UITextField的placeholderLabel

// 核心就是根据placeholderLabel在UITextField的实例变量中的偏移量直接拿到placeholderLabel的地址，然后强转，接下来就可以为所欲为了。。。
UILabel *placeholderLabel = [LDUtil getIvar:self.textField ivarName:@"_placeholderLabel"];
[placeholderLabel setTextColor:[UIColor redColor]];

+ (id)getIvar:(id)obj ivarName:(NSString *)name {
    unsigned int numIvars; // 成员变量个数
    Ivar *vars = class_copyIvarList([obj class], &numIvars);
    id resultObj;
    for(int i =0; i < numIvars; i++) {
        if ([[NSString stringWithUTF8String:ivar_getName(vars[i])] isEqualToString:name]) {
            ptrdiff_t offset = ivar_getOffset(vars[i]);
            // 核心代码就只有下面这一句，类似的其他类的其他变量，你懂的...
            resultObj = (__bridge id)((void *)(*((long *)((__bridge void *)obj + offset))));
            break;
        }
    }
    free(vars);
    return resultObj;
}