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Diffstat (limited to 'docs/html/resources/articles/drawable-mutations.jd')
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diff --git a/docs/html/resources/articles/drawable-mutations.jd b/docs/html/resources/articles/drawable-mutations.jd deleted file mode 100644 index c5818fc..0000000 --- a/docs/html/resources/articles/drawable-mutations.jd +++ /dev/null @@ -1,93 +0,0 @@ -page.title=Drawable Mutations -parent.title=Articles -parent.link=../browser.html?tag=article -@jd:body - -<p>Android's drawables are extremely useful to easily build applications. A -{@link android.graphics.drawable.Drawable Drawable} is a pluggable drawing -container that is usually associated with a View. For instance, a -{@link android.graphics.drawable.BitmapDrawable BitmapDrawable} is used to display -images, a {@link android.graphics.drawable.ShapeDrawable ShapeDrawable} to draw -shapes and gradients, and so on. You can even combine them to create complex -renderings.</p> - -<p>Drawables allow you to easily customize the rendering of the widgets without -subclassing them. As a matter of fact, they are so convenient that most of the -default Android apps and widgets are built using drawables; there are about 700 -drawables used in the core Android framework. Because drawables are used so -extensively throughout the system, Android optimizes them when they are loaded -from resources. For instance, every time you create a -{@link android.widget.Button Button}, a new drawable is loaded from the framework -resources (<code>android.R.drawable.btn_default</code>). This means all buttons -across all the apps use a different drawable instance as their background. -However, all these drawables share a common state, called the "constant state." -The content of this state varies according to the type of drawable you are -using, but it usually contains all the properties that can be defined by a -resource. In the case of a button, the constant state contains a bitmap image. -This way, all buttons across all applications share the same bitmap, which saves -a lot of memory.</p> - -<p>The following diagram shows what entities are -created when you assign the same image resource as the background of -two different views. As you can see, two drawables are created but they -both share the same constant state, hence the same bitmap:</p> - -<img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 307px; height: 400px;" src="images/shared_states.png" alt="" id="BLOGGER_PHOTO_ID_5331437883277472082" border="0"> - -<p>This state sharing feature is great to avoid wasting memory but it can cause -problems when you try to modify the properties of a drawable. Imagine an -application with a list of books. Each book has a star next to its name, totally -opaque when the user marks the book as a favorite, and translucent when the book -is not a favorite. To achieve this effect, you would probably write the -following code in your list adapter's <code>getView()</code> method:</p> - -<pre>Book book = ...; -TextView listItem = ...; - -listItem.setText(book.getTitle()); - -Drawable star = context.getResources().getDrawable(R.drawable.star); -if (book.isFavorite()) { - star.setAlpha(255); // opaque -} else { - star.setAlpha(70); // translucent -}</pre> - -<p>Unfortunately, this piece of code yields a rather strange result: -all of the drawables have the same opacity:</p> - -<img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 267px; height: 400px;" src="images/all_drawables_changed.png" alt="" id="BLOGGER_PHOTO_ID_5331438978390342066" border="0"> - -<p>This -result is explained by the constant state. Even though we are getting a -new drawable instance for each list item, the constant state remains -the same and, in the case of BitmapDrawable, the opacity is part of the -constant state. Thus, changing the opacity of one drawable instance -changes the opacity of all the other instances. Even worse, working -around this issue was not easy with Android 1.0 and 1.1.</p> - -<p>Android 1.5 and higher offers a very easy way to solve this issue -with the new {@link android.graphics.drawable.Drawable#mutate()} method</a>. -When you invoke this method on a drawable, the constant state of the -drawable is duplicated to allow you to change any property without -affecting other drawables. Note that bitmaps are still shared, even -after mutating a drawable. The diagram below shows what happens when -you invoke <code>mutate()</code> on a drawable:</p> - -<img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 307px; height: 400px;" src="images/mutated_states.png" alt="" id="BLOGGER_PHOTO_ID_5331440144116345074" border="0"> - -<p>Let's update our previous piece of code to make use of <code>mutate()</code>:</p> - -<pre>Drawable star = context.getResources().getDrawable(R.drawable.star); -if (book.isFavorite()) { - star.mutate().setAlpha(255); // opaque -} else { - star. mutate().setAlpha(70); // translucent -}</pre> - -<p>For convenience, <code>mutate()</code> -returns the drawable itself, which allows to chain method calls. It -does not however create a new drawable instance. With this new piece of -code, our application now behaves correctly:</p> - -<img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 267px; height: 400px;" src="images/correct_drawables.png" alt="" id="BLOGGER_PHOTO_ID_5331440757515573842" border="0"> |