ios - UIView 背景颜色始终为黑色

Question

当我以编程方式将 UIView 添加到 View Controller 时，我无法将背景颜色更改为任何其他颜色，它始终保持黑色。

- (void)drawRect:(CGRect)rect
{
    // Drawing code
    self.backgroundColor = [UIColor blueColor];

    UIBezierPath *path = [[UIBezierPath alloc] init];
    [path moveToPoint:CGPointMake(100, 33)];
    [path addLineToPoint:CGPointMake(200, 33)];
    path.lineWidth = 5;
    [[UIColor redColor] setStroke];
    [path stroke];
}

当我注释掉 drawrect: 并将 self.backgroundColor = [UIColor blueColor]; 添加到初始化程序时，颜色会发生变化:

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        // Initialization code
        self.backgroundColor = [UIColor blueColor]
    }
    return self;
}

这是为什么？我需要更改什么？其实我希望背景是透明的。

Answer 1

当您实现 drawRect: 时，这将覆盖 UIView 的任何默认绘图。

您必须将自定义 View 的 opaque 属性设置为 NO 并在呈现路径之前清除上下文:

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        // Initialization code
        self.opaque = NO;
    }
    return self;
}

- (void)drawRect:(CGRect)rect
{
    // Drawing code
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextClearRect(context, rect);

    UIBezierPath *path = [[UIBezierPath alloc] init];
    [path moveToPoint:CGPointMake(100, 33)];
    [path addLineToPoint:CGPointMake(200, 33)];
    path.lineWidth = 5;
    [[UIColor redColor] setStroke];
    [path stroke];
}

https://developer.apple.com/library/ios/documentation/uikit/reference/uiview_class/uiview/uiview.html#//apple_ref/occ/instp/UIView/opaque

An opaque view is expected to fill its bounds with entirely opaque content—that is, the content should have an alpha value of 1.0. If the view is opaque and either does not fill its bounds or contains wholly or partially transparent content, the results are unpredictable. You should always set the value of this property to NO if the view is fully or partially transparent.