UIPickerView 用法

两个代理
UIPickerViewDelegate
UIPickerViewDataSource

// 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:
//返回component列，row行的UIView，这里只有在定制情况下才有效，其他情况返回nil

- (UIView *)viewForRow:(NSInteger)row forComponent:(NSInteger)component;

// Reloading whole view or single component
//重装整个UIPickerView所有列和指定列的数据
- (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

@protocol UIPickerViewDataSource<NSObject>
@required

// returns the number of 'columns' to display.
//返回总共有几列
- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView;

// returns the # of rows in each component..

//返回指定列有几行

- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component;

@end

@protocol UIPickerViewDelegate<NSObject>
@optional

// returns width of column and height of row for each component.

//返回指定列的宽度

- (CGFloat)pickerView:(UIPickerView *)pickerView widthForComponent:(NSInteger)component;

//返回指定列的行的高度

- (CGFloat)pickerView:(UIPickerView *)pickerView rowHeightForComponent:(NSInteger)component;

// these methods return either a plain UIString, 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

//标准的UIPickerView内容，只有字符串
- (NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component;

//自定义的UIPickerView内容，给每列每行设置一个UIView
- (UIView *)pickerView:(UIPickerView *)pickerView viewForRow:(NSInteger)row forComponent:(NSInteger)component reusingView:(UIView *)view;

这里为列component，行row返回一个UIView来显示在UIPickerView中。reusingView:(UIView *) view 是IOS内核传入的一个缓冲的UIView。在程序中可以不用吧分配UIView，就可以重用IOS传来的view
//以上方法只能二选一

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

@end

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