Android的硬件加速及可能导致的问题

Sep 18th, 2012 | Comments

问题

在Android 4.0的某些设备上，在View刷新时会出现花屏和屏幕上的某些View错位的现象。经过调查后发现adb logcat中出现很多OpenGLRenderer: 0x501的错位：

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09-18 14:34:39.090: DEBUG/OpenGLRenderer(3104): GL error from OpenGLRenderer: 0x50109-18 14:34:39.386: DEBUG/OpenGLRenderer(3104): GL error from OpenGLRenderer: 0x50109-18 14:34:39.656: DEBUG/OpenGLRenderer(3104): GL error from OpenGLRenderer: 0x501

从这个日志，初步怀疑是硬件加速导致的问题。经过分析发现使用了比较复杂的自定义View，可能会导致硬件加速渲染出错。

硬件加速的优点与缺点

硬件加速能使用GPU来加速2D图像的渲染速度，但是硬件加速并不能完全支持所有的渲染操作，针对自定义的View，硬件加速可能导致渲染出现错误。如果有自定义的View，需要在硬件加速的设备上进行测试，如果出现渲染的问题，需要关闭硬件加速。

开启和关闭硬件加速

对硬件加速的开关可以在不同的级别进行控制：

• Application
• Activity
• Windows
• View

Application级别

在Applciation级别控制硬件加速的开关：

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<application android:hardwareAccelerated="true" ...>

Activity级别

可以对单个的Activity控制是否启用硬件加速：

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<application android:hardwareAccelerated="true">    <activity ... />    <activity android:hardwareAccelerated="false" /></application>

Window级别

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getWindow().setFlags(    WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED,    WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED);

View级别

在指定的View上关闭硬件加速：

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myView.setLayerType(View.LAYER_TYPE_SOFTWARE, null);

或者使用android:layerType="software"来关闭硬件加速：

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<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="fill_parent"    android:layout_height="fill_parent"    android:orientation="vertical"    android:paddingLeft="2dp"    android:layerType="software"    android:paddingRight="2dp" >

如何判断一个View是否启用了硬件加速

• View.isHardwareAccelerated() returns true if the View is attached to a hardware accelerated window.
• Canvas.isHardwareAccelerated() returns true if the Canvas is hardware accelerated

Android Drawing Models

When hardware acceleration is enabled, the Android framework utilizes a new drawing model that utilizes display lists to render your application to the screen. To fully understand display lists and how they might affect your application, it is useful to understand how Android draws views without hardware acceleration as well. The following sections describe the software-based and hardware-accelerated drawing models.

Software-based drawing model

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

1. Invalidate the hierarchy
2. 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.

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:

1. Invalidate the hierarchy
2. Record and update display lists
3. 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 changing it, which is an easier bug to find if it happens.

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.

参考

• http://developer.android.com/guide/topics/graphics/hardware-accel.html