我在 dart 操作员操作中遇到错误

Question

color:sensorver?.leftMotorSpeed <= 50 ? Colors.amber : Colors.red,

I want to change color using above code. If the incoming data is less than 50, I want to make it red if not amber. But I am getting an error in the operator part (<=). Can you help fix it?

Answer 1

check this

 color: sensorver!.leftMotorSpeed <= 50 ? Colors.amber : Colors.red,

Answer 2

You need to check fist sensorver is null or not as below:

color: sensorver!=null&&sensorver!.leftMotorSpeed <= 50 ? Colors.amber : Colors.red,

Answer 3

To expand on Sabahat's answer these are null safe operators in Dart .

sensorver?.leftMotorSpeed basically means 'give me the value of leftMotorSpeed if sensorver is not null.

Therefore you get an error when trying to see if it's less than or equal to 50 since you can't compare something that might be null to the number 50, which is definitely not null. This is dart's way of trying to ensure you don't encounter exceptions at runtime due to a null value.

There are two ways you can deal with this.

1. Dangerous option : Tell Dart you know sensorver is definitely not null (Sabahat's answer).

   You can tell dart you know for certain sensorver is not null with the ! operator. This is basically telling Dart 'shut up with you warnings, I know this will never be null'. Thus Sabahat's answer of color: sensorver.?leftMotorSpeed <= 50. Colors:amber. Colors,red, color: sensorver.?leftMotorSpeed <= 50. Colors:amber. Colors,red, will work.

   However if sensorver ever is null it will cause your entire program to crash

2. Safe option : Provide a fallback value.

   If sensorver could be null the safe option is to provide a fallback value that dart will use in the case it is null. This can be accomplished with the ?? operator. For example:

   color: sensorver?.leftMotorSpeed?? 0 <= 50? Colors.amber: Colors.red,

   This means that if sensorver is not null it will provide you the value of leftMotorSpeed . However if sensorver is null, it will provide you with the fallback value of 0 . The fallback value can be anything you like, as long as it's guaranteed to not be null.