Android Fresco图片处理库用法API英文原文文档2-1（Facebook开源Android图片库）

这是英文文档的第二部分（1）：DRAWEE GUIDE

由于第二部分内容多一些，所以分为2个文章发。方便大家查看。

Using Drawees in XML

Drawees have very extensive customization facilities. The best way to customize your Drawee is to do so in the XML.

Here is an example that sets nearly all possible options:

<com.facebook.drawee.view.SimpleDraweeView
    android:id="@+id/my_image_view"
    android:layout_width="20dp"
    android:layout_height="20dp"
    fresco:fadeDuration="300"
    fresco:actualImageScaleType="focusCrop"
    fresco:placeholderImage="@color/wait_color"
    fresco:placeholderImageScaleType="fitCenter"
    fresco:failureImage="@drawable/error"
    fresco:failureImageScaleType="centerInside"
    fresco:retryImage="@drawable/retrying"
    fresco:retryImageScaleType="centerCrop"
    fresco:progressBarImage="@drawable/progress_bar"
    fresco:progressBarImageScaleType="centerInside"
    fresco:progressBarAutoRotateInterval="1000"
    fresco:backgroundImage="@color/blue"
    fresco:overlayImage="@drawable/watermark"
    fresco:pressedStateOverlayImage="@color/red"
    fresco:roundAsCircle="false"
    fresco:roundedCornerRadius="1dp"
    fresco:roundTopLeft="true"
    fresco:roundTopRight="false"
    fresco:roundBottomLeft="false"
    fresco:roundBottomRight="true"
    fresco:roundWithOverlayColor="@color/corner_color"
    fresco:roundingBorderWidth="2dp"
    fresco:roundingBorderColor="@color/border_color"
  />
           

Height and width mandatory

You must declare both 

android:layout_width

 and 

android:layout_height

. Without both of these two, the view will not be able to lay the image out correctly.

wrap_content

Drawees do not support the 

wrap_content

 value for the 

layout_width

 and 

layout_height

attributes.

The reason for this is that the content's size changes. The size of your downloaded image can be different from your placeholder - and the failure and retry images, if any, can be still different.

Use of 

wrap_content

 would force Android to do another layout pass when your image comes in - and for the layout to change before users' eyes, creating a jarring effect.

Fixing the aspect ratio

This is the one time you should use 

wrap_content.

You can force a DraweeView to be laid out in a particular aspect ratio. If you want a width:height ratio of 4:3, for instance, do this:

<com.facebook.drawee.view.SimpleDraweeView
    android:id="@+id/my_image_view"
    android:layout_width="20dp"
    android:layout_height="wrap_content"
    <!-- other attributes -->
           

Then specify your aspect ratio in Java:

Using Drawees in Code

Change the image

The easy to way is to call

For more advanced requirements, use a controller builder.

Customizing the hierarchy

For most apps, specify the parameters of their hierarchy in XML provides all the customization they need. In some cases, though, you may need to go further.

We create an instance of the builder and then set it to the view:

List<Drawable> backgroundsList;
List<Drawable> overlaysList;
GenericDraweeHierarchyBuilder builder =
    new GenericDraweeHierarchyBuilder(getResources());
GenericDraweeHierarchy hierarchy = builder
    .setFadeDuration(300)
    .setPlaceholderImage(new MyCustomDrawable())
    .setBackgrounds(backgroundList)
    .setOverlays(overlaysList)
    .build();
mSimpleDraweeView.setHierarchy(hierarchy);
           

Do not call 

setHierarchy

 more than once on the same view, even if the view is recycled. The hierarchy is expensive to create and is intended to be used more than once. Use

setController

 or 

setImageURI

 to change the image shown in it.

Modifying the hierarchy in-place

Some attributes of the hierarchy can be changed while the application is running.

You would first need to get it from the View:

Change the placeholder

Then you could modify the placeholder, either with a resource id:

or a full-fledged Drawable:

Drawable drawable; 
// create your drawable
hierarchy.setPlaceholderImage(drawable);
           

Change the image display

You can change the scale type:

If you have chosen scale type 

focusCrop,

 you'll need to set a focus point:

You can add a color filter to the image:

ColorFilter filter;
// create your filter
hierarchy.setActualImageColorFilter(filter);
           

Rounding

All of the rounding related params, except the rounding method, can be modified. You get a

RoundingParams

 object from the hierarchy, modify it, and set it back again:

RoundingParams roundingParams = hierarchy.getRoundingParams();
roundingParams.setCornersRadius(10);
hierarchy.setRoundingParams(roundingParams);
           

Drawee Image Branches

Contents

• Definitions
• Actual
• Placeholder
• Failure
• Retry
• Progress Bar
• Backgrounds
• Overlays
• Pressed State Overlay

Definitions

This page outlines the different image branches that can be displayed in a Drawee, and how they are set.

Except for the actual image, all of them can be set by an XML attribute. The value in XML must be either an Android drawable or color resource.

They can also be set by a method in the GenericDraweeHierarchyBuilder class, if setting programmatically. In code, the value can either be from resources or be a custom subclass ofDrawable.

Some of the items can even be changed on the fly after the hierarchy has been built. These have a method in the GenericDraweeHierarchy class.

Several of the drawables can be scaled.

Actual

The actual image is the target; everything else is either an alternative or a decoration. This is specified using a URI, which can point to an image over the Internet, a local file, a resource, or a content provider.

This is a property of the controller, not the hierarchy. It therefore is not set by any of the methods used by the other Drawee components.

Instead, use the 

setImageURI

 method or set a controller programmatically.

In addition to the scale type, the hierarchy exposes other methods only for the actual image. These are:

• the focus point (used for the focusCrop scale type only)
• a color filter

Default scale type: 

centerCrop

Placeholder

The placeholder is shown in the Drawee when it first appears on screen. After you have called 

setController

 or 

setImageURI

 to load an image, the placeholder continues to be shown until the image has loaded.

In the case of a progressive JPEG, the placeholder only stays until your image has reached the quality threshold, whether the default, or one set by your app.

XML attribute: 

placeholderImage

Hierarchy builder method: 

setPlaceholderImage

Hierarchy method: 

setPlaceholderImage

Default value: a transparent ColorDrawable

Default scale type: 

centerInside

Failure

The failure image appears if there is an error loading your image. The most common cause of this is an invalid URI, or lack of connection to the network.

XML attribute: 

failureImage

Hierarchy builder method: 

setFailureImage

Default value: The placeholder image

Default scale type: 

centerInside

Retry

The retry image appears instead of the failure image if you have set your controller to enable the tap-to-retry feature.

You must build your own Controller to do this. Then add the following line

The image pipeline will then attempt to retry an image if the user taps on it. Up to four attempts are allowed before the failure image is shown instead.

XML attribute: 

retryImage

Hierarchy builder method: 

setRetryImage

Default value: The placeholder image

Default scale type: 

centerInside

Progress Bar

If specified, the progress bar image is shown as an overlay over the Drawee until the final image is set.

Currently the progress bar remains the same throughout the image load; actually changing in response to progress is not yet supported.

XML attribute: 

progressBarImage

Hierarchy builder method: 

setProgressBarImage

Default value: None

Default scale type: 

centerInside

Backgrounds

Background drawables are drawn first, "under" the rest of the hierarchy.

Only one can be specified in XML, but in code more than one can be set. In that case, the first one in the list is drawn first, at the bottom.

Background images don't support scale-types and are scaled to the Drawee size.

XML attribute: 

backgroundImage

Hierarchy builder method: 

setBackground,

setBackgrounds

Default value: None

Default scale type: N/A

Overlays

Overlay drawables are drawn last, "over" the rest of the hierarchy.

Only one can be specified in XML, but in code more than one can be set. In that case, the first one in the list is drawn first, at the bottom.

Overlay images don't support scale-types and are scaled to the Drawee size.

XML attribute: 

overlayImage

Hierarchy builder method: 

setOverlay,

setOverlays

Default value: None

Default scale type: N/A

Pressed State Overlay

The pressed state overlay is a special overlay shown only when the user presses the screen area of the Drawee. For example, if the Drawee is showing a button, this overlay could have the button change color when pressed.

The pressed state overlay doesn't support scale-types.

XML attribute: 

pressedStateOverlayImage

Hierarchy builder method: 

setPressedStateOverlay

Default value: None

Default scale type: N/A

Scaling

You can specify a different scale type for each of the different drawables in your Drawee. The

Available scale types

Scale Type	Explanation
center	Center the image in the view, but perform no scaling.
centerCrop	Scales the image so that both dimensions will be greater than or equal to the corresponding dimension of the parent.  One of width or height will fit exactly.  The image is centered within parent's bounds.
focusCrop	Same as centerCrop, but based around a caller-specified focus point instead of the center.
centerInside	Downscales the image so that it fits entirely inside the parent.  Unlike  fitCenter,  no upscaling will be performed.  Aspect ratio is preserved.  The image is centered within parent's bounds.
fitCenter	Scales the image so that it fits entirely inside the parent.  One of width or height will fit exactly.  Aspect ratio is preserved.  The image is centered within the parent's bounds.
fitStart	Scales the image so that it fits entirely inside the parent.  One of width or height will fit exactly.  Aspect ratio is preserved.  The image is aligned to the top-left corner of the parent.
fitEnd	Scales the image so that it fits entirely inside the parent.  One of width or height will fit exactly.  Aspect ratio is preserved.  The image is aligned to the bottom-right corner of the parent.
fitXY	Scales width and height independently, so that the image matches the parent exactly.  Aspect ratio is not preserved.
none	Used for Android's tile mode.

These are mostly the same as those supported by the Android ImageView class.

The one unsupported type is 

matrix.

 In its place, Fresco offers 

focusCrop,

 which will usually work better.

How to set

Actual, placeholder, retry, and failure images can all be set in XML, using attributes like

fresco:actualImageScaleType

. You can also set them in code using theGenericDraweeHierarchyBuilder class.

Even after your hierarchy is built, the actual image scale type can be modified on the fly using GenericDraweeHierarchy.

However, do not use the 

android:scaleType

 attribute, nor the 

.setScaleType

 method. These have no effect on Drawees.

focusCrop

Android, and Fresco, offer a 

centerCrop

 scale type, which will fill the entire viewing area while preserving the aspect ratio of the image, cropping as necessary.

This is very useful, but the trouble is the cropping doesn't always happen where you need it. If, for instance, you want to crop to someone's face in the bottom right corner of the image,

centerCrop

 will do the wrong thing.

By specifying a focus point, you can say which part of the image should be centered in the view. If you specify the focus point to be at the top of the image, such as (0.5f, 0f), we guarantee that, no matter what, this point will be visible and centered in the view as much as possible.

Focus points are specified in a relative coordinate system. That is, (0f, 0f) is the top-left corner, and (1f, 1f) is the bottom-right corner. Relative coordinates allow focus points to be scale-invariant, which is highly useful.

A focus point of (0.5f, 0.5f) is equivalent to a scale type of 

centerCrop.

To use focus points, you must first set the right scale type in your XML:

fresco:actualImageScaleType="focusCrop"
           

In your Java code, you must programmatically set the correct focus point for your image:

PointF focusPoint;
// your app populates the focus point
mSimpleDraweeView
    .getHierarchy()
    .setActualImageFocusPoint(focusPoint);
           

none#

If you are using Drawables that make use of Android's tile mode, you need to use the 

none

scale type for this to work correctly.

Rounded Corners and Circles

Not every image is a rectangle. Apps frequently need images that appear with softer, rounded corners, or as circles. Drawee supports a variety of scenarios, all without the memory overhead of copying bitmaps.

What

Images can be rounded in two shapes:

1. As a circle - set 

   roundAsCircle

    to true.
2. As a rectangle, but with rounded corners. Set 

   roundedCornerRadius

    to some value.

Rectangles support having each of the four corners have a different radius, but this must be specified in Java code rather than XML.

How

Images can be rounded with two different methods:

1. BITMAP_ONLY

    - Uses a shader to draw the bitmap with rounded corners. This is the default rounding method. This works only on the actual image and the placeholder. Other branches, like failure and retry images, are not rounded. Furthermore, this rounding method doesn't support animations.

2. OVERLAY_COLOR

    - Draws rounded corners by overlaying a solid color, specified by the caller. The Drawee's background should be static and of the same solid color. Use

   roundWithOverlayColor

    in XML, or 

   setOverlayColor

    in code, for this effect.

In XML

The 

SimpleDraweeView

 class will forward several attributes over to 

RoundingParams

:

<com.facebook.drawee.view.SimpleDraweeView
   ...
   fresco:roundedCornerRadius="5dp"
   fresco:roundBottomLeft="false"
   fresco:roundBottomRight="false"
   fresco:roundWithOverlayColor="@color/blue"
   fresco:roundingBorderWidth="1dp"
   fresco:roundingBorderColor="@color/red"
           

In code#

When constructing a hierarchy, you can pass an instance of RoundingParams to your

GenericDraweeHierarchyBuilder:

RoundingParams roundingParams = RoundingParams.fromCornersRadius(7f);
roundingParams.setOverlayColor(R.color.green);
// alternatively use fromCornersRadii or asCircle
genericDraweeHierarchyBuilder
    .setRoundingParams(roundingParams);
           

You can also change most of the rounding parameters on the fly:

RoundingParams roundingParams = 
    mSimpleDraweeView.getHierarchy().getRoundingParams();
roundingParams.setBorder(R.color.red, 1.0);
roundingParams.setRoundAsCircle(true);
mSimpleDraweeView.getHierarchy().setRoundingParams(roundingParams);
           

The one exception to this is that the 

RoundingMethod

 cannot be changed when changing dynamically. Attempting to do so will throw an 

IllegalStateException.