Creating & Using 9-patch images in Android

问题:

I recently heard about 9-patch images. I know its 9 tiled and is stretchable. I'd like to know more about it.

• How can I create a 9-patch image?

  Is there any tool? Can I create it from AndroidSDK or code?

• Main Advantages of 9-patch over regular png?

  (is it stretchable dynamically/ automatically according to screen?)

回答1:

Today I discovered nine patch images. The official documentation (linked to in the other answers) is ok I suppose, but it really lacks in examples.

This little tutorial has some great examples at the end that answer the second part of your question, explaining how the scaling works - not just for buttons - but also frames, and it has a complete example project that you can download and play with. The most important advantage over PNG is that nine patch images can specify non-contiguous areas to scale.

To answer the first part of your question, the SDK (and now Android Studio) ships with "Draw 9-patch" ("draw9patch" in the SDK tools folder) which is a simple editor. Here is a nicer one which is also open source. It has a simple but clever default image.

回答2:

Most of the examples talk about creating a 9-patch image, but implementation details are usually left at a high level.

Nick's post above - with the good 9-patch tutorial that provides a working project download file , saved the day.

Here are the main implementation details that worked for me (once you have a 9-patch image ready to go):

1. Reference the drawable with the name but don't include .9.png (auto-complete in eclipse will take care of this)

2. Make sure you only have 1 image under the main /drawable folder (not a version for each dpi folder)

3. The image must be specified using :background, not :src (this got me stuck for a while)

   android:background="@drawable/splash_logo"

4. Make sure the image and layout that contains it are using:

   android:layout_width="fill_parent"
android:layout_height="fill_parent"

回答3:

How can I create a 9-patch image? Is there any tool? Can I create it from AndroidSDK or code?

When you search a major search engine for android 9-patch tool, the very first hit is for the Android developer documentation page on the draw9patch tool.

Main Advantages of 9-patch over regular png? (is it stretchable dynamically/ automatically according to screen?)

The Android developer documentation contains other pages that describe nine-patch PNG files. This documentation includes passages like:

A NinePatchDrawable graphic is a stretchable bitmap image, which Android will automatically resize to accommodate the contents of the View in which you have placed it as the background. An example use of a NinePatch is the backgrounds used by standard Android buttons — buttons must stretch to accommodate strings of various lengths.

回答4:

With 9-patch image you can choose which part of your image may be stretched. It must be png image and name must end with .9.png (something.9.png)

http://developer.android.com/tools/help/draw9patch.html

Simple tool for all densities:

http://android-ui-utils.googlecode.com/hg/asset-studio/dist/nine-patches.html

回答5:

1.What are NinePatch images?

NinePatch images are PNG images that mark the parts of an image that can be stretched. They have an extension like image_name.9.png.

2.Where they are stored in android project ?

res/drawable/image_name.9.png

3.How to create NinePatch image for your anndroid app ?

Android SDK includes a WYSIWIG draw9patch.jar tool inside your Android SDK /tools folder.

回答6:

Main Advantages of 9-patch over regular png

Actually 9 Patch images are stretchable, repeatable images reduced to their smallest size.the image won't stretch and loose proportions in different screen sizes. One more and biggest advantage is memory.

Same small size memory can be reused for different screen size devices. Well-designed 9-patch images are less error-prone and have high re-usability .

https://developer.android.com/studio/write/draw9patch.html

回答7:

Step 1, Since you are not familiar yet, prepare a xxxhdpi png big image to play around first.

Step 2, That image desired expandable field must shorter (by removing redundant/repeated color part) as possible since 9-patch no such thing "reduce", but "expand" the image.

I personally use the ImageMagick command line tool to convert it, e.g.:

convert -trim 'my_image.png' png32:my_image_trim.png #trim extra transparent surrounded image
rm lala*; convert my_image_trim.png -crop 310  +repage  +adjoin  png32:lala%02d.png #cut redundant/repeat center part
convert +append lala02.png lala05.png png32:out_right.png #append left/right images side by side
convert -resize 144x144\! out_right.png png32:my_image.png #resize to the desired dpi

There is one pitfall when convert: I must prefix with png32, or else will get black 9-patch image, see this thread.

Step 3, Copy image to Android Studio drawable, then right-click and choose menu item "Create 9-Patch file...". A .9.png new image will be generated with the same image name. Now can just delete the original image. Careful when playing around with refactor to rename for backup since it will rename the XML image id too, and make you wonder why the 9-patch image not working since XML still referring non-9-patch image.

Step 4, Left and top 2 black lines form an expandable rectangular area, while right and bottom 2 black lines form a text rectangular area.

Your image size will grow on your declared expandable area as text growing. While the text area means text only allow in that area.

You don't have to draw the black dot/lines from scratch for simple usage, 4 black lines already existed behind the top, left, bottom, right on that image if your 9-Patch image generated by Android Studio. "Zoom in" to bigger if you can't see to drag that lines.

The original position of 2 black lines, vertical and horizontal, both declare an expandable area:

• The original position of left vertical line is between upper left to bottom left
• The original position of top horizontal line is between upper left to top right.

The original position of 2 black lines, vertical and horizontal, both declare a text area:

• The original position of right vertical line is between upper right to bottom right.
• The original position of bottom horizontal line is between bottom left to bottom right.

Above is the original position of black lines before you start to drag to narrow down the length, to adjust the start position and end position of that lines.

Both expandable and text areas can be different depends on your needs. But normally expandable area should equal OR less than text area, a classic example will be chat bubble image:

The image above has equal top and bottom black lines width but right black line is higher than left line, which also means that text consistently stay up to half of bottom curve in either minimum size or expanded size. And it only expand in body of text area.

Now you will know the two benefits of 9-Patch: the bottom and right line together form a text area which perfectly ensures text never overflow to outside of image curve ! And also defined which portion of text area responsible to expand the image as text growing, while keep the curve without scaling.

Hover the line will able to see the x,y positions in pixels, it help to measures both lines are equal or less in position.

You should remember the position of line when dragging, since left line can drag to right side, and the right line can drag to left side, and you may get lost of which line is expandable line and which line is text line.

There are 2 important checkboxes you should tick, i.e. "Show patches" and "Show bad patches" checkboxes.

In "Show bad patches" checkbox, if you drawing the expandable area but covering the curved line of your image instead of a straight line, then it will mark that area as red to warn you. You can narrow your line to dismiss that red warning, or just ignore it. Keep in mind that, the red warning may misleading which may actually be caused by the opposite line, in that case, you need to narrow the opposite line to dismiss that red warning.

Step 5, In xml, you can refer to that image as TextView background like below, use wrap_content to make it expandable:

<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@drawable/your_9_patch_image_name_excluded_.9"
    android:orientation="vertical">
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        />
    <2nd TextView />
</LinearLayout>

When first time playing around with it, ensures no padding on textview or fancy parent layout, or else you may wonder why not working as expected.

Button background example:

    <Button
        android:id="@+id/btn2"
        android:layout_width="wrap_content"
        android:layout_height="40dp"
        android:background="@drawable/your_9_patch_image_name_excluded_.9"
        android:layout_gravity="center"
        />

回答8:

This is good tool: Click here.

9 Patch images are stretchable, repeatable images reduced to their smallest size. The simplest example would be if you were to take a rounded div and slice it up into 9 squares like you would a tic-tac-toe board. The four corners wouldn't change sizes at all but would be static while the other 5 pieces would be stretched or repeated to allow the whole image to scale appropriately.

With that explanation and the advent of CSS3 you might think that there is no reason to use 9 patch images but the name '9 patch' is a misnomer. The images can be sliced up into even smaller pieces.

9 Patch images contain an index of which piece is what by adding a 1px border to the image. The colors in the border determine if a piece is static (doesn't scale), it stretches, or it repeats.

Google Slideshow: https://docs.google.com/present/view?id=dc7ghz8w_34f8338rcg

See also the Android developer info about 9-patch images: http://developer.android.com/guide/topics/graphics/2d-graphics.html#nine-patch