1.about VFL language
示例：
H
:
[
cancelButton
(72)]-12-[
acceptButton
(50)]
canelButton
宽
72
，
acceptButton
宽
50
，它们之间间距
12
H
:
[
wideView
(>=60@700)]
wideView
宽度大于等于
60point
，该约束条件优先级为
700
（优先级最大值为
1000
，优先级越高的约束越先被满足）
V
:[
redBox
][
yellowBox
(==
redBox
)]
竖直
方向上
，先
有
一个
redBox
，其下方紧接一个
高度
等于
redBox
高度
的
yellowBox
H
:|-
10-
[Find]-[
FindNext
]-[
FindField
(>=20)]-|
水平
方向上
，
Find
距离父
view
左边缘默认间隔宽度，之后是
FindNext
距离
Find
间隔默认宽度；再之后是宽度不小于
20
的
FindField
，它和
FindNext
以及父
view
右边缘的间距都是默认宽度。（
竖线“
|
”
表示
superview
的边缘）
VFL的使用：
使用VFL来创建约束数组
+ (NSArray *)constraintsWithVisualFormat:(NSString *)format options:(NSLayoutFormatOptions)opts metrics:(NSDictionary *)metrics views:(NSDictionary *)views;
format ：VFL语句
opts ：约束类型
metrics ：VFL语句中用到的具体数值
views ：VFL语句中用到的控件
创建一个字典（内部包含VFL语句中用到的控件）的快捷宏定义
NSDictionaryOfVariableBindings(...)
demo示例1：
- (void)test1
{
UIView *blueView = [[UIView alloc] init];
blueView.backgroundColor = [UIColor blueColor];
// 不要将AutoresizingMask转为Autolayout的约束
blueView.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:blueView];
// 添加水平方向的约束
NSString *vfl = @"H:|-20-[abc]-20-|";
NSDictionary *views = @{@"abc" : blueView};
NSArray *constraints = [NSLayoutConstraint constraintsWithVisualFormat:vfl options:kNilOptions metrics:nil views:views];
[self.view addConstraints:constraints];
// 添加竖直方向的间距
NSString *vfl2 = @"V:|-20-[abc(40)]";
NSDictionary *views2 = @{@"abc" : blueView};
NSArray *constraints2 = [NSLayoutConstraint constraintsWithVisualFormat:vfl2 options:kNilOptions metrics:nil views:views2];
[self.view addConstraints:constraints2];
}效果：
demo示例2：
- (void)test2
{
UIView *blueView = [[UIView alloc] init];
blueView.backgroundColor = [UIColor blueColor];
// 不要将AutoresizingMask转为Autolayout的约束
blueView.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:blueView];
// 间距
NSNumber *margin = @20;
// 添加水平方向的约束
NSString *vfl = @"H:|-margin-[blueView]-margin-|";
NSDictionary *views = NSDictionaryOfVariableBindings(blueView);
NSDictionary *mertrics = NSDictionaryOfVariableBindings(margin);
NSArray *constraints = [NSLayoutConstraint constraintsWithVisualFormat:vfl options:kNilOptions metrics:mertrics views:views];
[self.view addConstraints:constraints];
// 添加竖直方向的间距
NSNumber *height = @40;
NSString *vfl2 = @"V:|-margin-[blueView(height)]";
NSDictionary *mertrics2 = NSDictionaryOfVariableBindings(margin, height);
NSArray *constraints2 = [NSLayoutConstraint constraintsWithVisualFormat:vfl2 options:kNilOptions metrics:mertrics2 views:views];
[self.view addConstraints:constraints2];
}效果：
demo示例3：
-(void)test3{
UIView *blueView = [[UIView alloc] init];
blueView.backgroundColor = [UIColor blueColor];
// 不要将AutoresizingMask转为Autolayout的约束
blueView.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:blueView];
UIView *redView = [[UIView alloc] init];
redView.backgroundColor = [UIColor redColor];
// 不要将AutoresizingMask转为Autolayout的约束
redView.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:redView];
// 间距
NSNumber *margin = @20;
// 添加水平方向的约束
NSString *vfl = @"H:|-margin-[blueView]-margin-[redView(==blueView)]-margin-|";
NSDictionary *views = NSDictionaryOfVariableBindings(blueView, redView);
NSDictionary *mertrics = NSDictionaryOfVariableBindings(margin);
NSArray *constraints = [NSLayoutConstraint constraintsWithVisualFormat:vfl options:NSLayoutFormatAlignAllTop | NSLayoutFormatAlignAllBottom metrics:mertrics views:views];
[self.view addConstraints:constraints];
// 添加竖直方向的间距
NSNumber *height = @40;
NSString *vfl2 = @"V:[blueView(height)]-margin-|";
NSDictionary *mertrics2 = NSDictionaryOfVariableBindings(margin, height);
NSArray *constraints2 = [NSLayoutConstraint constraintsWithVisualFormat:vfl2 options:kNilOptions metrics:mertrics2 views:views];
[self.view addConstraints:constraints2];效果：
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