Masonry佈局控制元件，自動換行

masonry偏移量offset答疑：

假設子控制元件在父控制元件的內部，設定距離父控制元件的上左下右都為10，那麼相關程式碼為：

make.top.equalTo(self.view).offset(10);
make.left.equalTo(self.view).offset(10);
make.bottom.equalTo(self.view).offset(-10);
make.right.equalTo(self.view).offset(-10);
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上左為正，下右為負，這一切的一切都是因為在iOS系統中座標系的遠點是左上角，圖裡解釋的很清楚了。

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    //依次建立button 距離左右均為15
    NSArray *array = @[@"劉德華", @"劉德華，劉德華",@"劉德華華",
                       @"劉",@"劉德德華",@"劉德華，劉德華",
                       @"劉德華，劉德華", @"劉德華，劉德華", @"劉德華，劉德華",
                       @"劉德華，", @"劉德華，劉德華", @"劉德",
                       @"劉德華，劉德華", @"劉德華，", @"劉德華，劉德華",
                       @"劉德華，劉德華", @"劉德華，劉德華", @"劉德華，",
                       @"劉德華，劉德"];
    
    //存放上一個button
    UIButton *buttonBefore;
    //存放button距離左側的寬度，來確定是否需要換行
    __block CGFloat right = 0.0;
    //間距
    CGFloat margin = 15.f;
    for(int i=0; i<array.count; i++){
        NSString *str = [array objectAtIndex:i];
        CGFloat fitWidth = [str sizeWithAttributes:@{NSFontAttributeName:[UIFont systemFontOfSize:15]}].width + 30;
        UIButton *button = [[UIButton alloc] init];
        [button setTitle:str forState:UIControlStateNormal];
        button.backgroundColor = [UIColor lightGrayColor];
        [button setTitleColor:[UIColor greenColor] forState:UIControlStateNormal];
        [self.view addSubview:button];
        [button mas_makeConstraints:^(MASConstraintMaker *make) {
            make.width.mas_equalTo(fitWidth);
            make.height.mas_equalTo(20);
            if(i==0){
                make.left.equalTo(self.view).offset(margin);
                make.top.equalTo(self.view).offset(80);
                right = margin + fitWidth + margin;
            }else{
                if(right+fitWidth > [UIScreen mainScreen].bounds.size.width){
                    make.top.mas_equalTo(buttonBefore.mas_bottom).offset(margin);
                    make.left.equalTo(self.view).offset(margin);
                    right =  margin+fitWidth+margin;
                }else{
                    make.left.mas_equalTo(buttonBefore.mas_right).offset(margin);
                    make.top.equalTo(buttonBefore);
                    right += fitWidth+margin;
                }
            }
        }];
        buttonBefore = button;
    }
}
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