1.UIImageView 的 contentMode

UIImageView 的 contentMode 属性用来设置图片的显示方式,如居中,局左,是否缩放等,contentMode 是 UIViewContentMode(枚举类型) 的属性,主要有下面这些样式可供选择
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typedef NS_ENUM(NSInteger, UIViewContentMode) {
    UIViewContentModeScaleToFill,       //会导致图片变形
    UIViewContentModeScaleAspectFit,      // contents scaled to fit with fixed aspect. remainder is transparent -->保证图片比例不变的最大显示图片,可能会有留白部分
    UIViewContentModeScaleAspectFill,     // contents scaled to fill with fixed aspect. some portion of content may be clipped. --> 保证图片比例不变的情况下,填充整个 imageView 视图,可能只有部分图片显示
    UIViewContentModeRedraw,              // redraw on bounds change (calls -setNeedsDisplay)
    UIViewContentModeCenter,              // contents remain same size. positioned adjusted.
    UIViewContentModeTop,
    UIViewContentModeBottom,
    UIViewContentModeLeft,
    UIViewContentModeRight,
    UIViewContentModeTopLeft,
    UIViewContentModeTopRight,
    UIViewContentModeBottomLeft,
    UIViewContentModeBottomRight,
};

以上这些模式,带有 scale 的,当图片尺寸超过 imageView 的尺寸( frame)时,可能造成只有部分图片 内容会显示在 imageView 中,可以通过剪裁命令剪裁掉不在 imageView 视图之内的图片内容self.myImageView.clipsToBounds = YES;

2.Textfield 的 leftViewMode 和 rightViewMode

TextField 是自带左边视图和右边视图的,可以用来实现一些特殊的显示效果,默认是隐藏的,
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typedef NS_ENUM(NSInteger, UITextFieldViewMode) {
    UITextFieldViewModeNever,
    UITextFieldViewModeWhileEditing,
    UITextFieldViewModeUnlessEditing,
    UITextFieldViewModeAlways
};
下面代码演示了如何使用
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UIImageView *leftN = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"icon"]];
    UIImageView *leftP = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"lock"]];
    /** 给 textField 添加 左视图 小人 */
    leftN.frame = CGRectMake(0, 0, 55, 20);
    leftN.contentMode = UIViewContentModeCenter;//contentMode
    self.userNameField.leftView = leftN;//
    self.userNameField.leftViewMode = UITextFieldViewModeAlways;
    /** 给 textField 添加 左视图 小锁 */
    leftP.frame = CGRectMake(0, 0, 55, 20);
    leftP.contentMode = UIViewContentModeCenter;//contentMode
    self.userPasswordField.leftView = leftP;//
    self.userPasswordField.leftViewMode = UITextFieldViewModeAlways;

代码最终效果 最终是这个效果