Custom onDraw() method not called

问题:

<LinearLayout android:id="@+id/svLL" android:orientation="horizontal"
    android:layout_width="fill_parent" android:layout_height="fill_parent">
    <ScrollView android:id="@+id/sv"
        android:layout_width="wrap_content" android:layout_height="wrap_content"
        xmlns:android="http://schemas.android.com/apk/res/android">
        <!--
            <TextView android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@string/scrollbar_2_text" />
        -->
        <com.mypackage.MyDrawableView
            android:layout_width="fill_parent" android:layout_height="wrap_content" />
    </ScrollView>
</LinearLayout>

---

public class MyDrawableView extends View {

    Context thisContext;

    public MyDrawableView(Context context, AttributeSet attr) {
        super(context);
        thisContext = context;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        final Paint paint = new Paint();
        paint.setColor(Color.BLUE);
        paint.setTextSize(12);
        canvas.drawText("Blah blah", 0, 100, paint);
    }
}

---

public class MyActivity extends Activity {
    // Your member variable declaration here
    // Called when the activity is first created.
    @Override
    public void onCreate(Bundle savedInstanceState) {
        // Your code here super.onCreate(savedInstanceState);
        setContentView(R.layout.xmllayout);
        LinearLayout svll = (LinearLayout) findViewById(R.id.svLL);
        svll.setLayoutParams(new LinearLayout.LayoutParams(300, 300));
    }

} 

I am putting a breakpoint, but breakpoint is never hit inside onDraw() method, what's wrong ?

回答1:

Add setWillNotDraw(false) in MyDrawableView constructor. Original answer is here.

回答2:

Your View has a height of 0. You set your View to have height=wrap_content, but your don't override onMeasure() to tell the UI toolkit how big your View is.

回答3:

You need to @Override the OnMeasure(int, int) method to specify the size of your view by calling setMeasuredDimension(int,int).

If you don't, the default method will set your view to have size width=0 height=0. And therefore don't even try to call onDraw at all.

The OnMeasure method calls setMeasuredDimension(int,int) which tells how big is your view.

(So you need to get the size of the android screen on the OnMeasure method and call setMeasuredDimension accordingly).

回答4:

Just make layout_width and layout_height of the custom view some fix at initial time instead of wrap_content or fill_parent. That worked for me.

回答5:

Though I am new in android, still I believe the problem is because of

setContentView(R.layout.xmllayout); 

Instead of "R.layout.xmllayout", pass MyDrawableView object (using findViewById) and check. I have not tried, but hope, it will work.

回答6:

You need to use invalidate() in constructor method to get the onDraw Called.

回答7:

After you set the custom view width, you should set scroll width.

FMChart = (HKFMDrawChart)findViewById(R.id.fm_chart);
FMChart.setWidth(xxxx);
float scrollViewWidth = FMChart.getScrellViewWidth(); 
FMChart.setLayoutParams(new LinearLayout.LayoutParams(
      (int) (scrollViewWidth + HKApplication.getScreenWidth() / 2),
       LinearLayout.LayoutParams.WRAP_CONTENT));
FMChart.postInvalidate(); // onDraw(); 

回答8:

Probably

setMinimumHeight(width);
setMinimumWidth(height);

in constructor are not set or onMeasure() is not overriden.

回答9:

Here is one more reason that your view might not be appearing. If you are creating your custom ViewGroup with subviews, remember to add the subviews to your ViewGroup.

In your ViewGroup constructor call

addView(mySubView);

Also remember to measure and layout your subviews in onLayout()

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    mySubview.measure(...);
    mySubview.layout(...);
}