android 显示GIF动画

因为公司项目的需要，需要从后台获取gif动画，然后展示。Android本身并没有提供相关的直接显示gif动画的GifView之类的，通常使用webview显示，然后去搜索了下，发现有另外的实现思路，然后就借鉴了下。在这里总结下。另外，自己也动手写了一个gif加载框架，GifLoader，后面会开一篇讲解gifloader使用以及框架思路，已上传至jcenter 有需要的同学点这里

显示gif主要用到了android.graphics.Movie这个类

可以看到，movie提供了三个decode方法，分别从byte[]，file，InputStream中获取数据，并转换为movie对象，实际上movie对象中的数据即是要展示的gif数据，看看每个方法的具体含义

draw(Canvas canvas, float x, float y)  //canvas 绘制的画布，x，绘制左边起始位置，y，绘制的头起始部位置

draw(Canvas canvas, float x, float y， Paint paint) //同上，多了一个paint，自定义画笔

duration() //gif的动画时长

height() //gif高度，这里的高度是指，gif图片内容本身的高度，非view的高度

isOpaque() //是否透明

setTime(int relativeMilliseconds) //设置当前的时间，可以控制当前显示哪一帧

width()  // 同height()

那怎么使用呢？我们上GifView源码

import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Movie;
import android.os.Build;

import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.widget.ImageView;


public class GifView extends ImageView {
    private static final String TAG = "ImageView";
    private static final int DEFAULT_MOVIEW_DURATION = 2000;
    private int mMovieResourceId;
    private Movie mMovie;
    private long mMovieStart;
    private int mCurrentAnimationTime;
    /**
     * Position for drawing animation frames in the center of the view.
     */
    private float mLeft;
    private float mTop;

    private int mMeasureWidth;
    private int mMeasureHeight;

    /**
     * Scaling factor to fit the animation within view bounds.
     */
    private float mScale = 1;

    private volatile boolean mPaused;
    private boolean mVisible = true;

    public GifView(Context context) {
        this(context, null);
    }

    public GifView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public GifView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        setViewAttributes();
    }

    @SuppressLint("NewApi")
    public void setViewAttributes() {

        /**
         * Starting from HONEYCOMB(Api Level:11) have to turn off HW acceleration to draw
         * Movie on Canvas.
         */
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
            setLayerType(View.LAYER_TYPE_SOFTWARE, null);
        }
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if (mMovie != null) {
            if (!mPaused) {
                updateAnimationTime();
                drawMovieFrame(canvas);
                invalidateView();
            } else {
                drawMovieFrame(canvas);
            }
        }
    }

    /**
     * Draw current GIF frame
     */
    private void drawMovieFrame(Canvas canvas) {

        mMovie.setTime(mCurrentAnimationTime);

        canvas.save(Canvas.MATRIX_SAVE_FLAG);
        canvas.scale(mScale, mScale);
        mMovie.draw(canvas, mLeft / mScale, mTop / mScale);
        canvas.restore();
    }

    /**
     * Calculate current animation time
     */
    private void updateAnimationTime() {
        long now = android.os.SystemClock.uptimeMillis();

        if (mMovieStart == 0) {
            mMovieStart = now;
        }

        int dur = mMovie.duration();

        if (dur == 0) {
            dur = DEFAULT_MOVIEW_DURATION;
        }

        mCurrentAnimationTime = (int) ((now - mMovieStart) % dur);
    }

    /**
     * Invalidates view only if it is visible.
     * 

     * {@link #postInvalidateOnAnimation()} is used for Jelly Bean and higher.
     */
    @SuppressLint("NewApi")
    private void invalidateView() {
        if (mVisible) {
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) {
                postInvalidateOnAnimation();
            } else {
                invalidate();
            }
        }
    }

    @SuppressLint("NewApi")
    @Override
    public void onScreenStateChanged(int screenState) {
        super.onScreenStateChanged(screenState);
        mVisible = screenState == SCREEN_STATE_ON;
        invalidateView();
    }

    @SuppressLint("NewApi")
    @Override
    protected void onVisibilityChanged( View changedView, int visibility) {
        super.onVisibilityChanged(changedView, visibility);
        mVisible = visibility == View.VISIBLE;
        invalidateView();
    }

    @Override
    protected void onWindowVisibilityChanged(int visibility) {
        super.onWindowVisibilityChanged(visibility);
        mVisible = visibility == View.VISIBLE;
        invalidateView();
    }

    public void setMovieResource(int movieResId) {
        this.mMovieResourceId = movieResId;
        mMovie = Movie.decodeStream(getResources().openRawResource(mMovieResourceId));
        requestLayout();
    }

    public void setMovie(Movie movie) {
        this.mMovie = movie;
        setScale();
        invalidate();
    }

    private void setScale() {
        if (mMovie != null) {
            float scaleW = (float) getWidth() / mMovie.width();
            float scaleH = (float) getHeight() / mMovie.height();
            mScale = Math.min(scaleH, scaleW);
        }
    }

    public void setMovieTime(int time) {
        mCurrentAnimationTime = time;
        invalidate();
    }

    public void setPaused(boolean paused) {
        this.mPaused = paused;

        /**
         * Calculate new movie start time, so that it resumes from the same
         * frame.
         */
        if (!paused) {
            mMovieStart = android.os.SystemClock.uptimeMillis() - mCurrentAnimationTime;
        }

        invalidate();
    }

    public boolean isPaused() {
        return this.mPaused;
    }

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        if (changed) {
            if (mMovie != null) {
                float scaleW = (float) getWidth() / mMovie.width();
                float scaleH = (float) getHeight() / mMovie.height();
                mScale = Math.min(scaleH, scaleW);
            }
        }
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        if (mMovie != null) {
            int widthMode = MeasureSpec.getMode(widthMeasureSpec);
            int heightMode = MeasureSpec.getMode(heightMeasureSpec);
            int widthSize = MeasureSpec.getSize(widthMeasureSpec);
            int heightSize = MeasureSpec.getSize(heightMeasureSpec);
            if (widthMode == MeasureSpec.AT_MOST) {
                Log.e(TAG, "---moviewidth before" + mMovie.width());
                int movieWidth = mMovie.width();
                mMeasureWidth = Math.min(movieWidth, widthSize);
                Log.e(TAG, "---moviewidth after" + mMovie.width());
            } else {
                mMeasureWidth = widthSize;
            }
            if (heightMode == MeasureSpec.AT_MOST) {
                Log.e(TAG, "---height " + mMovie.height());
                int movieHieght = mMovie.height();
                mMeasureHeight = Math.min(movieHieght, heightSize);

            } else {
                mMeasureHeight = heightSize;
            }
            Log.e(TAG, "--wi : " + mMeasureWidth + "  h  : " + mMeasureHeight);
            setMeasuredDimension(mMeasureWidth, mMeasureHeight);
        } else {
            super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        }
    }
}

代码还是比较简洁清晰的
继承自ImageView是因为加载gif是一个比较耗时的工作（从网上下载），这个时候可以设置一张过渡图片，用户体验较好。
onMeasure()的作用是测量view大小，当设置wrap_content时，取movieSize和viewSize较小值
onLayout里面的代码是为了计算mScale,即gif内容部分需要放大或者缩小的比例，如果不计算则会出现，内容和控件尺寸不匹配的情况，ondraw则根据动画时长，和当前时间，计算出当前显示的帧，类似于动画差值器，然后去绘制当前显示的画面。
使用也比较简单，直接调用几个set方法，将movie，或者gif资源传给gifview即可。
最后上效果

效果还是挺流畅的，喜欢的同学可以拿去
另外也开源了gifloader加载网络gif gifloader框架 需要的同学可以在项目中使用，后面会专门介绍gifloader的使用以及框架的解析
欢迎拍砖!