did i use wrong with the TextEditingController? #36671

before i explain my issues, please check out its ok about the offical example.
https://api.flutter.dev/flutter/widgets/TextEditingController-class.html
This example doesn't seem to work.
just like this:

Steps to Reproduce

i just copy the code from the example, and run it.

I also have another problem. In my own code, I instantiate a _controller. When I assign a value to the controller property of TextField, the following problem occurs:

1. Open simulator;
2. Open keyboard;
3. Input values;
4. Click done;
5. The value in the TextField disappears;

I don't know why, but when I remove _controller, the problem disappears.

just a line code about the controller:

  TextEditingController _controller = TextEditingController();

Logs

[✓] Flutter (Channel stable, v1.7.8+hotfix.3, on Mac OS X 10.14.5 18F203, locale zh-Hans-CN)
• Flutter version 1.7.8+hotfix.3 at /Users/hejingguo/flutter
• Framework revision b712a17 (13 days ago), 2019-07-09 13:14:38 -0700
• Engine revision 54ad777fd2
• Dart version 2.4.0

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.0)
• Android SDK at /Users/hejingguo/Library/Android/sdk
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-29, build-tools 29.0.0
• Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 10.2.1)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 10.2.1, Build version 10E1001
• CocoaPods version 1.7.3

[✓] iOS tools - develop for iOS devices
• ios-deploy 1.9.4

[✓] Android Studio (version 3.4)
• Android Studio at /Applications/Android Studio.app/Contents
• Flutter plugin version 37.1.1
• Dart plugin version 183.6270
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)

[✓] IntelliJ IDEA Ultimate Edition (version 2019.1.1)
• IntelliJ at /Applications/IntelliJ IDEA.app
• Flutter plugin version 36.1.4
• Dart plugin version 191.7830

[✓] VS Code (version 1.36.0)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 3.2.0

[✓] Connected device (1 available)
• iPhone 6s • 4AE27175-9674-443D-B05D-0B437F6A927C • ios • com.apple.CoreSimulator.SimRuntime.iOS-12-2 (simulator)

• No issues found!