如果一個界面創(chuàng)建三個View，要求第一行兩個View等高等寬，第三個View與上面兩個View等高，無論是橫屏還是豎屏。
那該如何編寫使得View自動布局呢？那就要使用NSLayoutConstraint - 系統(tǒng)自動布局了
-----------------自動布局方法1 ----------------
[NSLayoutConstraint constraintWithItem:(id)item
attribute:(NSLayoutAttribute)attribute
relatedBy:(NSLayoutRelation)relation
toItem:(id)otherItem
attribute:(NSLayoutAttribute)otherAttribute
multiplier:(CGFloat)multiplier
constant:(CGFloat)constant]
1.參數(shù)說明:
第一個參數(shù):指定約束左邊的視圖view1
第二個參數(shù):指定view1的屬性attr1
第三個參數(shù):指定左右兩邊的視圖的關系relation
第四個參數(shù):指定約束右邊的視圖view2
第五個參數(shù):指定view2的屬性attr2
第六個參數(shù):指定一個與view2屬性相乘的乘數(shù)multiplier
第七個參數(shù):指定一個與view2屬性相加的浮點數(shù)constant
依據(jù)的公式是：view1.attr1 = view2.attr2*multiplier +constant
2.NSLayoutAttribute的類型：
| NSLayoutAttribute | 類型 |
|---|---|
| NSLayoutAttributeLeft | 視圖的左邊 |
| NSLayoutAttributeRight | 視圖的右邊 |
| NSLayoutAttributeTop | 視圖的上邊 |
| NSLayoutAttributeBottom | 視圖的下邊 |
| NSLayoutAttributeLeading | 視圖的前邊 |
| NSLayoutAttributeTrailing | 視圖的后邊 |
| NSLayoutAttributeWidth | 視圖的寬度 |
| NSLayoutAttributeHeight | 視圖的高度 |
| NSLayoutAttributeCenterX | 視圖的中點的X值 |
| NSLayoutAttributeCenterY | 視圖中點的Y值 |
| NSLayoutAttributeBaseline | 視圖的基準線 |
| NSLayoutAttributeNotAnAttribute | 無屬性 |
3.NSLayoutRelation的類型：
| NSLayoutRelation | 類型 |
|---|---|
| NSLayoutRelationLessThanOrEqual | 視圖關系小于或等于 |
| NSLayoutRelationEqual | 視圖關系等于 |
| NSLayoutRelationGreaterThanOrEqual | 視圖關系大于或等于 |
核心代碼如下：
- (void)createThreeViews{
//1 創(chuàng)建3個view的對象
UIView *leftView = [[UIView alloc]init];
UIView *rightView = [[UIView alloc]init];
UIView *bottomView = [[UIView alloc]init];
//2 設置背景顏色
leftView.backgroundColor = [UIColor greenColor];
rightView.backgroundColor = [UIColor purpleColor];
bottomView.backgroundColor = [UIColor orangeColor];
//3 添加視圖上顯示
[self.view addSubview:leftView];
[self.view addSubview:rightView];
[self.view addSubview:bottomView];
//4 關閉系統(tǒng)的自定義布局
leftView.translatesAutoresizingMaskIntoConstraints = NO;
rightView.translatesAutoresizingMaskIntoConstraints = NO;
bottomView.translatesAutoresizingMaskIntoConstraints = NO;
//leftView的上 = self.view的上+20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:leftView
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeTop
multiplier:1.
constant:20]];
// leftView的左 = self.view的左 + 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:leftView
attribute:NSLayoutAttributeLeft
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeLeft
multiplier:1.
constant:20]];
// rightView的上 = leftView的上
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:rightView
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeTop
multiplier:1.
constant:0]];
// rightView的左 = leftView的右 + 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:rightView
attribute:NSLayoutAttributeLeft
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeRight
multiplier:1.
constant:20]];
// rightView的右 = self.view的右 - 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:rightView
attribute:NSLayoutAttributeRight
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeRight
multiplier:1.
constant:-20]];
// rightView的高 = leftView的高
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:rightView
attribute:NSLayoutAttributeHeight
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeHeight
multiplier:1.
constant:0]];
// rightView的寬 = leftView的寬
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:rightView
attribute:NSLayoutAttributeWidth
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeWidth
multiplier:1.0
constant:0]];
//bottomView的左 = self.view的左 + 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:bottomView
attribute:NSLayoutAttributeLeft
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeLeft
multiplier:1
constant:20]];
//bottomView的右 = self.view的右 - 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:bottomView
attribute:NSLayoutAttributeRight
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeRight
multiplier:1
constant:-20]];
//bottomView的上 = leftView的下 + 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:bottomView
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeBottom
multiplier:1
constant:20]];
//bottomView的高 = leftView的高
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:bottomView
attribute:NSLayoutAttributeHeight
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeHeight
multiplier:1
constant:0]];
//bottomView的下 = self.view的下 + 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:bottomView
attribute:NSLayoutAttributeBottom
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeBottom
multiplier:1
constant:-20]];
}
-----------------自動布局方法2 ----------------
自動布局使用可視化語言:VFL(Visual Format Language)
1. 自動布局的相關參數(shù)
NSArray *constrArray = [NSLayoutConstraint constraintsWithVisualFormat:@"string類型" options: metrics: views:]；
2. 解釋：
a) 第一個參數(shù)：使用VFL格式化的字符串，可以參見官方的幫助文檔；
b) 第二個參數(shù)：指定VFL中所有對象的布局屬性和方向。舉例：有2個視圖使用VFL進行布局，可以使用NSLayoutFormatAlignAllLeft，就讓兩個視圖左對齊；
c) 第三個參數(shù)：度量或者指標的字典，字典里面有相關的鍵值對來控制相關的度量指標，通過key獲?。?/p>
d) 第四個參數(shù)：指定約束的視圖：一個或多個。
3. VFL語言的規(guī)則
a) "H" 表示水平方向，"V"表示垂直方向；
b) "|" 表示superview的邊界；
c) "[]" 表示view，"()"表示尺寸，它們可以多個條件組合，中間使用逗號分隔，舉例：[view(>=70, <=100)];
d) "-" 表示間隙；
e) "@"表示優(yōu)先級。舉例：V:|-50@750-[view(55)]
核心代碼如下：
- (void)createThreeViews{
//1 創(chuàng)建3個view的對象
UIView *leftView = [[UIView alloc]init];
UIView *rightView = [[UIView alloc]init];
UIView *bottomView = [[UIView alloc]init];
//2 設置背景顏色
leftView.backgroundColor = [UIColor greenColor];
rightView.backgroundColor = [UIColor purpleColor];
bottomView.backgroundColor = [UIColor orangeColor];
//3 添加視圖上顯示
[self.view addSubview:leftView];
[self.view addSubview:rightView];
[self.view addSubview:bottomView];
//4 關閉系統(tǒng)的自定義布局
leftView.translatesAutoresizingMaskIntoConstraints = NO;
rightView.translatesAutoresizingMaskIntoConstraints = NO;
bottomView.translatesAutoresizingMaskIntoConstraints = NO;
//創(chuàng)建VFL約束字符串
NSString *hVFL = @"H:|-space-[leftView(==rightView)]-space1-[rightView]-space-|";
NSString *hVFL1 = @"H:|-space-[bottomView]-space-|";
NSString *vVFL = @"V:|-space-[leftView(==bottomView)]-space-[bottomView]-space-|";
NSString *vVFL1 = @"V:|-space-[rightView(==bottomView)]-space-[bottomView]-space-|";
//創(chuàng)建鍵值映射
NSDictionary *metircs = @{@"space":@20,@"space1":@30};
NSDictionary *views = NSDictionaryOfVariableBindings(leftView,rightView,bottomView);
//創(chuàng)建約束
NSArray *hconstraint = [NSLayoutConstraint constraintsWithVisualFormat:hVFL options:NSLayoutFormatDirectionLeadingToTrailing metrics:metircs views:views];
NSArray *hconstraint1 = [NSLayoutConstraint constraintsWithVisualFormat:hVFL1 options:NSLayoutFormatDirectionLeadingToTrailing metrics:metircs views:views];
NSArray *vconstraint = [NSLayoutConstraint constraintsWithVisualFormat:vVFL options:NSLayoutFormatDirectionLeadingToTrailing metrics:metircs views:views];
NSArray *vconstraint1 = [NSLayoutConstraint constraintsWithVisualFormat:vVFL1 options:NSLayoutFormatDirectionLeadingToTrailing metrics:metircs views:views];
//添加約束
[self.view addConstraints:hconstraint];
[self.view addConstraints:hconstraint1];
[self.view addConstraints:vconstraint];
[self.view addConstraints:vconstraint1];
}
收藏 @學長的日常 的簡書
http://www.itdecent.cn/p/bab69690cb1a
并添加 Demo: https://github.com/policx/autoLayout
總結：http://www.itdecent.cn/p/bab69690cb1a 學長的日常
通過一個小小的編程，使大家初步了解了NSLayoutConstraint的使用規(guī)則和作用。這兩種方式各有特點，本人比較喜歡用第二種，因為感覺代碼較少。在使用NSLayoutConstraint 系統(tǒng)自動布局時，注意的地方很多，細心一點，用起來還是很方便的。
