Saying that the first impression in the end how important?
For example, walking across a slovenly stranger, you probably will not see more;
for example, a new show on television, too perfunctory titles, you may immediately change the channel;
for APP is also the same, first impressions you can help draw attention to the user in the shortest possible time -
when the user downloads a new APP, that he at least interested. Therefore, the registration / sign this first hurdle, it is particularly important!
Some of the long registration process APP to no friends, ah, not only to set up a new nickname, enter a password, enter the password again, also asked to fill in a mailbox, verify email, phone number, phone verification codes to complete the series of operations, the user's patience value was jumping style burst down! 
Users not had time to get to know you, you've waved goodbye ......
In order to avoid this risk, APP try to reduce the number of steps to register / log in, the way popular are the following:
1, mobile phone number + password.
This belongs to the normal operation mode, less error-prone. But the disadvantage is that the user needs to remember the cost. Today, as many as dozens of mobile phone software, registered with the site also has hundreds of APP, in order to reduce the cost of memory, many people choose the same password, it is easy to leak risks.
Coupled with the different requirements of different APP passwords, capitalization, and add special symbols, and so do not allow pure digital adds to the complexity of the password, eventually leading to subtle differences in the user's password, once forgotten, retrieve and process very complicated, the user lost ......
2, code + phone number
SMS verification code to sign in, there is still a mainstream way, and password compared to the more time-efficient, increased security, but also can prevent malicious registrations. 
Enter the phone number and wait for a verification code, enter the verification code, click login, this process is very simple, but there are serious delays or can not be received, the registration of new users of APP transforming negative impact. Of course, if the code was leaked, there are also security risks.
3, third-party login
emergence of third-party login, really brings convenience for the public, do not need to go fill out the information, remember complex passwords.
In this way the surface looks very good, but in fact, a lot of APP after the user authorization is still required to bind phone number, enter the verification code, but rather to increase the flow, and the phenomenon of "a number of people more" more serious .
With constantly updated login, the user experience has been optimized. And compared to the three traditional ways,
"one-click login" for developers, operations staff has brought new hope!
Users only need to click login button, you can easily log in to complete the registration process, without a password, without having to wait for a verification code, the original process requires more than 20 seconds, just two seconds to complete! This new way to reduce churn and enhance user stickiness old have played a significant gain effect.
Of course, to use a key login, you need access to the operator's SDK, this step for many companies is a troublesome thing. Now, however, third-party tools Shareinstall have solved this problem, just contact the business to obtain the SDK package, registered Shareinstall account, you can create lines - on the application of the test 
Shareinstall "one-click login" functional integration of the three operators network data, a key unauthorized access, providing system according to the customer's specific business needs, increase hardware resources and channel resources; multi-layered encryption to ensure data security, high concurrency processing function to provide protection for performance, support for 20000 + QPS, support elastic expansion based on customer demand; exclusive customer will provide all-weather "7 * 24" five-star customer service, and to provide SMS traffic in real-time early warning to avoid the loss of the company, and provide a more optimized messaging support program for the company. And Shareinstall launch of the "first month free" welfare is hot ~
When your APP in Register / Log In this step, the user left a good first impression, I believe that to increase user stickiness will play a great help. Moreover, there is no doubt, a key login will be one of the main landing approach.