iOS開發UI篇—懶載入

iOS開發UI篇—懶載入

1.懶載入基本

懶載入——也稱為延遲載入，即在須要的時候才載入（效率低，占用內存小）。所謂懶載入，寫的是其get方法.

注意：假設是懶載入的話則一定要注意先推斷是否已經有了。假設沒有那么再去進行實例化

2.使用懶載入的優點：

（1）不必將創建對象的代碼所有寫在viewDidLoad方法中，代碼的可讀性更強

（2）每一個控件的getter方法中分別負責各自的實例化處理，代碼彼此之間的獨立性強，松耦合

3.代碼演示樣例

//
//  YYViewController.m
//  03-圖片瀏覽器初步
//
//  Created by apple on 14-5-21.
//  Copyright (c) 2014年 itcase. All rights reserved.
//

#import "YYViewController.h"

#define POTOIMGW    200
#define POTOIMGH    300
#define POTOIMGX    60
#define  POTOIMGY    50

@interface YYViewController ()

@property(nonatomic,strong)UILabel *firstlab;
@property(nonatomic,strong)UILabel *lastlab;
@property(nonatomic,strong)UIImageView *icon;
@property(nonatomic,strong)UIButton *leftbtn;
@property(nonatomic,strong)UIButton *rightbtn;
@property(nonatomic,strong)NSArray *array;
@property(nonatomic ,assign)int i;
-(void)change;
@end



@implementation YYViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self change];
}

-(void)change
{
    [self.firstlab setText:[NSString stringWithFormat:@"%d/5",self.i+1]];
    //先get再set
    
    self.icon.image=[UIImage imageNamed:self.array[self.i][@"name"]];
    self.lastlab.text=self.array[self.i][@"desc"];
  
    self.leftbtn.enabled=(self.i!=0);
    self.rightbtn.enabled=(self.i!=4);
}

//延遲載入
/**1.圖片的序號標簽*/
-(UILabel *)firstlab
{
    //推斷是否已經有了，若沒有。則進行實例化
    if (!_firstlab) {
        _firstlab=[[UILabel alloc]initWithFrame:CGRectMake(20, 10, 300, 30)];
        [_firstlab setTextAlignment:NSTextAlignmentCenter];
        [self.view addSubview:_firstlab];
    }
    return _firstlab;
}

/**2.圖片控件的延遲載入*/
-(UIImageView *)icon
{
     //推斷是否已經有了，若沒有，則進行實例化
    if (!_icon) {
        _icon=[[UIImageView alloc]initWithFrame:CGRectMake(POTOIMGX, POTOIMGY, POTOIMGW, POTOIMGH)];
        UIImage *image=[UIImage imageNamed:@"biaoqingdi"];
        _icon.image=image;
        [self.view addSubview:_icon];
    }
    return _icon;
}

/**3.描寫敘述控件的延遲載入*/
-(UILabel *)lastlab
{
     //推斷是否已經有了。若沒有。則進行實例化
    if (!_lastlab) {
        _lastlab=[[UILabel alloc]initWithFrame:CGRectMake(20, 400, 300, 30)];
        [_lastlab setTextAlignment:NSTextAlignmentCenter];
        [self.view addSubview:_lastlab];
    }
    return _lastlab;
}

/**4.左鍵按鈕的延遲載入*/
-(UIButton *)leftbtn
{
     //推斷是否已經有了。若沒有。則進行實例化
    if (!_leftbtn) {
        _leftbtn=[UIButton buttonWithType:UIButtonTypeCustom];
        _leftbtn.frame=CGRectMake(0, self.view.center.y, 40, 40);
        [_leftbtn setBackgroundImage:[UIImage imageNamed:@"left_normal"] forState:UIControlStateNormal];
        [_leftbtn setBackgroundImage:[UIImage imageNamed:@"left_highlighted"] forState:UIControlStateHighlighted];
        [self.view addSubview:_leftbtn];
        [_leftbtn addTarget:self action:@selector(leftclick:) forControlEvents:UIControlEventTouchUpInside];
    }
    return _leftbtn;

}

/**5.右鍵按鈕的延遲載入*/
-(UIButton *)rightbtn
{
    if (!_rightbtn) {
        _rightbtn=[UIButton buttonWithType:UIButtonTypeCustom];
        _rightbtn.frame=CGRectMake(POTOIMGX+POTOIMGW+10, self.view.center.y, 40, 40);
        [_rightbtn setBackgroundImage:[UIImage imageNamed:@"right_normal"] forState:UIControlStateNormal];
        [_rightbtn setBackgroundImage:[UIImage imageNamed:@"right_highlighted"] forState:UIControlStateHighlighted];
        [self.view addSubview:_rightbtn];
        [_rightbtn addTarget:self action:@selector(rightclick:) forControlEvents:UIControlEventTouchUpInside];
    }
    return _rightbtn;
}

//array的get方法
-(NSArray *)array
{
    if (_array==nil) {
        NSString *path=[[NSBundle mainBundle] pathForResource:@"data" ofType:@"plist"];
        _array=[[NSArray alloc]initWithContentsOfFile:path];
    }
    return _array;
}

-(void)rightclick:(UIButton *)btn
{
    self.i++;
    [self change];
}

-(void)leftclick:(UIButton *)btn
{
    self.i--;
    [self change];
}

@end