TextView: onDraw getting called only once

问题

I have a situation where I want to debug calls to TextView.onDraw() so I subclassed it like this:

public class MyTextView extends AppCompatTextView {

    View parent;

    public MyTextView(Context context) {
        super(context);
    }

    public MyTextView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public MyTextView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    public void storeParent(View parent) {
        this.parent = parent;
    }

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

and put it up inside a hierarchy like this:(see MyTextView)

<?xml version="1.0" encoding="utf-8"?>
<ScrollView 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="com.example.scrollviewtest1.MainActivity">

    <LinearLayout
        android:id="@+id/scroll_container"
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <com.example.scrollviewtest1.MyTextView
            android:id="@+id/text2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/large_text" />

        <TextView
            android:id="@+id/text1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Hello World!" />

    </LinearLayout>


</ScrollView>

Lastly, I put a very large piece of text for this TextView so that it becomes scrollable.

Now, if I put a breakpoint inside the onDraw() method, I only get one call to it. As per my understanding, I should be getting call to it as I scroll up and down. Why is this happening?

Side note: I have already tried setting setWillNotDraw(false) with no change in the outcome.

回答1:

Credit goes to @pskink for suggesting turning off H/W acceleration for this; it worked. Here is what docs say about this:

Hardware accelerated drawing model

The Android system still uses invalidate() and draw() to request screen updates and to render views, but handles the actual drawing differently. Instead of executing the drawing commands immediately, the Android system records them inside display lists, which contain the output of the view hierarchy’s drawing code. Another optimization is that the Android system only needs to record and update display lists for views marked dirty by an invalidate() call. Views that have not been invalidated can be redrawn simply by re-issuing the previously recorded display list. The new drawing model contains three stages:

Invalidate the hierarchy
Record and update display lists
Draw the display lists

With this model, you cannot rely on a view intersecting the dirty region to have its draw() method executed. To ensure that the Android system records a view’s display list, you must call invalidate(). Forgetting to do so causes a view to look the same even after it has been changed.

Using display lists also benefits animation performance because setting specific properties, such as alpha or rotation, does not require invalidating the targeted view (it is done automatically). This optimization also applies to views with display lists (any view when your application is hardware accelerated.) For example, assume there is a LinearLayout that contains a ListView above a Button. The display list for the LinearLayout looks like this:

DrawDisplayList(ListView)
DrawDisplayList(Button)

Assume now that you want to change the ListView's opacity. After invoking setAlpha(0.5f) on the ListView, the display list now contains this:

SaveLayerAlpha(0.5)
DrawDisplayList(ListView)
Restore
DrawDisplayList(Button)

The complex drawing code of ListView was not executed. Instead, the system only updated the display list of the much simpler LinearLayout. In an application without hardware acceleration enabled, the drawing code of both the list and its parent are executed again.

VS

Software-based drawing model

In the software drawing model, views are drawn with the following two steps:

Invalidate the hierarchy
Draw the hierarchy

Whenever an application needs to update a part of its UI, it invokes invalidate() (or one of its variants) on any view that has changed content. The invalidation messages are propagated all the way up the view hierarchy to compute the regions of the screen that need to be redrawn (the dirty region). The Android system then draws any view in the hierarchy that intersects with the dirty region. Unfortunately, there are two drawbacks to this drawing model:

First, this model requires execution of a lot of code on every draw pass. For example, if your application calls invalidate() on a

button and that button sits on top of another view, the Android system redraws the view even though it hasn't changed. The second issue is that the drawing model can hide bugs in your application. Since the Android system redraws views when they intersect the dirty region, a view whose content you changed might be redrawn even though invalidate() was not called on it. When this happens, you are relying on another view being invalidated to obtain the proper behavior. This behavior can change every time you modify your application. Because of this, you should always call invalidate() on your custom views whenever you modify data or state that affects the view’s drawing code.

Note: Android views automatically call invalidate() when their properties change, such as the background color or the text in a TextView.

来源：https://stackoverflow.com/questions/46987711/textview-ondraw-getting-called-only-once