我的应用程序每次单击floataction按钮时动态创建（最多）8个按钮，但是当屏幕旋转时，所有按钮都会消失。我知道这背后的原因，那是因为我没有在生成的视图上设置onSaveInstanceState。我尝试使用outState.putInt设置onSaveInstanceState（因为我认为这是合适的，因为按钮ID是int类型）但它不起作用。
这是我的Java。
public class MainActivity extends AppCompatActivity {
int counter = 0;
FloatingActionButton addingSemester;
Button semesterButton;
LinearLayout semesterLayout;
GridLayout semesterGridLayout;
LinearLayout.LayoutParams portraitLayoutParams = new LinearLayout.LayoutParams(
AppBarLayout.LayoutParams.MATCH_PARENT,
AppBarLayout.LayoutParams.WRAP_CONTENT);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
addingSemester = (FloatingActionButton) findViewById(R.id.addActionButton);
semesterLayout = (LinearLayout) findViewById(R.id.main_layout);
semesterGridLayout = (GridLayout)findViewById(R.id.semester_grid_layout);
semesterButton = new Button(MainActivity.this);
}
@Override
public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
outState.putInt("semesterButton", semesterButton.getId());
super.onSaveInstanceState(outState, outPersistentState);
}
@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
savedInstanceState.getInt("semesterButton", semesterButton.getId());
super.onRestoreInstanceState(savedInstanceState);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater menuInflater = getMenuInflater();
menuInflater.inflate(R.menu.main, menu);
return super.onCreateOptionsMenu(menu);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.delete) {
new AlertDialog.Builder(MainActivity.this)
.setTitle("Delete entry")
.setMessage("Are you sure you want to delete everything?")
.setCancelable(true)
.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
if (MainActivity.this.getResources().getBoolean(R.bool.is_landscape)) {
semesterGridLayout.removeAllViews();
} else if (!MainActivity.this.getResources().getBoolean(R.bool.is_landscape)) {
semesterLayout.removeAllViews();
}
counter = 0;
}
})
.setNegativeButton("No", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
dialog.cancel();
}
})
.show();
return true;
}
return super.onOptionsItemSelected(item);
}
public void onFloatActionButtonClick(View view) {
Display display = getWindowManager().getDefaultDisplay();
Point size = new Point();
display.getSize(size);
double width = (size.x)/3;
semesterButton = new Button(MainActivity.this);
if (counter < 8) {
semesterButton.setId(counter + 1);
semesterButton.setText("Semester " + (counter + 1));
semesterButton.setBackgroundColor(getColor(R.color.colorPrimary));
semesterButton.setTextColor(Color.WHITE);
portraitLayoutParams.setMargins(24, 24, 24, 24);
if (MainActivity.this.getResources().getBoolean(R.bool.is_landscape)) {
GridLayout.LayoutParams params = new GridLayout.LayoutParams();
params.setMargins(24, 24, 24, 24);
params.width = (int) width;
params.height = GridLayout.LayoutParams.WRAP_CONTENT;
semesterButton.setLayoutParams(params);
semesterGridLayout.addView(semesterButton);
} else if (!MainActivity.this.getResources().getBoolean(R.bool.is_landscape)) {
semesterLayout.addView(semesterButton);
semesterButton.setLayoutParams(portraitLayoutParams);
}
// these lines moved outside the if statement blocks
counter++;
setOnLongClickListenerForSemesterButton();
} else if (counter == 8) {
Toast.makeText(MainActivity.this, "You cannot add more than 8 semesters", Toast.LENGTH_SHORT).show();
}
}
private void setOnLongClickListenerForSemesterButton() {
semesterButton.setOnLongClickListener(new View.OnLongClickListener() {
@Override
public boolean onLongClick(View v) {
final Button b = (Button) v;
b.setTag(b.getText().toString());
b.setBackgroundColor(Color.RED);
b.setText("Delete");
AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
builder.setTitle("Delete entry");
builder.setMessage("Are you sure you want to delete this entry?");
builder.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
if (MainActivity.this.getResources().getBoolean(R.bool.is_landscape)) {
semesterGridLayout.removeView(b);
for (int i = 0; i < semesterGridLayout.getChildCount(); i++) {
((Button) semesterGridLayout.getChildAt(i)).setText("Semester " + (i + 1));
}
} else if (!MainActivity.this.getResources().getBoolean(R.bool.is_landscape)) {
semesterLayout.removeView(b);
for (int i = 0; i < semesterLayout.getChildCount(); i++) {
((Button) semesterLayout.getChildAt(i)).setText("Semester " + (i + 1));
}
}
counter--;
}
});
builder.setNegativeButton("No", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
b.cancelLongPress();
b.setBackgroundColor(ContextCompat.getColor(MainActivity.this, R.color.colorPrimary));
b.setText(b.getTag().toString());
dialog.cancel();
}
});
builder.show();
return true;
}
});
}
}
UPDATE：对于我的onSaveInstaceState，我这样做了：
@Override
public void onSaveInstanceState(Bundle outsave) {
super.onSaveInstanceState(outsave);
if (MainActivity.this.getResources().getBoolean(R.bool.is_landscape)) {
outsave.putInt("NUMBEROFBUTTONS", semesterGridLayout.getChildCount());
} else if (!MainActivity.this.getResources().getBoolean(R.bool.is_landscape)) {
outsave.putInt("NUMBEROFBUTTONS", semesterLayout.getChildCount());
}
}
对于我的onCreate，这就是我所做的：
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
addingSemester = (FloatingActionButton) findViewById(R.id.addActionButton);
semesterLayout = (LinearLayout) findViewById(R.id.main_layout);
semesterGridLayout = (GridLayout)findViewById(R.id.semester_grid_layout);
semesterButton = new Button(MainActivity.this);
if(savedInstanceState!=null) {
int numberofbuttons = savedInstanceState.getInt("NUMBEROFBUTTONS");
if (MainActivity.this.getResources().getBoolean(R.bool.is_landscape)) {
for(int i = 0; i < numberofbuttons; i++){
semesterLayout.addView(semesterButton);
}
} else if (!MainActivity.this.getResources().getBoolean(R.bool.is_landscape)) {
for(int i = 0; i < numberofbuttons; i++){
semesterGridLayout.addView(semesterButton);
}
}
}
}
这就是我在stacktrace上遇到的问题
FATAL EXCEPTION: main
Process: myapp.onur.journeygpacalculator, PID: 10463
java.lang.NullPointerException: Attempt to invoke virtual method 'int android.widget.GridLayout.getChildCount()' on a null object reference
at myapp.onur.journeygpacalculator.MainActivity.onSaveInstanceState(MainActivity.java:68)
基本上问题是我无法获取GridLayout对象的getChildCount（）。
答案 0 :(得分：0)
您无法在配置更改之间保存与UI相关的变量（这是旋转屏幕时发生的情况）。
但是，您有多种选择可以做您想做的事情：
您可以使用片段。如果您使用＆＃34; setRetainInstance＆＃34;在按钮片段上，它应该在配置更改之间保持相同，但是这种方法不允许您处理backstack（用于使用后退按钮恢复更改）。亲：最简单的交易方式。缺点：meh，setRetainInstance用于保存没有UI元素的片段。
您可以在编写时保存onSaveInstanceState中的按钮数，但是您需要检查＆＃34; onCreate＆＃34;上的包，如果不是null，请获取该数字，并相应地添加按钮。亲：容易。缺点：如果你从按钮交换到任何东西或者按钮依赖于某些外部数据，这样做可能会改变按钮的外观。
您可以自行管理屏幕旋转。我不知道该怎么做但我不推荐这个选项，因为如果用户交换语言，区域或导致配置更改的任何内容，问题仍然存在。但是，即使这是最糟糕的（IMO），也需要提及此选项。
编辑：根据要求提供有关second-＆gt;
的更多详细信息首先，覆盖onSaveInstanceState：
public void onSaveInstanceState(Bundle outsave) {
super.onSaveInstanceState(outsave);
outsave.putInt("NUMBEROFBUTTONS", number_of_buttons);
}
然后，在onCreate期间：
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if(savedInstanceState!=null) {
int number_of_button = savedInstanceState.getInt("NUMBEROFBUTTONS");
display_X_buttons(number_of_button); /*sum up : display number_of_button buttons as you wish display them.*/
}
}