实例015 实现背景透明

实例说明

本实例的功能是，使用UIView的属性alpha来改变指定视图的透明度。首先在屏幕上方设置了label区域，并显示了指定的文本“将白色文字背景设为红”。在下方设置了1个“透明化”按钮。每当单击一次“透明化”按钮，会设置上方label标签的alpha值以0.1为单位递减，从而逐渐实现透明效果。

具体实现

实例文件UIkitPrjAlpha.h的实现代码如下所示。

#import "SampleBaseController.h"
@interface UIKitPrjAlpha : SampleBaseController
{
 @private
UILabel* label_;
}
@end

实例文件UIkitPrjAlpha.m的实现代码如下所示。

#import "UIKitPrjAlpha.h"
@implementation UIKitPrjAlpha
- (void)dealloc {
  [label_ release];
  [super dealloc];
}
- (void)viewDidLoad {
  [super viewDidLoad];
  // 背景设为白
  self.view.backgroundColor = [UIColor whiteColor];
  // 追加画面上方的标签
  label_ = [[UILabel alloc] initWithFrame:CGRectMake( 0, 0, 320, 200 )];
  label_.textAlignment = UITextAlignmentCenter;
  label_.backgroundColor = [UIColor redColor];
  label_.textColor = [UIColor whiteColor];
  label_.adjustsFontSizeToFitWidth = YES;
  label_.text = @"将白色文字背景设为红";
  [self.view addSubview:label_];
  // 追加染色按钮
  UIButton* alphaButton = [UIButton buttonWithType:UIButtonTypeRoundedRect];
  alphaButton.frame = CGRectMake( 0, 0, 100, 40 );
  CGPoint newPoint = self.view.center;
  newPoint.y = self.view.frame.size.height -70;
  alphaButton.center = newPoint;
  [alphaButton setTitle:@"透明化" forState:UIControlStateNormal];
  [alphaButton addTarget:self
              action:@selector(alphaDidPush)
      forControlEvents:UIControlEventTouchUpInside];
  [self.view addSubview:alphaButton];
}
- (void)alphaDidPush {
  // 标签的alpha值以0.1为单位递减
  // 0.0时恢复为1.0
  if ( label_.alpha < 0.09 ) {
    label_.alpha = 1.0;
  } else {
    label_.alpha -= 0.1;
  }
}
- (void)touchesEnded:(NSSet*)touches withEvent:(UIEvent*)event {
  [self.navigationController setNavigationBarHidden:NO animated:YES];
}
@end

执行后的效果如图2-19所示。每当单击一次“透明化”逐渐实现透明效果。