Will someone please explain RESULT_FIRST_USER

问题:

I don't understand the meaning, value, or importance of RESULT_FIRST_USER, other than that my own result codes must be greater than 1. Will someone please explain it? RESULT_OK and RESULT_CANCELED make perfect sense to an english speaker. But what in the world of android is RESULT_FIRST_USER? All the documentations says about it is

Start of user-defined activity results.

回答1:

When you finish an activity, you can call setResult(RESULT_CODE) to send back data to another activity. If you don't call this method, the default value will be RESULT_CANCELLED (which equals 0)

Example of returning data:

Intent intent= new Intent();
intent.putExtra("data",data);
setResult(YOUR_RESULT_CODE,intent);
finish();

回答2:

When an activity ends, it can call setResult(int) to return data back to its parent.

It must always supply a result code, which can be the standard results

• RESULT_CANCELED (Standard activity result: Operation canceled. Constant Value: 0)
• RESULT_OK (Standard activity result: operation succeeded. Constant Value: -1), or any custom values starting at RESULT_FIRST_USER (Start of user-defined activity results. Constant Value: 1). In addition, it can optionally return back an Intent containing any additional data it wants.

So, bottom line since you must supply the result code Android "helps' you a bit by saying: please state if the result code of this Activity is OK, CANCELED or you have your own, custom made, result.

回答3:

The answer to the question is actually the combination of comments from @CommonsWare and @Chris. So, for the sake of progeny, I am going to consolidate the comments and make it available in one place.

Basically, there are two predefined constants for the requestCode and they are Activity.RESULT_OK and Activity.RESULT_CANCELLED. However, android developers can also set custom codes for their apps by using the offset Activity.RESULT_FIRST_USER. Doing so ensures that there are no clashes between constants set at the OS level and the app level.

Purely, my opinion, I think that the FIRST USER suffix is meant to refer to developers – just like how end consumer refers to consumers of a said product – who are the first users before the app users.

Below is an example of how you can use this offset,

public static final int MY_RESULT_CODE = Activity.RESULT_FIRST_USER + 1;