Difference between setAlpha and setImageAlpha

问题:

ImageView has two methods related methods: setAlpha and setImageAlpha. The former is available since API level 1, but is deprecated since level 16. The latter is available since level 16. There's also another setAlpha method, from the View class and this is introduced in API level 11.

Is the difference between ImageView#setAlpha and ImageView#setImageAlpha only in the naming? Is there any behavioral difference? What's the relationship between View#setAlpha and ImageView#setAlpha?

回答1:

ImageView.setAlpha(int) has been renamed to ImageView.setImageAlpha(int) to avoid confusion with the new method View.setAlpha(float) introduced in API level 11.

View.setAlpha(float) is a general method available on all Views, including ImageView. It applies the specified opacity to the whole view. To achieve this, by default the system creates a temporary buffer (a hardware layer) where the View is drawn as usual, then the buffer is drawn on the screen with the specified alpha value. It's a two-pass mechanism which requires the initial allocation of a buffer, so it's somewhat slower. See this video for more information and how to change the default behavior: Hidden Cost of Transparency. It's important to note that ImageView includes by default an optimization that will avoid this buffer allocation if it has no background, so in practice there will be no performance penalty when calling ImageView.setAlpha(float) if the ImageView has no background.

ImageView.setImageAlpha(int) (and ImageView.setAlpha(int)) are methods proper to the ImageView. They control the alpha value which is used to draw the content image (bitmap or other) directly on the screen, with no intermediate pass, so this is the preferred method to use to apply transparency to an image displayed by an ImageView. Of course if you set a background Drawable on your ImageView that you also want to be translucent, this method will not produce the expected result.

回答2:

• View.setAlpha(float) accepts a float as input and expects a value in the range 0..1 inclusive.
• ImageView.setAlpha(int) accepts an int as input and expects a value on the range 0..255 inclusive. ImageView.setAlpha(int) is deprecated. This is probably because they wanted to remove the conflict with the underlying View.setAlpha(float)
• As other responders have pointed out ImageView.setImageAlpha(int) simply calls through to the deprecated ImageView.setAlpha(int). You should expect that ImageView.setAlpha(int) will be removed in a future API update and should therefore avoid using it.

回答3:

yes, it is only a naming difference - the current implementation in the Android source in API level 16 is:

/**
 * Sets the alpha value that should be applied to the image.
 *
 * @param alpha the alpha value that should be applied to the image
 *
 * @see #getImageAlpha()
 */
@RemotableViewMethod
public void setImageAlpha(int alpha) {
    setAlpha(alpha);
}

回答4:

I believe only the naming is a difference. Because setImageAlpha() is more specific than setAlpha(). For the View#setAlpha and ImageView#setAlpha/setImageAlpha there is not a direct relation.. the View class has a setAlpha because it could function as a parent of a View#ImageView so it functions like a container.

With the setAlpha you could set the opacity of the whole container therefor it needs that method. with the setImageAlpha you could set the alpha of only the image and not the whole container.

Besides this i can't really think of a reason they have both an alpha method.