实现ios手机QQ空间导航栏控制器时隐时现效果，kvo的应用

// 这段代码可以实现导航栏时隐时现效果，kvo的应用

// ViewController.m

#import "ViewController.h"

#import "TableViewCell.h"

@interface
ViewController ()<UITableViewDataSource,UITableViewDelegate>
{

UITableView * _tableView;

}

@end

@implementation ViewController

- (void)viewDidLoad {

[super
viewDidLoad];

//self.view.backgroundColor = [UIColor blueColor];

self.navigationController.edgesForExtendedLayout =
UIRectEdgeNone;

self.navigationController.navigationBar.translucent
= YES;

[self.navigationController.navigationBar
setBarTintColor:[UIColor
orangeColor]];

_tableView = [[UITableView
alloc]initWithFrame:CGRectMake(0,
0, self.view.frame.size.width,
self.view.frame.size.height-64)
style:UITableViewStylePlain];

_tableView.delegate =
self;

_tableView.dataSource =
self;

_tableView.backgroundColor = [UIColor
clearColor];

_tableView.bounces =
YES;
[self.view
addSubview:_tableView];

[_tableView
addObserver: self
forKeyPath: @"contentOffset"
options: NSKeyValueObservingOptionNew
context:nil];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString
*,id> *)change context:(void *)context{

CGFloat offset=_tableView.contentOffset.y;

NSLog(@"offset:%f",offset);//越来越大
想要实现效果：一开始全显示，后来颜色变浅

CGFloat delta=1-offset/100.f;
delta=MIN(1,delta);

NSLog(@"%f",delta);

self.navigationController.navigationBar.alpha=MIN(1,delta);

}

-(NSInteger)numberOfSectionsInTableView:(UITableView *)tableView{

return 1;
}

-(NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section{

return 100;
}

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath
*)indexPath{

TableViewCell * cell = [[[NSBundle
mainBundle]loadNibNamed:@"TableViewCell"
owner:self
options:nil]firstObject];

if (indexPath.row==0) {
cell.myImage.hidden =
NO;
}else{
cell.myImage.hidden =
YES;

}

return cell;

}

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath
*)indexPath{

if (indexPath.row==0) {

return 180.0f;
}

return 60.0f;
}

- (void)didReceiveMemoryWarning {

[super
didReceiveMemoryWarning];

// Dispose of any resources that can be recreated.
}

@end



另附一则参考别人相反的实现

//添加监听者

[self.tableView addObserver: self forKeyPath: @"contentOffset" options: NSKeyValueObservingOptionNew context: nil];

/**

* 监听属性值发生改变时回调

*/

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context

{

CGFloat offset = self.tableView.contentOffset.y;

CGFloat delta = offset / 64.f + 1.f;

delta = MAX(0, delta);

[self alphaNavController].barAlpha = MIN(1, delta);

}