SVProgressHUD源码记二:要点学习

如何表示弃用的API

  + (void)showWithMaskType:(SVProgressHUDMaskType)maskType __attribute__((deprecated("Use show and setDefaultMaskType: instead.")));

使用 __attribute__, deprecated 关键字表示已过时的API

单例创建方式

    static dispatch_once_t once;
    static SVProgressHUD *sharedView;
    dispatch_once(&once, ^{
      sharedView = [[self alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
      });
    return sharedView;

使用GCD的dispatch_once函数

数组元素的倒序遍历

    NSEnumerator *frontToBackWindows = [UIApplication.sharedApplication.windows reverseObjectEnumerator];
    for (UIWindow *window in frontToBackWindows){
        BOOL windowOnMainScreen = window.screen == UIScreen.mainScreen; // 为设备屏幕
        BOOL windowIsVisible = !window.hidden && window.alpha > 0;
        BOOL windowLevelNormal = window.windowLevel == UIWindowLevelNormal;
        
        if(windowOnMainScreen && windowIsVisible && windowLevelNormal){
            [window addSubview:self.overlayView];
            break;
        }
    }

枚举器,反向遍历reverseObjectEnumerator;
objectEnumerator为正向遍历,返回NSEnumerator<Object>对象,Object表示真正的对象类型.

if-else条件编译宏的使用

#if conditionA 
  // ...
#elif conditionB
  // ...
#else 
  // ...
#endif

将浮点数取值的C函数

ceilf(5.3) // 6
floorf(5.6) // 5

ceil表示向上取整;floorf表示向下取整

Accessibility 支持

  self.accessibilityIdentifier = @"SVProgressHUD";
  self.accessibilityLabel = @"SVProgressHUD";     
  self.isAccessibilityElement = YES;

让控件或者对象自定义的唯一可识别的标签,并且关联user interface

计算文字的尺寸

  stringRect = [string boundingRectWithSize:constraintSize
                                    options:(NSStringDrawingOptions)(NSStringDrawingUsesFontLeading|NSStringDrawingTruncatesLastVisibleLine|NSStringDrawingUsesLineFragmentOrigin)
                                 attributes:@{NSFontAttributeName: self.stringLabel.font}
                                    context:NULL];

首先需要大致限制尺寸,和NSStringDrawingOptions表示了绘制的文字在计算高度和宽度时所参考的起始点和结束点,涉及到印刷字体的排版相关知识

动画事务的使用

  [CATransaction begin];
  [CATransaction setDisableActions:YES];
  [_hudView.layer removeAllAnimations];
  // ....  
  [CATransaction commit];

CATransaction使用begin开始,并对应commit结束; setDisableActions:YES用于当此transaction内动画属性变化需要中断此次事务