Android ImageView setAlpha(float)、setAlpha(int)及setImageAlpha(int)的区别及踩过的坑

做项目中踩过的一个坑，记录一下

需求是先把ImageView设置成透明不可见，然后在某个条件下再设置成可见，代码如下

mImageView.setAlpha(0); // 设置成透明
mImageView.setAlpha(1f); // 设置成不透明

结果是设置成透明可以，但是要再设置成不透明显示控件却死活不可以，后来改成如下代码就可以了

mImageView.setAlpha(0f); // 设置成透明
mImageView.setAlpha(1f); // 设置成不透明

原来setAlpha(0)和setAlpha(0f)分别调用的是不同的方法

setAlpha(int)

这是在ImageView中定义的方法，已经被废弃了，取值范围是0-255，

0表示透明

255表示不透明

所以如果要设置ImageView为不透明，正确的值是255，而不是1

/**
 * Sets the alpha value that should be applied to the image.
 *
 * @param alpha the alpha value that should be applied to the image
 *
 * @deprecated use #setImageAlpha(int) instead
 */
@Deprecated
@RemotableViewMethod
public void setAlpha(int alpha) {
    alpha &= 0xFF;          // keep it legal
    if (mAlpha != alpha) {
        mAlpha = alpha;
        mColorMod = true;
        applyColorMod();
        invalidate();
    }
}

setAlpha(float)

这其实是在ImageView的父类View中定义的方法，推荐用此方法，取值范围为0.0f-1.0f

0.0f表示透明

1.0f表示不透明

/**
 * Sets the opacity of the view to a value from 0 to 1, where 0 means the view is
 * completely transparent and 1 means the view is completely opaque.
 *
 * <p class="note"><strong>Note:</strong> setting alpha to a translucent value (0 < alpha < 1)
 * can have significant performance implications, especially for large views. It is best to use
 * the alpha property sparingly and transiently, as in the case of fading animations.</p>
 *
 * <p>For a view with a frequently changing alpha, such as during a fading animation, it is
 * strongly recommended for performance reasons to either override
 * {@link #hasOverlappingRendering()} to return <code>false</code> if appropriate, or setting a
 * {@link #setLayerType(int, android.graphics.Paint) layer type} on the view for the duration
 * of the animation. On versions {@link android.os.Build.VERSION_CODES#M} and below,
 * the default path for rendering an unlayered View with alpha could add multiple milliseconds
 * of rendering cost, even for simple or small views. Starting with
 * {@link android.os.Build.VERSION_CODES#M}, {@link #LAYER_TYPE_HARDWARE} is automatically
 * applied to the view at the rendering level.</p>
 *
 * <p>If this view overrides {@link #onSetAlpha(int)} to return true, then this view is
 * responsible for applying the opacity itself.</p>
 *
 * <p>On versions {@link android.os.Build.VERSION_CODES#LOLLIPOP_MR1} and below, note that if
 * the view is backed by a {@link #setLayerType(int, android.graphics.Paint) layer} and is
 * associated with a {@link #setLayerPaint(android.graphics.Paint) layer paint}, setting an
 * alpha value less than 1.0 will supersede the alpha of the layer paint.</p>
 *
 * <p>Starting with {@link android.os.Build.VERSION_CODES#M}, setting a translucent alpha
 * value will clip a View to its bounds, unless the View returns <code>false</code> from
 * {@link #hasOverlappingRendering}.</p>
 *
 * @param alpha The opacity of the view.
 *
 * @see #hasOverlappingRendering()
 * @see #setLayerType(int, android.graphics.Paint)
 *
 * @attr ref android.R.styleable#View_alpha
 */
public void setAlpha(@FloatRange(from=0.0, to=1.0) float alpha) {
    ensureTransformationInfo();
    if (mTransformationInfo.mAlpha != alpha) {
        setAlphaInternal(alpha);
        if (onSetAlpha((int) (alpha * 255))) {
            mPrivateFlags |= PFLAG_ALPHA_SET;
            // subclass is handling alpha - don't optimize rendering cache invalidation
            invalidateParentCaches();
            invalidate(true);
        } else {
            mPrivateFlags &= ~PFLAG_ALPHA_SET;
            invalidateViewProperty(true, false);
            mRenderNode.setAlpha(getFinalAlpha());
        }
    }
}

setImageAlpha(int)

看源码可知setImageAlpha(int)实际内部调用的是setAlpha(int)，所以取值返回也是0-255，表示从透明到不透明

/**
 * Sets the alpha value that should be applied to the image.
 *
 * @param alpha the alpha value that should be applied to the image (between
 * 0 and 255 inclusive, with 0 being transparent and 255 being opaque)
 *
 * @see #getImageAlpha()
 */
@RemotableViewMethod
public void setImageAlpha(int alpha) {
    setAlpha(alpha);
}

总结

• setAlpha(float)推荐使用，取值范围为0.0f-1.0f，表示透明到不透明
• setAlpha(int)，已废弃，不推荐使用，取值范围为1-255，表示透明到不透明
• setImageAlpha(int)，内部实际调用setAlpha(int)，取值范围也是1-255，表示透明到不透明