cocoa touch——UIPickerView

概述

UIPickerView，选择器控件，可以实现单列或多列选择

UIPickerView

appearance

@property(nonatomic)        BOOL                       showsSelectionIndicator;   // default is NO

注意：
ios7之前，此属性控制selection indicator显示，默认值为NO，ios7及以后，此属性不再起作用，无论设置YES或NO，selection indicator总显示

selection

// 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

dataSource

@property(nullable,nonatomic,weak) id<UIPickerViewDataSource> dataSource;                // default is nil. weak reference

component&row

view

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

dataSource

// 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;

注意：
view获取的是cache值
dataSource定制view的component和row

delegate

@property(nullable,nonatomic,weak) id<UIPickerViewDelegate>   delegate;                  // default is nil. weak reference

cell appearance

view

// info that was fetched and cached from the delegate
- (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;

delegate

// 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;

注意：
view获取的只是cache值
delegate定制view的cell appearance
delegate提供了三个版本的cell appearance：NSString，NSAttributedString，UIView，分别表示UILabel(plain)，UILabel(attributed)，UIView，优先级：UIView>NSAttributedString>NSString

cell select

delegate

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

reload

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

注意：
view绘制前会调用一次reloadAllComponents，component从默认值0改变为自定义值
reloadAllComponents会调用dataSource的定制component和row方法，delegate的定制cell appearance的其中一个版本，如果component值发生改变，会调用delegate的定制cell size方法，如果component值没有改变，不会调用delegate的定制cell size方法
reloadComponent会调用dataSource的指定component的定制row方法，delegate的指定component的定制cell appearance的其中一个版本