。這會(huì)涉及到 contentsGravity 是如何顯示和拉伸的。他的值為rect，值按單位計(jì)算［0，1］。

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self addView:self.oneView rect:CGRectMake(0, 0, .5, .5)];
    [self addView:self.twoView rect:CGRectMake(.5, 0, .5, .5)];
    [self addView:self.threeView rect:CGRectMake(0, .5, .5, .5)];
    [self addView:self.fourView rect:CGRectMake(.5, .5, .5, .5)];
    
}

- (void)addView:(UIView *)view rect:(CGRect)rect{
    
    UIImage *img = [UIImage imageNamed:@"5"];
    
    view.layer.contents = (id)img.CGImage;
    view.layer.contentsGravity = kCAGravityResizeAspect;
    view.layer.contentsScale = [UIScreen mainScreen].scale;
    view.layer.contentsRect = rect;   
}

7.contentsCenter

contentsCenter 工作起來的效果和 UIImage 中 -resizableImageWithCapInsets:方法非常相似，只是它可以用到人和寄宿圖，甚至包括Core Graphics。用圖片rect區(qū)域來進(jìn)行填充。保護(hù)邊角不備拉伸。{0, 0, 1, 1}
可以在xib上設(shè)置，不用寫代碼。view －> srewtching

UIImage *imgzero = [UIImage imageNamed:@"44"];
    self.zeroView.layer.contents = (id)imgzero.CGImage;
//    self.zeroView.layer.contentsGravity = kCAGravityResizeAspect;
    self.zeroView.layer.contentsCenter = CGRectMake(0.5, 0.5, 0.1, 0.1);

8.Custom Drawing(自定義繪圖)

給contents賦CGImage的值不是唯一的設(shè)置寄宿圖的方法。我們也可以直接用Core Graphics直接繪制寄宿圖。能夠通過繼承UIView并實(shí)現(xiàn)drawRect:方法來自定義繪制。
-drawRect: 方法沒有默認(rèn)的實(shí)現(xiàn)，因?yàn)閷?duì)UIView來說，寄宿圖并不是必須的，它不在意那到底是單調(diào)的顏色還是有一個(gè)圖片的實(shí)例。如果UIView檢測(cè)到-drawRect:方法被調(diào)用了，它就會(huì)為視圖分配一個(gè)寄宿圖，這個(gè)寄宿圖的像素尺寸等于視圖大小乘以contentsScale的值。

當(dāng)需要被重繪時(shí)，CALayer會(huì)請(qǐng)求它的代理給他一個(gè)寄宿圖來顯示。它通過調(diào)用下面這個(gè)方法做到的:
(void)displayLayer:(CALayerCALayer *)layer;
趁著這個(gè)機(jī)會(huì)，如果代理想直接設(shè)置contents屬性的話，它就可以這么做，不然沒有別的方法可以調(diào)用了。如果代理不實(shí)現(xiàn)-displayLayer:方法，CALayer就會(huì)轉(zhuǎn)而嘗試調(diào)用下面這個(gè)方法：- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx;
在調(diào)用這個(gè)方法之前，CALayer創(chuàng)建了一個(gè)合適尺寸的空寄宿圖（尺寸由bounds和contentsScale決定）和一個(gè)Core Graphics的繪制上下文環(huán)境，為繪制寄宿圖做準(zhǔn)備，他作為ctx參數(shù)傳入。

@implementation ViewController
- (void)viewDidLoad
{
  [super viewDidLoad];
  
  //create sublayer
  CALayer *blueLayer = [CALayer layer];
  blueLayer.frame = CGRectMake(50.0f, 50.0f, 100.0f, 100.0f);
  blueLayer.backgroundColor = [UIColor blueColor].CGColor;

  //set controller as layer delegate
  blueLayer.delegate = self;

  //ensure that layer backing image uses correct scale
  blueLayer.contentsScale = [UIScreen mainScreen].scale; //add layer to our view
  [self.layerView.layer addSublayer:blueLayer];

  //force layer to redraw
  [blueLayer display];
}

- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx
{
  //draw a thick red circle
  CGContextSetLineWidth(ctx, 10.0f);
  CGContextSetStrokeColorWithColor(ctx, [UIColor redColor].CGColor);
  CGContextStrokeEllipseInRect(ctx, layer.bounds);
}
@end