UIView是如何顯示一個頁面的？

1. CALayer通過代理，將需要繪制的信息傳遞給View
2. UIView通過CoreGraphics進行繪制的操作
3. 繪制好的內(nèi)容交給CALayer，方法有兩種

• layer.contents = image
• 存儲在CABackingStore

4. 通過OpenGL ES/Metal(GPU)進行顯示

CALayer的調(diào)用流程

CALayer調(diào)用流程，分為布局和繪制兩部分。
布局部分，先會調(diào)用CALayer的layerSublayers方法，再調(diào)用view的layoutSubViews
繪制部分，通過設置layer的setNeedsDisplay標記需要刷新，然后調(diào)用CALayer的display方法，再判斷UIView的displayLayer:
方法是否實現(xiàn)，如果實現(xiàn)走自定義繪制流程，否則進入系統(tǒng)繪制流程先調(diào)用drawLayer:InContext:，再執(zhí)行UIView的DrawRect

通過在drawRect進行斷點，查看函數(shù)調(diào)用棧

 * thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
 * frame #0: 0x0000000103de7317 CALayerProcessDemo`-[LCustomLayerView drawRect:](self=0x00007ff154508f80, _cmd="drawRect:", rect=(origin = (x = 0, y = 0), size = (width = 200, height = 200))) at LCustomLayerView.m:13:5
 frame #1: 0x00000001083704d3 UIKitCore`-[UIView(CALayerDelegate) drawLayer:inContext:] + 550
 frame #2: 0x00000001098f393d QuartzCore`-[CALayer drawInContext:] + 285
 frame #3: 0x00000001097ee969 QuartzCore`CABackingStoreUpdate_ + 173 //后臺存儲區(qū)
 frame #4: 0x00000001098fa3fc QuartzCore`___ZN2CA5Layer8display_Ev_block_invoke + 44
 frame #5: 0x00000001098f3439 QuartzCore`-[CALayer _display] + 2443
 frame #6: 0x0000000109904e77 QuartzCore`CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 359
 frame #7: 0x000000010987424a QuartzCore`CA::Context::commit_transaction(CA::Transaction*) + 328
 frame #8: 0x00000001098ab606 QuartzCore`CA::Transaction::commit() + 610
 frame #9: 0x0000000107eab2c3 UIKitCore`__34-[UIApplication _firstCommitBlock]_block_invoke_2 + 128
 frame #10: 0x0000000105081cbc CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
 frame #11: 0x0000000105081480 CoreFoundation`__CFRunLoopDoBlocks + 336
 frame #12: 0x000000010507bd04 CoreFoundation`__CFRunLoopRun + 1252
 frame #13: 0x000000010507b4d2 CoreFoundation`CFRunLoopRunSpecific + 626
 frame #14: 0x000000010d6bf2fe GraphicsServices`GSEventRunModal + 65
 frame #15: 0x0000000107e91fc2 UIKitCore`UIApplicationMain + 140
 frame #16: 0x0000000103de76a0 CALayerProcessDemo`main(argc=1, argv=0x00007ffeebe17ea8) at main.m:14:16
 frame #17: 0x0000000106a00541 libdyld.dylib`start + 1
 frame #18: 0x0000000106a00541 libdyld.dylib`start + 1

從調(diào)用棧中可以看到在調(diào)用drawRect之前，還會調(diào)用-[CALayer _display]（視圖繪制）、-[CALayer drawInContext:]（默認調(diào)用）、-[UIView(CALayerDelegate) drawLayer:inContext:]（自定義繪制流程）等方法，我們試著實現(xiàn)drawLayer:inContext時，發(fā)現(xiàn)drawRect不會再走

下面我們通過自定義CALayer，來分析下系統(tǒng)的CALayer到底做了什么

@protocol LLayerDelegate <NSObject>

- (void)l_layoutSublayersOfLayer:(CALayer *)layer;
- (void)l_displayLayer:(CALayer *)layer;
- (void)l_drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx;

- (CGContextRef)l_beginDraw;
- (void)l_endDraw;

@end
@interface LLayer : CALayer

@end
@implementation LLayer
- (void)layoutSublayers { //這個會調(diào)用UIView layoutSubviews
    __strong id <LLayerDelegate> delegate = (id<LLayerDelegate>)self.delegate;
    if ([delegate respondsToSelector:@selector(l_layoutSublayersOfLayer:)]) {
        //調(diào)用代理的實現(xiàn)
    }
}

- (void)display {
    //詢問代理繪制
    __strong id <LLayerDelegate> delegate = (id<LLayerDelegate>)self.delegate;
    CGContextRef context = [delegate l_beginDraw];
    //如何繪制
    [delegate l_drawLayer:self inContext:context];
        
    [delegate l_displayLayer:self];
        
    [delegate l_endDraw];
}
@end

@interface LCustomLayerView()<LLayerDelegate>
@end
@implementation LCustomLayerView
//下面我們模擬下layer的執(zhí)行流程
//layer走自定義的
+ (Class)layerClass {
    return [LLayer class];
}

#pragma -mark LLayerDelegate
- (void)l_layoutSublayersOfLayer:(CALayer *)layer {
    [self layoutSubviews];
}

- (CGContextRef)l_beginDraw {
    //開始繪制，并返回context
    UIGraphicsBeginImageContextWithOptions(self.bounds.size, self.layer.opaque, self.layer.contentsScale);
    CGContextRef cRef = UIGraphicsGetCurrentContext();
    return cRef;
}

- (void)l_drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx {
    [[UIColor blueColor] set];
    UIBezierPath *path = [UIBezierPath bezierPathWithRect:CGRectMake(self.bounds.size.width/2.0 - 30, self.bounds.size.height/2.0 - 30, 60, 60)];
    CGContextAddPath(ctx, path.CGPath);
    CGContextFillPath(ctx);
}

//自定義繪制
- (void)l_displayLayer:(CALayer *)layer {
    UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
    dispatch_async(dispatch_get_main_queue(), ^{
        layer.contents = (__bridge id _Nullable)(img.CGImage);
    });
}

- (void)l_endDraw {
    UIGraphicsEndImageContext();
}

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    [self.view addSubview:self.clView];
}
- (LCustomLayerView *)clView {
    if (!_clView) {
        _clView = [LCustomLayerView new];
        _clView.frame = CGRectMake(50, 100, 200, 200);
        _clView.backgroundColor = [UIColor orangeColor];
    }
    return _clView;
}

@end

重新布局方法，layoutSubviews調(diào)用時機

• 延時執(zhí)行
  • 被添加到父視圖中
  • 添加子控件
  • 尺寸發(fā)生變化
  • 子控件尺寸發(fā)生變化
  • 滾動UIScrollView
  • 旋轉屏幕
  • setNeedsLayout，且有布局需要更新
• 立即執(zhí)行
  • 調(diào)用layoutIfNeeded時且有變化會立即觸發(fā)

重新繪制方法，drawRect調(diào)用時機

• 在loadView，viewDidLoad方法之后調(diào)用
• View初始化時沒有設置rect大小，將直接導致drawRect不會被調(diào)用
• 在調(diào)用sizeThatFits后被調(diào)用，可以先調(diào)用sizeToFit計算出size，然后系統(tǒng)自動調(diào)用drawRect
• 通過設置contentMode為UIViewContentModeRedraw，每次設置或更改frame的時候自動調(diào)用drawRect
• rect不為0，調(diào)用setNeedsDisplay或setNeedsDisplayInRect會觸發(fā)

生活如此美好，今天就點到為止。。。