在 CALayer 中定位内容答案

【问题标题】：Positioning content in a CALayer在 CALayer 中定位内容
【发布时间】：2013-01-30 22:57:45
【问题描述】：

如何在 CALayer 中定位内容，使其缩放以适应宽度（保留纵横比）并位于图层顶部？

更具体地说，我正在尝试使用 AVCaptureVideoPreviewLayer 来实现此定位。如果我将预览层的 videoGravity 设置为 AVLayerVideoGravityResizeAspectFill，那么它会填充宽度但将内容垂直居中在框架中（切断顶部和底部）。

【问题讨论】：

标签： ios objective-c calayer avcapture

【解决方案1】：

尝试使用 .contentGravity 属性 -

一个常量，指定层的内容如何定位或在其范围内缩放

<CALayer>.contentsGravity = kCAGravityResizeAspect;

可能

CA_EXTERN NSString * const kCAGravityCenter
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
CA_EXTERN NSString * const kCAGravityTop
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
CA_EXTERN NSString * const kCAGravityBottom
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
CA_EXTERN NSString * const kCAGravityLeft
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
CA_EXTERN NSString * const kCAGravityRight
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
CA_EXTERN NSString * const kCAGravityTopLeft
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
CA_EXTERN NSString * const kCAGravityTopRight
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
CA_EXTERN NSString * const kCAGravityBottomLeft
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
CA_EXTERN NSString * const kCAGravityBottomRight
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
CA_EXTERN NSString * const kCAGravityResize
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
CA_EXTERN NSString * const kCAGravityResizeAspect
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
CA_EXTERN NSString * const kCAGravityResizeAspectFill
    __OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);

【讨论】：

为这个简单的答案点赞！
kCAGravityResizeAspect 可能不会填满整个宽度（取决于高度）。 kCAGravityResizeAspectFill 确实对齐中心 - 而不是顶部。所以我仍然想知道如何拥有 kCAGravityResizeAspectFill 和顶部。

【解决方案2】：

尝试使用这组代码：

将此行添加到图层的初始布局中：

[previewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];

然后添加这些：

- (void) viewDidLayoutSubviews
{
    [self layoutPreviewInView:_photoImageView];
}

-(void)layoutPreviewInView:(UIView *) aView
{
    AVCaptureVideoPreviewLayer *layer = [self previewInView:aView];
    if (!layer) return;

    UIDeviceOrientation orientation = [UIDevice currentDevice].orientation;
    CATransform3D transform = CATransform3DIdentity;
    if (orientation == UIDeviceOrientationPortrait) ;
    else if (orientation == UIDeviceOrientationLandscapeLeft)
        transform = CATransform3DMakeRotation(-M_PI_2, 0.0f, 0.0f, 1.0f);
    else if (orientation == UIDeviceOrientationLandscapeRight)
        transform = CATransform3DMakeRotation(M_PI_2, 0.0f, 0.0f, 1.0f);
    else if (orientation == UIDeviceOrientationPortraitUpsideDown)
        transform = CATransform3DMakeRotation(M_PI, 0.0f, 0.0f, 1.0f);

    layer.transform = transform;
    layer.frame = aView.bounds;
}

-(AVCaptureVideoPreviewLayer *) previewInView: (UIView *) view
{
    for (CALayer *layer in view.layer.sublayers)
    {
        if ([layer isKindOfClass:[AVCaptureVideoPreviewLayer class]])
            return (AVCaptureVideoPreviewLayer *) layer;
    }
    return nil;
}

【讨论】：

【解决方案3】：

我通常使用自动布局来做到这一点：

-(void)awakeFromNib{
  [super awakeFromNib];
  self.backgroundColor = [UIColor clearColor];
  CAGradientLayer* gradient = (CAGradientLayer*) self.layer;
  gradient.frame = self.bounds;
  gradient.colors = [NSArray arrayWithObjects:
                     (id)[UIColor colorWithRed:0 green:0 blue:0 alpha:0].CGColor,
                     (id)[UIColor colorWithRed:0 green:0 blue:0 alpha:1].CGColor, nil];

}

+ (Class) layerClass {
  return [CAGradientLayer class];
}

【讨论】：