# IOS自動布局
## 簡介
自動布局在iOS 6.0中引入，僅可以支持IOS6.0 及 更高版本。它可以幫助我們創建用于多個種設備的界面。
### 實例步驟
1.創建一個簡單的 View based application
2.修改 ViewController.m 的文件內容，如下所示
```
#import "ViewController.h"
@interface ViewController ()
@property (nonatomic, strong) UIButton *leftButton;
@property (nonatomic, strong) UIButton *rightButton;
@property (nonatomic, strong) UITextField *textfield;
@end
@implementation ViewController
- (void)viewDidLoad{
[super viewDidLoad];
UIView *superview = self.view;
/*1\. Create leftButton and add to our view*/
self.leftButton = [UIButton buttonWithType:UIButtonTypeRoundedRect];
self.leftButton.translatesAutoresizingMaskIntoConstraints = NO;
[self.leftButton setTitle:@"LeftButton" forState:UIControlStateNormal];
[self.view addSubview:self.leftButton];
/* 2\. Constraint to position LeftButton's X*/
NSLayoutConstraint *leftButtonXConstraint = [NSLayoutConstraint
constraintWithItem:self.leftButton attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationGreaterThanOrEqual toItem:superview attribute:
NSLayoutAttributeCenterX multiplier:1.0 constant:-60.0f];
/* 3\. Constraint to position LeftButton's Y*/
NSLayoutConstraint *leftButtonYConstraint = [NSLayoutConstraint
constraintWithItem:self.leftButton attribute:NSLayoutAttributeCenterY
relatedBy:NSLayoutRelationEqual toItem:superview attribute:
NSLayoutAttributeCenterY multiplier:1.0f constant:0.0f];
/* 4\. Add the constraints to button's superview*/
[superview addConstraints:@[ leftButtonXConstraint,
leftButtonYConstraint]];
/*5\. Create rightButton and add to our view*/
self.rightButton = [UIButton buttonWithType:UIButtonTypeRoundedRect];
self.rightButton.translatesAutoresizingMaskIntoConstraints = NO;
[self.rightButton setTitle:@"RightButton" forState:UIControlStateNormal];
[self.view addSubview:self.rightButton];
/*6\. Constraint to position RightButton's X*/
NSLayoutConstraint *rightButtonXConstraint = [NSLayoutConstraint
constraintWithItem:self.rightButton attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationGreaterThanOrEqual toItem:superview attribute:
NSLayoutAttributeCenterX multiplier:1.0 constant:60.0f];
/*7\. Constraint to position RightButton's Y*/
rightButtonXConstraint.priority = UILayoutPriorityDefaultHigh;
NSLayoutConstraint *centerYMyConstraint = [NSLayoutConstraint
constraintWithItem:self.rightButton attribute:NSLayoutAttributeCenterY
relatedBy:NSLayoutRelationGreaterThanOrEqual toItem:superview attribute:
NSLayoutAttributeCenterY multiplier:1.0f constant:0.0f];
[superview addConstraints:@[centerYMyConstraint,
rightButtonXConstraint]];
//8\. Add Text field
self.textfield = [[UITextField alloc]initWithFrame:
CGRectMake(0, 100, 100, 30)];
self.textfield.borderStyle = UITextBorderStyleRoundedRect;
self.textfield.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:self.textfield];
//9\. Text field Constraints
NSLayoutConstraint *textFieldTopConstraint = [NSLayoutConstraint
constraintWithItem:self.textfield attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationGreaterThanOrEqual toItem:superview
attribute:NSLayoutAttributeTop multiplier:1.0 constant:60.0f];
NSLayoutConstraint *textFieldBottomConstraint = [NSLayoutConstraint
constraintWithItem:self.textfield attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationGreaterThanOrEqual toItem:self.rightButton
attribute:NSLayoutAttributeTop multiplier:0.8 constant:-60.0f];
NSLayoutConstraint *textFieldLeftConstraint = [NSLayoutConstraint
constraintWithItem:self.textfield attribute:NSLayoutAttributeLeft
relatedBy:NSLayoutRelationEqual toItem:superview attribute:
NSLayoutAttributeLeft multiplier:1.0 constant:30.0f];
NSLayoutConstraint *textFieldRightConstraint = [NSLayoutConstraint
constraintWithItem:self.textfield attribute:NSLayoutAttributeRight
relatedBy:NSLayoutRelationEqual toItem:superview attribute:
NSLayoutAttributeRight multiplier:1.0 constant:-30.0f];
[superview addConstraints:@[textFieldBottomConstraint ,
textFieldLeftConstraint, textFieldRightConstraint,
textFieldTopConstraint]];
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
@end
```
### 輸出
運行應用程序，在 iPhone 模擬器上會有下面的輸出結果
當我們更改模擬器為橫向的方向時，輸出結果如下
我們在 iPhone 5 模擬器上運行同一應用程序時,輸出結果如下
當我們更改模擬器為橫向的方向時，輸出結果如下:
