我正在尝试使用MPAndroidChart绘制实时数据图，并遵循代码示例here和here。我过去有带有静态数据的图表。
在第一个示例之后，我在onCreate方法中初始化了全局声明的mChart变量。但是，在调用onResume时，mChart变量再次设置为NULL，并且由于NULL异常而导致我的addEntry（）方法失败。
我需要做什么以防止将mChart设置为NULL（来自XML的其他变量在onCreate中初始化，并将其值保留在onResume中）。
顺便说一句，此应用程序还具有BLE服务，该服务使用源于AndroidStudio BLE示例的源代码运行。
这是mChart看起来像onCreate的结尾（包含了mDataField以进行比较）。
mChart = {LineChart@830034242904} "com.github.mikephil.charting.charts.LineChart{41e56d58 V.ED.... ......ID 0,0-0,0 #7f0b0066 app:id/chart}"
mDataField = {TextView@830035362136} "android.widget.TextView{41f68158 V.ED.... ......ID 0,0-0,0 #7f0b0068 app:id/data_value}"
mChart被声明为类私有变量：
private LineChart mChart;
我的onCreate（）方法（根据AndroidStudio调试器，mChart在此方法的末尾具有非NULL值）：
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.gatt_services_characteristics);
final Intent intent = getIntent();
mDeviceName = intent.getStringExtra(EXTRAS_DEVICE_NAME);
mDeviceAddress = intent.getStringExtra(EXTRAS_DEVICE_ADDRESS);
// Sets up UI references.
((TextView) findViewById(R.id.device_address)).setText(mDeviceAddress);
mGattServicesList = (ExpandableListView) findViewById(R.id.gatt_services_list);
mGattServicesList.setOnChildClickListener(servicesListClickListner);
mConnectionState = (TextView) findViewById(R.id.connection_state);
mDataField = (TextView) findViewById(R.id.data_value);
getActionBar().setTitle(mDeviceName);
getActionBar().setDisplayHomeAsUpEnabled(true);
Intent gattServiceIntent = new Intent(this, BluetoothLeService.class);
bindService(gattServiceIntent, mServiceConnection, BIND_AUTO_CREATE);
// https://github.com/PhilJay/MPAndroidChart/wiki/Getting-Started
// in this example, a LineChart is initialized from xml
LineChart mChart = (LineChart) findViewById(R.id.chart);
// mChart.setOnChartValueSelectedListener(this);
// ---------------------------------------------------------
// Set up chart formatting
// ---------------------------------------------------------
mChart.setDescription(new Description());
mChart.setNoDataText("No Data Yet");
// enable value highlighting
mChart.setHighlightPerDragEnabled(true);
mChart.setHighlightPerTapEnabled(true);
mChart.setTouchEnabled(true);
// enable scaling and dragging
mChart.setDragEnabled(true);
mChart.setScaleEnabled(true);
mChart.setDrawGridBackground(false);
// enable pinch zoom to avoid scaling x and y axis separately
mChart.setPinchZoom(true);
// alternative background color
mChart.setBackgroundColor(Color.LTGRAY);
LineData data = new LineData();
data.setValueTextColor(Color.WHITE);
// add data to line chart
mChart.setData(data);
// get legend object
Legend l = mChart.getLegend();
// customize legend
l.setForm(Legend.LegendForm.LINE);
l.setTextColor(Color.WHITE);
XAxis x1 = mChart.getXAxis();
x1.setTextColor(Color.WHITE);
x1.setDrawGridLines(false);
x1.setAvoidFirstLastClipping(true);
YAxis y1 = mChart.getAxisLeft();
y1.setTextColor(Color.WHITE);
y1.setAxisMaxValue(120f);
y1.setDrawGridLines(true);
YAxis y12 = mChart.getAxisRight();
y12.setEnabled(false);
final Button emailButton = (Button) findViewById(R.id.button_email);
emailButton.setOnClickListener(new View.OnClickListener(){
public void onClick(View view){
showEmailDialog();
}
} );
}
而且，我的onResume。根据调试器，在执行对super.onResume（）的调用之前，mChart等于NULL。
@Override
protected void onResume() {
super.onResume();
registerReceiver(mGattUpdateReceiver, makeGattUpdateIntentFilter());
if (mBluetoothLeService != null) {
final boolean result = mBluetoothLeService.connect(mDeviceAddress);
Log.d(TAG, "Connect request result=" + result);
}
// Simulate real time data
new Thread( new Runnable(){
@Override
public void run(){
// add 100 entries
for(int i = 0; i<100; i++){
runOnUiThread(new Runnable(){
@Override
public void run() {
addEntry(); // Chart is notified of update in addEntry method
}
});
// pause between adds
try {
Thread.sleep(600);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}).start();
}
最后，这是我的XML：
<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2013 The Android Open Source Project
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
-->
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp">
<LinearLayout android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp">
<TextView android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/label_device_address"
android:textSize="18sp"/>
<Space android:layout_width="5dp"
android:layout_height="wrap_content"/>
<TextView android:id="@+id/device_address"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:textSize="18sp"/>
</LinearLayout>
<com.github.mikephil.charting.charts.LineChart
android:id="@+id/chart"
android:layout_width="match_parent"
android:layout_height="200sp" />
<LinearLayout android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp">
<TextView android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/label_state"
android:textSize="12sp"/>
<Space android:layout_width="5dp"
android:layout_height="wrap_content"/>
<TextView android:id="@+id/connection_state"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="@string/disconnected"
android:textSize="12sp"/>
</LinearLayout>
<LinearLayout android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp">
<TextView android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/label_data"
android:textSize="12sp"/>
<Space android:layout_width="5dp"
android:layout_height="wrap_content"/>
<TextView android:id="@+id/data_value"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="@string/no_data"
android:textSize="12sp"/>
</LinearLayout>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal"
android:gravity="bottom|right"
android:layout_centerInParent="true"
android:weightSum="5"
android:layout_alignParentBottom="true">
<Button
android:id="@+id/button_email"
android:layout_width="0dp"
android:layout_weight="1"
android:layout_height="wrap_content"
android:textAllCaps="true"
android:maxLines="1"
android:textSize="10sp"
android:text="@string/button_email_name"/>
</LinearLayout>
<ExpandableListView android:id="@+id/gatt_services_list"
android:layout_width="match_parent"
android:layout_height="wrap_content"/>
</LinearLayout>
最佳答案
您正在onCreate中将其定义为局部变量
LineChart mChart = (LineChart) findViewById(R.id.chart);
将该行更改为
mChart = (LineChart) findViewById(R.id.chart);
由于
mChart被定义为字段