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
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(); } }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()); } } }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); }
