MPAndroidchart mChart在onCreate和onResume之間設置爲null

Question

我想用MPAndroidChart來繪製實時數據，並且遵循代碼示例here和here。我有過去的靜態數據圖表。

繼第一個示例之後，我在onCreate方法中初始化全局聲明的mChart變量。但是，當調用onResume時，mChart變量再次設置爲NULL，並且由於NULL異常，我的addEntry（）方法失敗。

我需要做些什麼來保持mChart不被設置爲NULL（XML中的其他變量在onCreate中初始化並在onResume中保留它們的值）。

順便說一下，這個應用程序還有一個BLE服務，運行的源代碼源自AndroidStudio BLE示例。

這就是mChart看起來像onCreate的結尾（包含mDataField用於比較）。

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被聲明爲類私有變量：

private LineChart mChart; 

我onCreate（）方法（根據AndroidStudio調試器，mChart具有在該方法的最後一個非空值）：

@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(); 
      } 
     }); 
    } 

而我的onResume。根據調試器，在調用super.onResume（）之前，mChart等於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(); 

} 

最後，這裏是我的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> 

Answer 1

你將它定義爲一個局部變量onCreate

LineChart mChart = (LineChart) findViewById(R.id.chart); 

把上面一行

mChart = (LineChart) findViewById(R.id.chart); 

由於mChart已定義作爲字段