Question

我在使用Model（GraphView）和Controller（GraphViewController）之间的协议/委托方法时遇到了一些麻烦。 GraphView.m中 drawRect：中的NSLog告诉我表达式是 null

变量表达式包含一个值（ViewDidLoad中的NSLog证明了这一点）。我错过了一些明显的东西吗？

GraphView.h

@class GraphView;

@protocol GraphViewDelegate
-(NSString *) expressionForGraphView:(GraphView *) requestor; 
@end

@interface GraphView : UIView
{
      id <GraphViewDelegate> delegate;
}

@property (assign) id <GraphViewDelegate> delegate;

@end

GraphView.m

#import "GraphView.h"

@implementation GraphView

@synthesize delegate;

- (void)drawRect:(CGRect)rect
{
      NSString *expression = [self.delegate expressionForGraphView:self];
      NSLog(@"%@", expression);

}

GraphViewController.h

#import <UIKit/UIKit.h>
#import "GraphView.h"

@interface GraphViewController : UIViewController <GraphViewDelegate>
{
    GraphView *graphView;
    NSString *expression;
}

@property (retain) IBOutlet GraphView *graphView;
@property (retain) NSString *expression;

@end

GraphViewController.m

#import "GraphViewController.h"

@implementation GraphViewController

@synthesize graphView;
@synthesize expression;

-(NSString *) expressionForGraphView:(GraphView *) requestor
{
      NSString *tempString;
      if (requestor == self.graphView)
        tempString = self.expression;
      else 
        tempString = nil;

      return tempString;
}

- (void)viewDidLoad
{
     [super viewDidLoad];
     self.graphView.delegate = self;
     NSLog(@"%@", self.expression);
}

Answer 1

线索在方法名称

中

- (void)viewDidLoad

在设置委托之前已经加载了视图。因此，当drawRect被调用时，委托是nil。

在相关的初始化方法或IB

中设置委托

View和Controller之间的协议和委托

1 个答案: