什么不多说,见代码

首先要封装一个类 NIDropDown
这个类继承与UIView;

屏幕快照 2016-06-07 下午1.57.34.png

关于h文件里面的东西

import <UIKit/UIKit.h>

@class NIDropDown;
@protocol NIDropDownDelegate

• (void) niDropDownDelegateMethod: (NIDropDown *) sender;
  @end

@interface NIDropDown : UIView <UITableViewDelegate, UITableViewDataSource>

@property (nonatomic, retain) id <NIDropDownDelegate> delegate;

-(void)hideDropDown:(UIButton *)b;

• (id)showDropDown:(UIButton *)b:(CGFloat *)height:(NSArray *)arr;

@end

关于m文件里面的东西

import "NIDropDown.h"

import "QuartzCore/QuartzCore.h"

@interface NIDropDown ()
@property(nonatomic,strong)UITableView *table;
@property(nonatomic,strong)UIButton *btnSender;
@property(nonatomic,retain)NSArray *list;
@end

@implementation NIDropDown

@synthesize table;
@synthesize btnSender;
@synthesize list;
@synthesize delegate;

-(id)showDropDown:(UIButton *)b :(CGFloat *)height :(NSArray *)arr
{
btnSender = b;
if(self){
CGRect btn = b.frame;
//self.frame = CGRectMake(btn.origin.x, btn.origin.y+btn.size.height, btn.size.width, 0);
[self initWithFrame:CGRectMake(btn.origin.x, btn.origin.y+btn.size.height, btn.size.width, 0)];
self.list = [NSArray arrayWithArray:arr];
self.layer.masksToBounds = NO;
self.layer.cornerRadius = 8;
self.layer.shadowOffset = CGSizeMake(-5, 5);
self.layer.shadowRadius = 5;
self.layer.shadowOpacity = 0.5;

    table = [[UITableView alloc] initWithFrame:CGRectMake(0, 0, btn.size.width, 0)];
    table.delegate = self;
    table.dataSource = self;
    table.layer.cornerRadius = 5;
    table.backgroundColor = [UIColor redColor];
    table.separatorStyle = UITableViewCellSeparatorStyleSingleLine;
    table.separatorColor = [UIColor grayColor];
    
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:0.5];
    self.frame = CGRectMake(btn.origin.x, btn.origin.y + btn.size.height, btn.size.width, *height);
    
    table.frame = CGRectMake(0, 0, btn.size.width, *height);
    [UIView commitAnimations];
    
    [b.superview addSubview:self];
    [self addSubview:table];
}
return self;

}

-(void)hideDropDown:(UIButton *)b
{
CGRect btn = b.frame;
[UIView beginAnimations:nil context:nil];
[UIView setAnimationDuration:0.5];
self.frame = CGRectMake(btn.origin.x, btn.origin.y + btn.size.height, btn.size.width, 0);
table.frame = CGRectMake(0, 0, btn.size.width, 0);
[UIView commitAnimations];
}

-(NSInteger)numberOfSectionsInTableView:(nonnull UITableView *)tableView
{
return 1;
}

-(CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
return 40;
}

• (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
  return [self.list count];
  }

• (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
  {
  static NSString *CellIdentifier = @"Cell";

  UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
  if(cell == nil){
  cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
  cell.textLabel.font = [UIFont systemFontOfSize:15];
  cell.textLabel.textAlignment = UITextAlignmentCenter;
  }
  cell.textLabel.text = [list objectAtIndex:indexPath.row];
  cell.textLabel.textColor = [UIColor redColor];

  UIView *v = [[UIView alloc] init];
  v.backgroundColor = [UIColor grayColor];
  cell.selectedBackgroundView = v;

  return cell;
  }

-(void)tableView:(nonnull UITableView *)tableView didSelectRowAtIndexPath:(nonnull NSIndexPath *)indexPath
{
[self hideDropDown:btnSender];
UITableViewCell *c = [tableView cellForRowAtIndexPath:indexPath];
[btnSender setTitle:c.textLabel.text forState:(UIControlStateNormal)];
[self myDelegate];
}

-(void)myDelegate
{
[self.delegate niDropDownDelegateMethod:self];

}

写完这个类以后,回到自己的viewController里面
h文件
引入NIDropDown这个类

import <UIKit/UIKit.h>

import "NIDropDown.h"

@interface ViewController : UIViewController<NIDropDownDelegate>
{
NIDropDown *dropDown;
}

@property(nonatomic,strong)UIButton *button;

-(void)rel;

@end

m文件
button点击方法:
-(void)buttonAction:(UIButton *)sender
{
NSArray * arr = [[NSArray alloc] init];
arr = [NSArray arrayWithObjects:@"Hello 0", @"Hello 1", @"Hello 2", @"Hello 3", @"Hello 4", @"Hello 5", @"Hello 6", @"Hello 7", @"Hello 8", @"Hello 9",nil];
if(dropDown == nil) {
CGFloat f = 200;
dropDown = [[NIDropDown alloc]showDropDown:sender :&f :arr];
dropDown.delegate = self;
}
else {
[dropDown hideDropDown:sender];
[self rel];
}

}

-(void)niDropDownDelegateMethod:(NIDropDown *)sender
{
[self rel];
}
-(void)rel{
dropDown = nil;
}