我正在尝试使用MPAndroidChart来绘制实时数据，并遵循代码示例here和here。我有过静态数据的图表。
在第一个例子之后，我在onCreate方法中初始化我的全局声明的mChart变量。但是，当调用onResume时，mChart变量再次设置为NULL，并且我的addEntry（）方法由于NULL异常而失败。
我需要做什么才能使mChart不被设置为NULL（来自XML的其他变量在onCreate中初始化并在onResume中保留它们的值）。
顺便说一句，这个应用程序还有一个运行源代码的BLE服务，源自AndroidStudio BLE样本。这就是mChart看起来像onCreate的结尾（包括mDataField用于比较）。
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在此方法的末尾有一个非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>
答案 0 :(得分：1)
您将其定义为onCreate
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