作者:Mitchell 

一搔耕、代碼添加約束順序###

• 創(chuàng)建控件
• 將控件添加到父控件中
• 關(guān)閉需要添加約束的控件的Autoresizing屬性
• 創(chuàng)建約束

二、注意約束添加到哪里###

• 如果是自身的屬性囚聚，如寬高，添加到自身
• 如果是關(guān)于父視圖的約束，添加到父視圖
• 如果是兩個(gè)控件珍语，添加到兩個(gè)控件共同的父視圖。

三竖幔、需求案例###

• 案例1：

// 1.創(chuàng)建一個(gè)控件
    UIView *redView = [[UIView alloc] init];
    redView.backgroundColor = [UIColor redColor];
    [self.view addSubview:redView];
    
    // 1.如果是通過(guò)代碼添加Autolayout, 那么必須在添加約束之前禁用Autoresizing
    // 2.禁用Autoresizing時(shí), 必須是給誰(shuí)添加就禁用誰(shuí)的, 也就是說(shuō)如果禁用了父控件無(wú)效
    // 3.添加約束之前, 必須保證被約束的控件已經(jīng)添加到父控件中了
    //    self.view.translatesAutoresizingMaskIntoConstraints = NO;
    redView.translatesAutoresizingMaskIntoConstraints = NO;
    
    // 2.創(chuàng)建約束對(duì)象
    /*
     Item == 第一個(gè)控件
     attribute == 第一個(gè)控件的什么屬性
     relatedBy == 等于/小于等于/大于等于
     toItem == 第二個(gè)控件
     attribute == 第二個(gè)控件的什么屬性
     multiplier == 乘以多少
     constant == 加上多少
     */
    // 2.1頂部約束
    NSLayoutConstraint *topCos = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:20];
    [self.view addConstraint:topCos];
    
    // 2.1左邊約束
    NSLayoutConstraint *leftCos = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:20];
    [self.view addConstraint:leftCos];
    
    // 2.1底部約束
    NSLayoutConstraint *bottomCos = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0 constant:-20];
    [self.view addConstraint:bottomCos];
    
    // 2.1右邊約束
    NSLayoutConstraint *rightCos = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0 constant:-20];
    [self.view addConstraint:rightCos];
    
    //    寫(xiě)在這里不行, 必須在設(shè)置約束之前添加
    //    [self.view addSubview:redView];

• 案例2：

    // 1.創(chuàng)建兩個(gè)控件
    UIView *redView = [[UIView alloc] init];
    redView.backgroundColor = [UIColor redColor];
    
    UIView *blueView = [[UIView alloc] init];
    blueView.backgroundColor = [UIColor blueColor];
    
    // 2.將兩個(gè)控件添加到父控件中
    [self.view addSubview:redView];
    [self.view addSubview:blueView];
    
    // 3.關(guān)閉需要添加約束的控件的Autoresizing屬性
    redView.translatesAutoresizingMaskIntoConstraints = NO;
    blueView.translatesAutoresizingMaskIntoConstraints = NO;
    
    // 4.創(chuàng)建約束
    // 4.1藍(lán)色約束
    // 4.1.1頂部約束
    NSLayoutConstraint *blueTopCos = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:20];
    [self.view addConstraint:blueTopCos];
    
    // 4.1.2左邊約束
    NSLayoutConstraint *blueLeftCos = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:20];
    [self.view addConstraint:blueLeftCos];
    
    // 4.1.3右邊約束
    NSLayoutConstraint *blueRightCos = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0 constant:-20];
    [self.view addConstraint:blueRightCos];
    
    // 4.1.4高度約束
    NSLayoutConstraint *blueHeightCos = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:kNilOptions multiplier:1.0 constant:50];
    [blueView addConstraint:blueHeightCos];
    
    // 4.2紅色約束
    // 4.2.1頂部約束
    NSLayoutConstraint *redTopCos = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeBottom multiplier:1.0 constant:20];
    [self.view addConstraint:redTopCos];
    
    // 4.2.2右邊約束
    NSLayoutConstraint *redRightCos = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0 constant:-20];
    [self.view addConstraint:redRightCos];
    
    // 4.2.3高度約束
    NSLayoutConstraint *redHeightCos = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeHeight multiplier:1.0 constant:0.0];
    [self.view addConstraint:redHeightCos];
    
    // 4.2.4寬度約束
    NSLayoutConstraint *redWidthCos = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeWidth multiplier:0.5 constant:0.0];
    [self.view addConstraint:redWidthCos];