具有自定义视图的活动在启动时崩溃

Question

我正在尝试使用自定义视图创建绘画活动，但是当我尝试启动活动时应用程序崩溃。从布局文件中删除myDrawView时，它不会崩溃。我的猜测是布局文件以某种方式无法与我创建的自定义绘图视图连接。

这里是活动：

public class PaintActivity extends AppCompatActivity {

    MyDrawView myDrawView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_paint);
        RelativeLayout parent = (RelativeLayout) findViewById(R.id.drawing);
        myDrawView = new MyDrawView(this);
        parent.addView(myDrawView);
    }
    public void back(View view){

        super.onBackPressed();
    }
}

布局文件：

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".view.PaintActivity">

    <ImageButton
        android:id="@+id/back"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="44dp"
        android:onClick="back"
        app:layout_constraintBottom_toTopOf="@+id/drawing"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:srcCompat="@drawable/back" />

    <info.jonas.notes.view.MyDrawView
        android:id="@+id/drawing"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginBottom="63dp"
        android:layout_marginEnd="61dp"
        android:layout_marginLeft="61dp"
        android:layout_marginRight="61dp"
        android:layout_marginStart="61dp"
        android:layout_weight="1"
        android:background="#FFFFFFFF"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/back" />

</android.support.constraint.ConstraintLayout>

MyDrawView.java类：

public class MyDrawView extends View {

    private Bitmap  mBitmap;
    private Canvas  mCanvas;
    private Path    mPath;
    private Paint   mBitmapPaint;
    private Paint   mPaint;

    public MyDrawView(Context c) {
        super(c);

        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);

        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setDither(true);
        mPaint.setColor(0xFF000000);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeWidth(3);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        mCanvas = new Canvas(mBitmap);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);

        canvas.drawPath(mPath, mPaint);
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y) {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }
    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
            mX = x;
            mY = y;
        }
    }
    private void touch_up() {
        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        // kill this so we don't double draw
        mPath.reset();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
        }
        return true;
    }

    public void clear(){
        mBitmap.eraseColor(Color.TRANSPARENT);
        invalidate();
        System.gc();
    }}

这是堆栈跟踪：

10-07 15:12:26.797 20682-20682/info.androidhive.sqlite E/AndroidRuntime: FATAL EXCEPTION: main
    Process: info.androidhive.sqlite, PID: 20682
    java.lang.RuntimeException: Unable to start activity ComponentInfo{info.androidhive.sqlite/info.jonas.notes.view.PaintActivity}: android.view.InflateException: Binary XML file line #20: Binary XML file line #20: Error inflating class info.jonas.notes.view.MyDrawView
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: android.view.InflateException: Binary XML file line #20: Binary XML file line #20: Error inflating class info.jonas.notes.view.MyDrawView
     Caused by: android.view.InflateException: Binary XML file line #20: Error inflating class info.jonas.notes.view.MyDrawView
     Caused by: java.lang.NoSuchMethodException: <init> [class android.content.Context, interface android.util.AttributeSet]
        at java.lang.Class.getConstructor0(Class.java:2327)
        at java.lang.Class.getConstructor(Class.java:1725)
        at android.view.LayoutInflater.createView(LayoutInflater.java:615)
        at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:790)
        at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:730)
        at android.view.LayoutInflater.rInflate(LayoutInflater.java:863)
        at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:824)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:515)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:423)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:374)
        at android.support.v7.app.AppCompatDelegateImplV9.setContentView(AppCompatDelegateImplV9.java:287)
        at android.support.v7.app.AppCompatActivity.setContentView(AppCompatActivity.java:139)
        at info.jonas.notes.view.PaintActivity.onCreate(PaintActivity.java:18)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

请帮助我找到问题。

Answer 1

我看到您有两个问题。第一个在堆栈跟踪中，第二个在修复第一个之后会遇到。

首先，您需要向自定义视图添加另一个构造函数。否则，Android无法应用XML中的任何属性。那就是崩溃发生的地方。将您当前的构造函数更改为：

public MyDrawView(Context c, AttributeSet attrs) {
    super(c, attrs);

    mPath = new Path();
    mBitmapPaint = new Paint(Paint.DITHER_FLAG);

    mPaint = new Paint();
    mPaint.setAntiAlias(true);
    mPaint.setDither(true);
    mPaint.setColor(0xFF000000);
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setStrokeJoin(Paint.Join.ROUND);
    mPaint.setStrokeCap(Paint.Cap.ROUND);
    mPaint.setStrokeWidth(3);
}

并添加另一个构造函数：

public MyDrawView(Context c) {
    this(c, null);
}

第二个问题是您的整个onCreate()方法：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_paint);
    RelativeLayout parent = (RelativeLayout) findViewById(R.id.drawing); 
    myDrawView = new MyDrawView(this);
    parent.addView(myDrawView);
}

R.id.drawing是您的自定义视图，它不会扩展RelativeLayout。实际上，您的XML中甚至没有RelativeLayout。然后，您要创建View的新实例，并尝试将其添加到此不存在的RelativeLayout吗？如果您已经有了XML的自定义视图，则不需要任何代码。全部删除。

如果您需要引用您的视图：

myDrawView = findViewById(R.id.drawing);

Answer 2

您需要覆盖XML膨胀时使用的其他构造函数：

  public MyDrawView(Context context, AttributeSet attrs) {....

attrs包含视图属性，如果需要，您也可以创建自己的属性。

Answer 3

根据堆栈跟踪，您的MyDrawView需要重写一个构造函数。

public class MyDrawView extends View {

private Bitmap mBitmap;
private Canvas mCanvas;
private Path mPath;
private Paint   mBitmapPaint;
private Paint mPaint;

public MyDrawView(Context c) {
    super(c);
    init();
}

public MyDrawView(Context context, @Nullable AttributeSet attrs) {
    super(context, attrs);
    init();
}

public void init() {
    mPath = new Path();
    mBitmapPaint = new Paint(Paint.DITHER_FLAG);

    mPaint = new Paint();
    mPaint.setAntiAlias(true);
    mPaint.setDither(true);
    mPaint.setColor(0xFF000000);
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setStrokeJoin(Paint.Join.ROUND);
    mPaint.setStrokeCap(Paint.Cap.ROUND);
    mPaint.setStrokeWidth(3);
}
.........
.........
//Rest of the code

Answer 4

这段代码有两个问题：

第一：您需要添加缺少的构造函数

public MyDrawView(Context context) {
        this(context, null);
    }

    public MyDrawView(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

public MyDrawView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

2nd：错误的投射

findViewById(R.id.drawing);不是RelativeLayout，而是您的自定义视图，即MyDrawView