iOS － UIPickerView文字大小颜色修改，无限轮播，无限循环滚动

最近项目里用到了UIPickerView，要求无限轮播，简单看了看UIPickView的所有属性和所有代理方法，没有无限轮播的设置，遂去百度了一把，发现网上流传着一个帖子，大家各种转载，看了看后，明白其中道理，原来是这样！

UIPickerView，使用规则与UITableView很像，下边是UIPickerView的.h文件，包涵两个代理的描述：

//
//  UIPickerView.h
//  UIKit
//
//  Copyright (c) 2006-2015 Apple Inc. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <CoreGraphics/CoreGraphics.h>
#import <UIKit/UIView.h>
#import <UIKit/UIKitDefines.h>

NS_ASSUME_NONNULL_BEGIN

@protocol UIPickerViewDataSource, UIPickerViewDelegate;

NS_CLASS_AVAILABLE_IOS(2_0) __TVOS_PROHIBITED @interface UIPickerView : UIView <NSCoding>

@property(nullable,nonatomic,weak) id<UIPickerViewDataSource> dataSource;                // default is nil. weak reference
@property(nullable,nonatomic,weak) id<UIPickerViewDelegate>   delegate;                  // default is nil. weak reference
@property(nonatomic)        BOOL                       showsSelectionIndicator;   // default is NO

// info that was fetched and cached from the data source and delegate
@property(nonatomic,readonly) NSInteger numberOfComponents;
- (NSInteger)numberOfRowsInComponent:(NSInteger)component;
- (CGSize)rowSizeForComponent:(NSInteger)component;

// returns the view provided by the delegate via pickerView:viewForRow:forComponent:reusingView:
// or nil if the row/component is not visible or the delegate does not implement 
// pickerView:viewForRow:forComponent:reusingView:
- (nullable UIView *)viewForRow:(NSInteger)row forComponent:(NSInteger)component;

// Reloading whole view or single component
- (void)reloadAllComponents;
- (void)reloadComponent:(NSInteger)component;

// selection. in this case, it means showing the appropriate row in the middle
- (void)selectRow:(NSInteger)row inComponent:(NSInteger)component animated:(BOOL)animated;  // scrolls the specified row to center.

- (NSInteger)selectedRowInComponent:(NSInteger)component;                                   // returns selected row. -1 if nothing selected

@end


__TVOS_PROHIBITED
@protocol UIPickerViewDataSource<NSObject>
@required

// returns the number of 'columns' to display.
- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView;

// returns the # of rows in each component..
- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component;
@end

__TVOS_PROHIBITED
@protocol UIPickerViewDelegate<NSObject>
@optional

// returns width of column and height of row for each component. 
- (CGFloat)pickerView:(UIPickerView *)pickerView widthForComponent:(NSInteger)component __TVOS_PROHIBITED;
- (CGFloat)pickerView:(UIPickerView *)pickerView rowHeightForComponent:(NSInteger)component __TVOS_PROHIBITED;

// these methods return either a plain NSString, a NSAttributedString, or a view (e.g UILabel) to display the row for the component.
// for the view versions, we cache any hidden and thus unused views and pass them back for reuse. 
// If you return back a different object, the old one will be released. the view will be centered in the row rect  
- (nullable NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component __TVOS_PROHIBITED;
- (nullable NSAttributedString *)pickerView:(UIPickerView *)pickerView attributedTitleForRow:(NSInteger)row forComponent:(NSInteger)component NS_AVAILABLE_IOS(6_0) __TVOS_PROHIBITED; // attributed title is favored if both methods are implemented
- (UIView *)pickerView:(UIPickerView *)pickerView viewForRow:(NSInteger)row forComponent:(NSInteger)component reusingView:(nullable UIView *)view __TVOS_PROHIBITED;

- (void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component __TVOS_PROHIBITED;

@end

NS_ASSUME_NONNULL_END
           

你会发现，其中方法很少，也很容易理解，我不做过多解释。我们接着说文字大小和颜色的修改～

注意代理又一个这样的方法：

- (nullable NSAttributedString *)pickerView:(UIPickerView *)pickerView attributedTitleForRow:(NSInteger)row forComponent:(NSInteger)component NS_AVAILABLE_IOS(6_0) __TVOS_PROHIBITED; // attributed title is favored if both methods are implemented
           

OK，就是这个，这个方法返回一个NSAttributedString类型数据，这个数据在我的微博里已经转载了一篇详细文章：http://blog.csdn.net/icefishlily/article/details/52692141

上代码：

- (NSAttributedString *)pickerView:(UIPickerView *)pickerView attributedTitleForRow:(NSInteger)row forComponent:(NSInteger)component {
    NSDictionary* titleTextAttributes = [NSDictionary dictionaryWithObjectsAndKeys:
                                         kCL6, NSForegroundColorAttributeName,
                                         [UIFont systemFontOfSize:30.0f], NSFontAttributeName,
                                         nil];
    
    NSInteger count = self.dataList.count;
    NSString *str = [NSString stringWithFormat:@"%.1f%%", [self.dataList[row%count] doubleValue]];
    NSAttributedString *restr = [[NSAttributedString alloc] initWithString:str attributes:titleTextAttributes];
    return restr;
}
           

用了上边这个方法，就不要用下边的方法了。

- (nullable NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component __TVOS_PROHIBITED;
           

注意上边代码中字典的定义，用到了两个东西：

NSForegroundColorAttributeName  文字颜色值
NSFontAttributeName             文字font
           

好了，不用多说了，颜色和大小搞定了。

接着说无限轮播，其实UIPickerView是个假的无限轮播，其原理是将你需要展示的几个cell（这里暂且叫cell，好理解）复制很多份（例如200份），然后都放到UIPickerView里边展示，并且在pickerView最开始出现的时候，就让它默认拨到中间位置，这样上下都有100组同样的数据，在不滑倒边界时，就像是无限轮播了。

代码如下：

//下边代码放在初始化的位置即可
        self.dataList = [[NSMutableArray alloc] init];
        for (int i=0; i<=20; i++) {
            [self.dataList addObject:[NSNumber numberWithDouble:i*0.1]];
        }
        //拨到中间位置
        [self.pickView selectRow:self.dataList.count*100 inComponent:0 animated:YES];

//下边代码是UIPickerView的部分代理
#pragma mark UIPickerViewDelegate, UIPickerViewDataSource
- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView {
    return 1;
    
}

- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component {
    return self.dataList.count * 200;
}

- (NSAttributedString *)pickerView:(UIPickerView *)pickerView attributedTitleForRow:(NSInteger)row forComponent:(NSInteger)component {
    NSDictionary* titleTextAttributes = [NSDictionary dictionaryWithObjectsAndKeys:
                                         kCL6, NSForegroundColorAttributeName,
                                         [UIFont systemFontOfSize:30.0f], NSFontAttributeName,
                                         nil];
    
    NSInteger count = self.dataList.count;
    NSString *str = [NSString stringWithFormat:@"%.1f%%", [self.dataList[row%count] doubleValue]];
    NSAttributedString *restr = [[NSAttributedString alloc] initWithString:str attributes:titleTextAttributes];
    return restr;
}